The heating value (or energy value or calorific value ) of a substance , usually a fuel or food (see food energy ), is the amount of heat released during the combustion of a specified amount of it.
120-462: The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions . The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities: There are two kinds of enthalpy of combustion, called high(er) and low(er) heat(ing) value, depending on how much
240-548: A basal metabolic rate of 80 watts. For example, if our bodies run (on average) at 80 watts, then a light bulb running at 100 watts is running at 1.25 human equivalents (100 ÷ 80) i.e. 1.25 H-e. For a difficult task of only a few seconds' duration, a person can put out thousands of watts, many times the 746 watts in one official horsepower. For tasks lasting a few minutes, a fit human can generate perhaps 1,000 watts. For an activity that must be sustained for an hour, output drops to around 300; for an activity kept up all day, 150 watts
360-462: A battery (from chemical energy to electric energy ), a dam (from gravitational potential energy to kinetic energy of moving water (and the blades of a turbine ) and ultimately to electric energy through an electric generator ), and a heat engine (from heat to work). Examples of energy transformation include generating electric energy from heat energy via a steam turbine, or lifting an object against gravity using electrical energy driving
480-503: A physical system , recognizable in the performance of work and in the form of heat and light . Energy is a conserved quantity —the law of conservation of energy states that energy can be converted in form, but not created or destroyed; matter and energy may also be converted to one another. The unit of measurement for energy in the International System of Units (SI) is the joule (J). Forms of energy include
600-435: A Lagrangian; for example, dissipative systems with continuous symmetries need not have a corresponding conservation law. In the context of chemistry , energy is an attribute of a substance as a consequence of its atomic, molecular, or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is usually accompanied by a decrease, and sometimes an increase, of
720-474: A biological cell or organelle of a biological organism. Energy used in respiration is stored in substances such as carbohydrates (including sugars), lipids , and proteins stored by cells . In human terms, the human equivalent (H-e) (Human energy conversion) indicates, for a given amount of energy expenditure, the relative quantity of energy needed for human metabolism , using as a standard an average human energy expenditure of 12,500 kJ per day and
840-515: A bound system is discrete (a set of permitted states, each characterized by an energy level ) which results in the concept of quanta . In the solution of the Schrödinger equation for any oscillator (vibrator) and for electromagnetic waves in a vacuum, the resulting energy states are related to the frequency by Planck's relation : E = h ν {\displaystyle E=h\nu } (where h {\displaystyle h}
960-411: A change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer . When transfer of matter into or out of the system is also prevented and no electrical or mechanical (stirring shaft or lift pumping) work is done, at constant pressure the enthalpy change equals the energy exchanged with the environment by heat . In chemistry,
1080-501: A combustion of fuel, measured as a unit of energy per unit mass or volume of substance. In contrast to the HHV, the LHV considers energy losses such as the energy used to vaporize water - although its exact definition is not uniformly agreed upon. One definition is simply to subtract the heat of vaporization of the water from the higher heating value. This treats any H 2 O formed as a vapor that
1200-786: A constant external pressure, which is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work W {\displaystyle W} that was done against constant external pressure P ext {\displaystyle P_{\text{ext}}} to establish the system's physical dimensions from V system, initial = 0 {\displaystyle V_{\text{system, initial}}=0} to some final volume V system, final {\displaystyle V_{\text{system, final}}} (as W = P ext Δ V {\displaystyle W=P_{\text{ext}}\Delta V} ), i.e. to make room for it by displacing its surroundings. The pressure-volume term
1320-573: A constant number of particles at constant pressure, the difference in enthalpy is the maximum amount of thermal energy derivable from an isobaric thermodynamic process. The total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead. Enthalpy change is defined by the following equation: Δ H = H f − H i , {\displaystyle \Delta H=H_{\mathsf {f}}-H_{\mathsf {i}}\,,} where For an exothermic reaction at constant pressure ,
SECTION 10
#17327824945961440-417: A core concept. Work , a function of energy, is force times distance. This says that the work ( W {\displaystyle W} ) is equal to the line integral of the force F along a path C ; for details see the mechanical work article. Work and thus energy is frame dependent . For example, consider a ball being hit by a bat. In the center-of-mass reference frame, the bat does no work on
1560-492: A crane motor. Lifting against gravity performs mechanical work on the object and stores gravitational potential energy in the object. If the object falls to the ground, gravity does mechanical work on the object which transforms the potential energy in the gravitational field to the kinetic energy released as heat on impact with the ground. The Sun transforms nuclear potential energy to other forms of energy; its total mass does not decrease due to that itself (since it still contains
1680-556: A few exceptions, like those generated by volcanic events for example. An example of a solar-mediated weather event is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, suddenly give up some of their thermal energy to power a few days of violent air movement. In a slower process, radioactive decay of atoms in the core of the Earth releases heat. This thermal energy drives plate tectonics and may lift mountains, via orogenesis . This slow lifting represents
1800-407: A fuel of composition C c H h O o N n , the (higher) heat of combustion is 419 kJ/mol × ( c + 0.3 h − 0.5 o ) usually to a good approximation (±3%), though it gives poor results for some compounds such as (gaseous) formaldehyde and carbon monoxide , and can be significantly off if o + n > c , such as for glycerine dinitrate, C 3 H 6 O 7 N 2 . By convention,
1920-474: A given temperature T . This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation . The activation energy necessary for a chemical reaction can be provided in the form of thermal energy. In biology , energy is an attribute of all biological systems, from the biosphere to the smallest living organism. Within an organism it is responsible for growth and development of
2040-1280: A homogeneous system in which only reversible processes or pure heat transfer are considered, the second law of thermodynamics gives δ Q = T d S , with T the absolute temperature and d S the infinitesimal change in entropy S of the system. Furthermore, if only p V work is done, δ W = p d V . As a result, d U = T d S − p d V . {\displaystyle \mathrm {d} U=T\,\mathrm {d} S-p\,\mathrm {d} V~.} Adding d( p V ) to both sides of this expression gives d U + d ( p V ) = T d S − p d V + d ( p V ) , {\displaystyle \mathrm {d} U+\mathrm {d} (p\,V)=T\,\mathrm {d} S-p\,\mathrm {d} V+\mathrm {d} (p\,V)\;,} or d ( U + p V ) = T d S + V d p . {\displaystyle \mathrm {d} (U+p\,V)=T\,\mathrm {d} S+V\,\mathrm {d} p~.} So d H ( S , p ) = T d S + V d p {\displaystyle \mathrm {d} H(S,\,p)=T\,\mathrm {d} S+V\,\mathrm {d} p~} and
2160-477: A kind of gravitational potential energy storage of the thermal energy, which may later be transformed into active kinetic energy during landslides, after a triggering event. Earthquakes also release stored elastic potential energy in rocks, a store that has been produced ultimately from the same radioactive heat sources. Thus, according to present understanding, familiar events such as landslides and earthquakes release energy that has been stored as potential energy in
2280-412: A steel container at 25 °C (77 °F) is initiated by an ignition device and the reactions allowed to complete. When hydrogen and oxygen react during combustion, water vapor is produced. The vessel and its contents are then cooled to the original 25 °C and the higher heating value is determined as the heat released between identical initial and final temperatures. When the lower heating value (LHV)
2400-425: A system. This property is responsible for the inertia and strength of gravitational interaction of the system ("mass manifestations"), and is also responsible for the potential ability of the system to perform work or heating ("energy manifestations"), subject to the limitations of other physical laws. In classical physics , energy is a scalar quantity, the canonical conjugate to time. In special relativity energy
2520-550: A thermodynamic system is its entropy, as a function, S [ p ]( H , p , {N i } ) , of the same list of variables of state, except that the entropy, S [ p ] , is replaced in the list by the enthalpy, H . It expresses the entropy representation . The state variables H , p , and { N i } are said to be the natural state variables in this representation. They are suitable for describing processes in which they are experimentally controlled. For example, H and p can be controlled by allowing heat transfer, and by varying only
SECTION 20
#17327824945962640-426: A tiny fraction of the original chemical energy is used for work : It would appear that living organisms are remarkably inefficient (in the physical sense) in their use of the energy they receive (chemical or radiant energy); most machines manage higher efficiencies. In growing organisms the energy that is converted to heat serves a vital purpose, as it allows the organism tissue to be highly ordered with regard to
2760-481: Is a derived unit that is equal to the energy expended, or work done, in applying a force of one newton through a distance of one metre. However energy can also be expressed in many other units not part of the SI, such as ergs , calories , British thermal units , kilowatt-hours and kilocalories , which require a conversion factor when expressed in SI units. The SI unit of power , defined as energy per unit of time,
2880-460: Is about the maximum. The human equivalent assists understanding of energy flows in physical and biological systems by expressing energy units in human terms: it provides a "feel" for the use of a given amount of energy. Sunlight's radiant energy is also captured by plants as chemical potential energy in photosynthesis , when carbon dioxide and water (two low-energy compounds) are converted into carbohydrates, lipids, proteins and oxygen. Release of
3000-434: Is also a scalar (although not a Lorentz scalar but a time component of the energy–momentum 4-vector ). In other words, energy is invariant with respect to rotations of space , but not invariant with respect to rotations of spacetime (= boosts ). Energy may be transformed between different forms at various efficiencies . Items that transform between these forms are called transducers . Examples of transducers include
3120-445: Is burned in an open flame, e.g. H 2 O (g), Br 2 (g), I 2 (g) and SO 2 (g). In both definitions the products for C, F, Cl and N are CO 2 (g), HF (g), Cl 2 (g) and N 2 (g), respectively. The heating value of a fuel can be calculated with the results of ultimate analysis of fuel. From analysis, percentages of the combustibles in the fuel ( carbon , hydrogen , sulfur ) are known. Since
3240-575: Is called the Lagrangian , after Joseph-Louis Lagrange . This formalism is as fundamental as the Hamiltonian, and both can be used to derive the equations of motion or be derived from them. It was invented in the context of classical mechanics , but is generally useful in modern physics. The Lagrangian is defined as the kinetic energy minus the potential energy. Usually, the Lagrange formalism
3360-473: Is defined in terms of the energy operator (Hamiltonian) as a time derivative of the wave function . The Schrödinger equation equates the energy operator to the full energy of a particle or a system. Its results can be considered as a definition of measurement of energy in quantum mechanics. The Schrödinger equation describes the space- and time-dependence of a slowly changing (non-relativistic) wave function of quantum systems. The solution of this equation for
3480-473: Is derived from the Greek word enthalpein , which means "to heat". The enthalpy H of a thermodynamic system is defined as the sum of its internal energy and the product of its pressure and volume: H = U + p V , {\displaystyle H=U+pV,} where U is the internal energy , p is pressure , and V is the volume of the system; p V is sometimes referred to as
3600-424: Is determined, cooling is stopped at 150 °C and the reaction heat is only partially recovered. The limit of 150 °C is based on acid gas dew-point. Note: Higher heating value (HHV) is calculated with the product of water being in liquid form while lower heating value (LHV) is calculated with the product of water being in vapor form. The difference between the two heating values depends on the chemical composition of
3720-418: Is directly proportional to the mass of the body: E 0 = m 0 c 2 , {\displaystyle E_{0}=m_{0}c^{2},} where For example, consider electron – positron annihilation, in which the rest energy of these two individual particles (equivalent to their rest mass) is converted to the radiant energy of the photons produced in the process. In this system
Heat of combustion - Misplaced Pages Continue
3840-516: Is either from gravitational collapse of matter (usually molecular hydrogen) into various classes of astronomical objects (stars, black holes, etc.), or from nuclear fusion (of lighter elements, primarily hydrogen). The nuclear fusion of hydrogen in the Sun also releases another store of potential energy which was created at the time of the Big Bang . At that time, according to theory, space expanded and
3960-510: Is extremely large relative to ordinary human scales, the conversion of an everyday amount of rest mass (for example, 1 kg) from rest energy to other forms of energy (such as kinetic energy, thermal energy, or the radiant energy carried by light and other radiation) can liberate tremendous amounts of energy (~ 9 × 10 16 {\displaystyle 9\times 10^{16}} joules = 21 megatons of TNT), as can be seen in nuclear reactors and nuclear weapons. Conversely,
4080-400: Is invariant with altitude. (Correspondingly, the system's gravitational potential energy density also varies with altitude.) Then the enthalpy summation becomes an integral : H = ∫ ( ρ h ) d V , {\displaystyle H=\int \left(\rho \,h\right)\,\mathrm {d} V\;,} where The integral therefore represents the sum of
4200-623: Is mathematically more convenient than the Hamiltonian for non-conservative systems (such as systems with friction). Noether's theorem (1918) states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. Noether's theorem has become a fundamental tool of modern theoretical physics and the calculus of variations. A generalisation of the seminal formulations on constants of motion in Lagrangian and Hamiltonian mechanics (1788 and 1833, respectively), it does not apply to systems that cannot be modeled with
4320-402: Is no friction or other losses, the conversion of energy between these processes would be perfect, and the pendulum would continue swinging forever. Energy is also transferred from potential energy ( E p {\displaystyle E_{p}} ) to kinetic energy ( E k {\displaystyle E_{k}} ) and then back to potential energy constantly. This
4440-466: Is normally about 90% of its higher heating value. This table is in Standard cubic metres (1 atm , 15 °C), to convert to values per Normal cubic metre (1 atm, 0 °C), multiply above table by 1.0549. Energy Energy (from Ancient Greek ἐνέργεια ( enérgeia ) 'activity') is the quantitative property that is transferred to a body or to
4560-415: Is referred to as conservation of energy. In this isolated system , energy cannot be created or destroyed; therefore, the initial energy and the final energy will be equal to each other. This can be demonstrated by the following: The equation can then be simplified further since E p = m g h {\displaystyle E_{p}=mgh} (mass times acceleration due to gravity times
4680-430: Is released as a waste. The energy required to vaporize the water is therefore lost. LHV calculations assume that the water component of a combustion process is in vapor state at the end of combustion, as opposed to the higher heating value (HHV) (a.k.a. gross calorific value or gross CV ) which assumes that all of the water in a combustion process is in a liquid state after a combustion process. Another definition of
4800-696: Is the Planck constant and ν {\displaystyle \nu } the frequency). In the case of an electromagnetic wave these energy states are called quanta of light or photons . When calculating kinetic energy ( work to accelerate a massive body from zero speed to some finite speed) relativistically – using Lorentz transformations instead of Newtonian mechanics – Einstein discovered an unexpected by-product of these calculations to be an energy term which does not vanish at zero speed. He called it rest energy : energy which every massive body must possess even when being at rest. The amount of energy
4920-441: Is the heat capacity at constant pressure and α is the coefficient of (cubic) thermal expansion : α = 1 V ( ∂ V ∂ T ) p . {\displaystyle \alpha ={\frac {\,1\,}{V}}\left({\frac {\partial V}{\,\partial T\,}}\right)_{\mathsf {p}}~.} With this expression one can, in principle, determine
Heat of combustion - Misplaced Pages Continue
5040-496: Is the watt , which is a joule per second. Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. The CGS energy unit is the erg and the imperial and US customary unit is the foot pound . Other energy units such as the electronvolt , food calorie or thermodynamic kcal (based on the temperature change of water in a heating process), and BTU are used in specific areas of science and commerce. In 1843, French physicist James Prescott Joule , namesake of
5160-416: Is the work done in pushing against the ambient (atmospheric) pressure. In physics and statistical mechanics it may be more interesting to study the internal properties of a constant-volume system and therefore the internal energy is used. In chemistry , experiments are often conducted at constant atmospheric pressure , and the pressure–volume work represents a small, well-defined energy exchange with
5280-433: Is the chemical potential per particle for a type i particle, and N i is the number of such particles. The last term can also be written as μ i d n i (with d n i 0 the number of moles of component i added to the system and, in this case, μ i the molar chemical potential) or as μ i d m i (with d m i the mass of component i added to the system and, in this case, μ i
5400-445: Is the main input to Earth's energy budget which accounts for its temperature and climate stability. Sunlight may be stored as gravitational potential energy after it strikes the Earth, as (for example when) water evaporates from oceans and is deposited upon mountains (where, after being released at a hydroelectric dam, it can be used to drive turbines or generators to produce electricity). Sunlight also drives most weather phenomena, save
5520-415: Is the pressure, and v is specific volume , which is equal to 1 / ρ , where ρ is the density . An enthalpy change describes the change in enthalpy observed in the constituents of a thermodynamic system when undergoing a transformation or chemical reaction. It is the difference between the enthalpy after the process has completed, i.e. the enthalpy of the products assuming that
5640-439: Is the same as the thermodynamic heat of combustion since the enthalpy change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is condensed to a liquid. The higher heating value takes into account the latent heat of vaporization of water in the combustion products, and is useful in calculating heating values for fuels where condensation of
5760-424: Is transformed to what other kind) is often determined by entropy (equal energy spread among all available degrees of freedom ) considerations. In practice all energy transformations are permitted on a small scale, but certain larger transformations are not permitted because it is statistically unlikely that energy or matter will randomly move into more concentrated forms or smaller spaces. Energy transformations in
5880-433: Is trapped in a system with zero momentum, where it can be weighed. It is also equivalent to mass, and this mass is always associated with it. Mass is also equivalent to a certain amount of energy, and likewise always appears associated with it, as described in mass–energy equivalence . The formula E = mc ², derived by Albert Einstein (1905) quantifies the relationship between relativistic mass and energy within
6000-405: Is very small for solids and liquids at common conditions, and fairly small for gases. Therefore, enthalpy is a stand-in for energy in chemical systems; bond , lattice , solvation , and other chemical "energies" are actually enthalpy differences. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it. In
6120-524: The American Petroleum Institute (API), uses a reference temperature of 60 °F ( 15 + 5 ⁄ 9 °C). Another definition, used by Gas Processors Suppliers Association (GPSA) and originally used by API (data collected for API research project 44), is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature (API research project 44 used 25 °C. GPSA currently uses 60 °F), minus
SECTION 50
#17327824945966240-482: The International System of Units (SI), the unit of measurement for enthalpy is the joule . Other historical conventional units still in use include the calorie and the British thermal unit (BTU). The total enthalpy of a system cannot be measured directly because the internal energy contains components that are unknown, not easily accessible, or are not of interest for the thermodynamic problem at hand. In practice,
6360-436: The composite motion of the object's components – while potential energy reflects the potential of an object to have motion, generally being based upon the object's position within a field or what is stored within the field itself. While these two categories are sufficient to describe all forms of energy, it is often convenient to refer to particular combinations of potential and kinetic energy as its own form. For example,
6480-487: The gravitational collapse of supernovae to "store" energy in the creation of heavy isotopes (such as uranium and thorium ), and nuclear decay , a process in which energy is released that was originally stored in these heavy elements, before they were incorporated into the Solar System and the Earth. This energy is triggered and released in nuclear fission bombs or in civil nuclear power generation. Similarly, in
6600-578: The kinetic energy of a moving object, the potential energy stored by an object (for instance due to its position in a field ), the elastic energy stored in a solid object, chemical energy associated with chemical reactions , the radiant energy carried by electromagnetic radiation , the internal energy contained within a thermodynamic system , and rest energy associated with an object's rest mass . All living organisms constantly take in and release energy. The Earth's climate and ecosystems processes are driven primarily by radiant energy from
6720-416: The matter and antimatter (electrons and positrons) are destroyed and changed to non-matter (the photons). However, the total mass and total energy do not change during this interaction. The photons each have no rest mass but nonetheless have radiant energy which exhibits the same inertia as did the two original particles. This is a reversible process – the inverse process is called pair creation – in which
6840-662: The (higher) heat of combustion is defined to be the heat released for the complete combustion of a compound in its standard state to form stable products in their standard states: hydrogen is converted to water (in its liquid state), carbon is converted to carbon dioxide gas, and nitrogen is converted to nitrogen gas. That is, the heat of combustion, Δ H ° comb , is the heat of reaction of the following process: Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and SO 2 or SO 3 gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids , respectively, when
6960-571: The Earth's gravitational field or elastic strain (mechanical potential energy) in rocks. Prior to this, they represent release of energy that has been stored in heavy atoms since the collapse of long-destroyed supernova stars (which created these atoms). In cosmology and astronomy the phenomena of stars , nova , supernova , quasars and gamma-ray bursts are the universe's highest-output energy transformations of matter. All stellar phenomena (including solar activity) are driven by various kinds of energy transformations. Energy in such transformations
7080-427: The LHV is the amount of heat released when the products are cooled to 150 °C (302 °F). This means that the latent heat of vaporization of water and other reaction products is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below 150 °C (302 °F) cannot be put to use. One definition of lower heating value, adopted by
7200-409: The atmosphere, so that Δ H is the appropriate expression for the heat of reaction . For a heat engine , the change in its enthalpy after a full cycle is equal to zero, since the final and initial state are equal. In order to discuss the relation between the enthalpy increase and heat supply, we return to the first law for closed systems, with the physics sign convention: d U = δ Q − δ W , where
7320-510: The ball. But, in the reference frame of the person swinging the bat, considerable work is done on the ball. The total energy of a system is sometimes called the Hamiltonian , after William Rowan Hamilton . The classical equations of motion can be written in terms of the Hamiltonian, even for highly complex or abstract systems. These classical equations have direct analogs in nonrelativistic quantum mechanics. Another energy-related concept
SECTION 60
#17327824945967440-447: The case of a chemical explosion , chemical potential energy is transformed to kinetic and thermal energy in a very short time. Yet another example is that of a pendulum . At its highest points the kinetic energy is zero and the gravitational potential energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease in potential energy . If one (unrealistically) assumes that there
7560-965: The case of animals. The daily 1500–2000 Calories (6–8 MJ) recommended for a human adult are taken as food molecules, mostly carbohydrates and fats, of which glucose (C 6 H 12 O 6 ) and stearin (C 57 H 110 O 6 ) are convenient examples. The food molecules are oxidized to carbon dioxide and water in the mitochondria C 6 H 12 O 6 + 6 O 2 ⟶ 6 CO 2 + 6 H 2 O {\displaystyle {\ce {C6H12O6 + 6O2 -> 6CO2 + 6H2O}}} C 57 H 110 O 6 + ( 81 1 2 ) O 2 ⟶ 57 CO 2 + 55 H 2 O {\displaystyle {\ce {C57H110O6 + (81 1/2) O2 -> 57CO2 + 55H2O}}} and some of
7680-416: The change in internal energy, U , which includes activation energies , ionization energies, mixing energies, vaporization energies, chemical bond energies, and so forth. Together, these constitute the change in the enthalpy U + p V . For systems at constant pressure, with no external work done other than the p V work, the change in enthalpy is the heat received by the system. For a simple system with
7800-607: The coefficients of the natural variable differentials d S and d p are just the single variables T and V . The above expression of d H in terms of entropy and pressure may be unfamiliar to some readers. There are also expressions in terms of more directly measurable variables such as temperature and pressure: d H = C p d T + V ( 1 − α T ) d p . {\displaystyle \mathrm {d} H=C_{\mathsf {p}}\,\mathrm {d} T+V\,(1-\alpha T)\,\mathrm {d} p~.} Here C p
7920-409: The combustion is conducted in a bomb calorimeter containing some quantity of water. Zwolinski and Wilhoit defined, in 1972, "gross" and "net" values for heats of combustion. In the gross definition the products are the most stable compounds, e.g. H 2 O (l), Br 2 (l), I 2 (s) and H 2 SO 4 (l). In the net definition the products are the gases produced when the compound
8040-423: The complete conversion of matter (such as atoms) to non-matter (such as photons) is forbidden by conservation laws . Enthalpy Enthalpy ( / ˈ ɛ n θ əl p i / ) is the sum of a thermodynamic system 's internal energy and the product of its pressure and volume . It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at
8160-451: The complex organisms can occupy ecological niches that are not available to their simpler brethren. The conversion of a portion of the chemical energy to heat at each step in a metabolic pathway is the physical reason behind the pyramid of biomass observed in ecology . As an example, to take just the first step in the food chain : of the estimated 124.7 Pg/a of carbon that is fixed by photosynthesis , 64.3 Pg/a (52%) are used for
8280-553: The concept of special relativity. In different theoretical frameworks, similar formulas were derived by J.J. Thomson (1881), Henri Poincaré (1900), Friedrich Hasenöhrl (1904) and others (see Mass–energy equivalence#History for further information). Part of the rest energy (equivalent to rest mass) of matter may be converted to other forms of energy (still exhibiting mass), but neither energy nor mass can be destroyed; rather, both remain constant during any process. However, since c 2 {\displaystyle c^{2}}
8400-399: The conservation of energy is a consequence of the fact that the laws of physics do not change over time. Thus, since 1918, theorists have understood that the law of conservation of energy is the direct mathematical consequence of the translational symmetry of the quantity conjugate to energy, namely time. In the International System of Units (SI), the unit of energy is the joule . It
8520-421: The difference is much more significant as it includes the sensible heat of water vapor between 150 °C and 100 °C, the latent heat of condensation at 100 °C, and the sensible heat of the condensed water between 100 °C and 25 °C. In all, the higher heating value of hydrogen is 18.2% above its lower heating value (142 MJ/kg vs. 120 MJ/kg). For hydrocarbons, the difference depends on the hydrogen content of
8640-424: The differences are so small, reaction enthalpies are often described as reaction energies and analyzed in terms of bond energies . The specific enthalpy of a uniform system is defined as h = H / m , where m is the mass of the system. Its SI unit is joule per kilogram. It can be expressed in other specific quantities by h = u + p v , where u is the specific internal energy , p
8760-473: The energy is used to convert ADP into ATP : The rest of the chemical energy of the carbohydrate or fat are converted into heat: the ATP is used as a sort of "energy currency", and some of the chemical energy it contains is used for other metabolism when ATP reacts with OH groups and eventually splits into ADP and phosphate (at each stage of a metabolic pathway , some chemical energy is converted into heat). Only
8880-457: The energy stored during photosynthesis as heat or light may be triggered suddenly by a spark in a forest fire, or it may be made available more slowly for animal or human metabolism when organic molecules are ingested and catabolism is triggered by enzyme action. All living creatures rely on an external source of energy to be able to grow and reproduce – radiant energy from the Sun in the case of green plants and chemical energy (in some form) in
9000-610: The enthalpies of all the elements of the volume. The enthalpy of a closed homogeneous system is its energy function H ( S , p ) , with its entropy S [ p ] and its pressure p as natural state variables which provide a differential relation for d H of the simplest form, derived as follows. We start from the first law of thermodynamics for closed systems for an infinitesimal process: d U = δ Q − δ W , {\displaystyle \mathrm {d} U=\mathrm {\delta } \,Q-\mathrm {\delta } \,W\;,} where In
9120-751: The enthalpy if C p and V are known as functions of p and T . However the expression is more complicated than d H = T d S + V d p {\displaystyle \;\mathrm {d} H=T\,\mathrm {d} S+V\,\mathrm {d} p\;} because T is not a natural variable for the enthalpy H . At constant pressure, d P = 0 {\displaystyle \;\mathrm {d} P=0\;} so that d H = C p d T . {\displaystyle \;\mathrm {d} H=C_{\mathsf {p}}\,\mathrm {d} T~.} For an ideal gas , d H {\displaystyle \;\mathrm {d} H\;} reduces to this form even if
9240-413: The enthalpy is the sum of the enthalpies of the component subsystems: H = ∑ k H k , {\displaystyle H=\sum _{k}H_{k}\;,} where A closed system may lie in thermodynamic equilibrium in a static gravitational field , so that its pressure p varies continuously with altitude , while, because of the equilibrium requirement, its temperature T
9360-445: The enthalpy of the stoichiometric oxygen (O 2 ) at the reference temperature, minus the heat of vaporization of the vapor content of the combustion products. The definition in which the combustion products are all returned to the reference temperature is more easily calculated from the higher heating value than when using other definitions and will in fact give a slightly different answer. Gross heating value accounts for water in
9480-449: The exhaust leaving as vapor, as does LHV, but gross heating value also includes liquid water in the fuel prior to combustion. This value is important for fuels like wood or coal , which will usually contain some amount of water prior to burning. The higher heating value is experimentally determined in a bomb calorimeter . The combustion of a stoichiometric mixture of fuel and oxidizer (e.g. two moles of hydrogen and one mole of oxygen) in
9600-528: The external pressure on the piston that sets the volume of the system. The U term is the energy of the system, and the p V term can be interpreted as the work that would be required to "make room" for the system if the pressure of the environment remained constant. When a system, for example, n moles of a gas of volume V at pressure p and temperature T , is created or brought to its present state from absolute zero , energy must be supplied equal to its internal energy U plus p V , where p V
9720-830: The first law reads: d U = δ Q − p d V . {\displaystyle \mathrm {d} U=\mathrm {\delta } \,Q-p\,\mathrm {d} V~.} Now, d H = d U + d ( p V ) . {\displaystyle \mathrm {d} H=\mathrm {d} U+\mathrm {d} (p\,V)~.} So d H = δ Q + V d p + p d V − p d V = δ Q + V d p . {\displaystyle {\begin{aligned}\mathrm {d} H&=\mathrm {\delta } Q+V\,\mathrm {d} p+p\,\mathrm {d} V-p\,\mathrm {d} V\\&=\mathrm {\delta } Q+V\,\mathrm {d} p~.\end{aligned}}} If
9840-493: The first time in the work of Aristotle in the 4th century BC. In contrast to the modern definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure. In the late 17th century, Gottfried Leibniz proposed the idea of the Latin : vis viva , or living force, which defined as the product of the mass of an object and its velocity squared; he believed that total vis viva
9960-459: The fuel. For gasoline and diesel the higher heating value exceeds the lower heating value by about 10% and 7%, respectively, and for natural gas about 11%. A common method of relating HHV to LHV is: where H v is the heat of vaporization of water, n H 2 O ,out is the number of moles of water vaporized and n fuel,in is the number of moles of fuel combusted. Engine manufacturers typically rate their engines fuel consumption by
10080-408: The fuel. In the case of pure carbon or carbon monoxide, the two heating values are almost identical, the difference being the sensible heat content of carbon dioxide between 150 °C and 25 °C ( sensible heat exchange causes a change of temperature, while latent heat is added or subtracted for phase transitions at constant temperature. Examples: heat of vaporization or heat of fusion ). For hydrogen,
10200-466: The heat absorbed in the reaction. From the definition of enthalpy as H = U + p V , the enthalpy change at constant pressure is Δ H = Δ U + p Δ V . However, for most chemical reactions, the work term p Δ V is much smaller than the internal energy change Δ U , which is approximately equal to Δ H . As an example, for the combustion of carbon monoxide 2 CO(g) + O 2 (g) → 2 CO 2 (g) , Δ H = −566.0 kJ and Δ U = −563.5 kJ. Since
10320-430: The heat δ Q is supplied by conduction, radiation, Joule heating . We apply it to the special case with a constant pressure at the surface. In this case the work is given by p d V (where p is the pressure at the surface, d V is the increase of the volume of the system). Cases of long range electromagnetic interaction require further state variables in their formulation, and are not considered here. In this case
10440-495: The heat of combustion of these elements is known, the heating value can be calculated using Dulong's Formula: HHV [kJ/g]= 33.87m C + 122.3(m H - m O ÷ 8) + 9.4m S where m C , m H , m O , m N , and m S are the contents of carbon, hydrogen, oxygen, nitrogen, and sulfur on any (wet, dry or ash free) basis, respectively. The higher heating value (HHV; gross energy , upper heating value , gross calorific value GCV , or higher calorific value ; HCV ) indicates
10560-413: The height) and E k = 1 2 m v 2 {\textstyle E_{k}={\frac {1}{2}}mv^{2}} (half mass times velocity squared). Then the total amount of energy can be found by adding E p + E k = E total {\displaystyle E_{p}+E_{k}=E_{\text{total}}} . Energy gives rise to weight when it
10680-475: The higher heating value will be somewhat higher. The difference between HHV and LHV definitions causes endless confusion when quoters do not bother to state the convention being used. since there is typically a 10% difference between the two methods for a power plant burning natural gas. For simply benchmarking part of a reaction the LHV may be appropriate, but HHV should be used for overall energy efficiency calculations if only to avoid confusion, and in any case,
10800-404: The initial and final pressure and temperature correspond to the standard state. The value does not depend on the path from initial to final state because enthalpy is a state function . Enthalpies of chemical substances are usually listed for 1 bar (100 kPa) pressure as a standard state. Enthalpies and enthalpy changes for reactions vary as a function of temperature, but tables generally list
10920-442: The lower heating values since the exhaust is never condensed in the engine, and doing this allows them to publish more attractive numbers than are used in conventional power plant terms. The conventional power industry had used HHV (high heat value) exclusively for decades, even though virtually all of these plants did not condense exhaust either. American consumers should be aware that the corresponding fuel-consumption figure based on
11040-482: The marginalia of her French language translation of Newton's Principia Mathematica , which represented the first formulation of a conserved measurable quantity that was distinct from momentum , and which would later be called "energy". In 1807, Thomas Young was possibly the first to use the term "energy" instead of vis viva , in its modern sense. Gustave-Gaspard Coriolis described " kinetic energy " in 1829 in its modern sense, and in 1853, William Rankine coined
11160-424: The mass equivalent of an everyday amount energy is minuscule, which is why a loss of energy (loss of mass) from most systems is difficult to measure on a weighing scale, unless the energy loss is very large. Examples of large transformations between rest energy (of matter) and other forms of energy (e.g., kinetic energy into particles with rest mass) are found in nuclear physics and particle physics . Often, however,
11280-474: The metabolism of green plants, i.e. reconverted into carbon dioxide and heat. In geology , continental drift , mountain ranges , volcanoes , and earthquakes are phenomena that can be explained in terms of energy transformations in the Earth's interior, while meteorological phenomena like wind, rain, hail , snow, lightning, tornadoes and hurricanes are all a result of energy transformations in our atmosphere brought about by solar energy . Sunlight
11400-427: The molecules it is built from. The second law of thermodynamics states that energy (and matter) tends to become more evenly spread out across the universe: to concentrate energy (or matter) in one specific place, it is necessary to spread out a greater amount of energy (as heat) across the remainder of the universe ("the surroundings"). Simpler organisms can achieve higher energy efficiencies than more complex ones, but
11520-426: The pressure energy Ɛ p . Enthalpy is an extensive property ; it is proportional to the size of the system (for homogeneous systems). As intensive properties , the specific enthalpy , h = H / m , is referenced to a unit of mass m of the system, and the molar enthalpy , H m = H / n , where n is the number of moles . For inhomogeneous systems
11640-591: The process involves a pressure change, because α T = 1 . In a more general form, the first law describes the internal energy with additional terms involving the chemical potential and the number of particles of various types. The differential statement for d H then becomes d H = T d S + V d p + ∑ i μ i d N i , {\displaystyle \mathrm {d} H=T\,\mathrm {d} S+V\,\mathrm {d} p+\sum _{i}\mu _{i}\,\mathrm {d} N_{i}\;,} where μ i
11760-502: The products are allowed to cool and whether compounds like H 2 O are allowed to condense. The high heat values are conventionally measured with a bomb calorimeter . Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the heat of formation Δ H f of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion).. For
11880-457: The reaction products is practical (e.g., in a gas-fired boiler used for space heat). In other words, HHV assumes all the water component is in liquid state at the end of combustion (in product of combustion) and that heat delivered at temperatures below 150 °C (302 °F) can be put to use. The lower heating value (LHV; net calorific value ; NCV , or lower calorific value ; LCV ) is another measure of available thermal energy produced by
12000-422: The requirements for creating a system from "nothingness"; the mechanical work required, p V , differs based upon the conditions that obtain during the creation of the thermodynamic system . Energy must be supplied to remove particles from the surroundings to make space for the creation of the system, assuming that the pressure p remains constant; this is the p V term. The supplied energy must also provide
12120-405: The rest mass of particles is created from the radiant energy of two (or more) annihilating photons. In general relativity, the stress–energy tensor serves as the source term for the gravitational field, in rough analogy to the way mass serves as the source term in the non-relativistic Newtonian approximation. Energy and mass are manifestations of one and the same underlying physical property of
12240-474: The same total energy even in different forms) but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy . There are strict limits to how efficiently heat can be converted into work in a cyclic process, e.g. in a heat engine, as described by Carnot's theorem and the second law of thermodynamics . However, some energy transformations can be quite efficient. The direction of transformations in energy (what kind of energy
12360-467: The situation is the reverse. Chemical reactions are usually not possible unless the reactants surmount an energy barrier known as the activation energy . The speed of a chemical reaction (at a given temperature T ) is related to the activation energy E by the Boltzmann's population factor e ; that is, the probability of a molecule to have energy greater than or equal to E at
12480-485: The specific chemical potential). The enthalpy, H ( S [ p ], p , { N i } ) , expresses the thermodynamics of a system in the energy representation . As a function of state , its arguments include both one intensive and several extensive state variables . The state variables S [ p ] , p , and { N i } are said to be the natural state variables in this representation. They are suitable for describing processes in which they are determined by factors in
12600-406: The standard enthalpy of reaction is the enthalpy change when reactants in their standard states ( p = 1 bar ; usually T = 298 K ) change to products in their standard states. This quantity is the standard heat of reaction at constant pressure and temperature, but it can be measured by calorimetric methods even if the temperature does vary during the measurement, provided that
12720-544: The standard heats of formation of substances at 25 °C (298 K). For endothermic (heat-absorbing) processes, the change Δ H is a positive value; for exothermic (heat-releasing) processes it is negative. The enthalpy of an ideal gas is independent of its pressure or volume, and depends only on its temperature, which correlates to its thermal energy. Real gases at common temperatures and pressures often closely approximate this behavior, which simplifies practical thermodynamic design and analysis. The word "enthalpy"
12840-526: The sum of translational and rotational kinetic and potential energy within a system is referred to as mechanical energy , whereas nuclear energy refers to the combined potentials within an atomic nucleus from either the nuclear force or the weak force , among other examples. The word energy derives from the Ancient Greek : ἐνέργεια , romanized : energeia , lit. 'activity, operation', which possibly appears for
12960-420: The sun . The energy industry provides the energy required for human civilization to function, which it obtains from energy resources such as fossil fuels , nuclear fuel , and renewable energy . The total energy of a system can be subdivided and classified into potential energy , kinetic energy , or combinations of the two in various ways. Kinetic energy is determined by the movement of an object – or
13080-414: The surroundings. For example, when a virtual parcel of atmospheric air moves to a different altitude, the pressure surrounding it changes, and the process is often so rapid that there is too little time for heat transfer. This is the basis of the so-called adiabatic approximation that is used in meteorology . Conjugate with the enthalpy, with these arguments, the other characteristic function of state of
13200-417: The system is under constant pressure , d p = 0 and consequently, the increase in enthalpy of the system is equal to the heat added: d H = δ Q {\displaystyle \mathrm {d} H=\mathrm {\delta } \,Q} This is why the now-obsolete term heat content was used for enthalpy in the 19th century. In thermodynamics, one can calculate enthalpy by determining
13320-400: The system's change in enthalpy, Δ H , is negative due to the products of the reaction having a smaller enthalpy than the reactants, and equals the heat released in the reaction if no electrical or shaft work is done. In other words, the overall decrease in enthalpy is achieved by the generation of heat. Conversely, for a constant-pressure endothermic reaction, Δ H is positive and equal to
13440-429: The term " potential energy ". The law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system . It was argued for some years whether heat was a physical substance, dubbed the caloric , or merely a physical quantity, such as momentum . In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat. These developments led to
13560-461: The theory of conservation of energy, formalized largely by William Thomson ( Lord Kelvin ) as the field of thermodynamics . Thermodynamics aided the rapid development of explanations of chemical processes by Rudolf Clausius , Josiah Willard Gibbs , and Walther Nernst . It also led to a mathematical formulation of the concept of entropy by Clausius and to the introduction of laws of radiant energy by Jožef Stefan . According to Noether's theorem ,
13680-420: The total energy of the substances involved. Some energy may be transferred between the surroundings and the reactants in the form of heat or light; thus the products of a reaction have sometimes more but usually less energy than the reactants. A reaction is said to be exothermic or exergonic if the final state is lower on the energy scale than the initial state; in the less common case of endothermic reactions
13800-406: The unit of measure, discovered that the gravitational potential energy lost by a descending weight attached via a string was equal to the internal energy gained by the water through friction with the paddle. In classical mechanics, energy is a conceptually and mathematically useful property, as it is a conserved quantity . Several formulations of mechanics have been developed using energy as
13920-421: The universe cooled too rapidly for hydrogen to completely fuse into heavier elements. This meant that hydrogen represents a store of potential energy that can be released by fusion. Such a fusion process is triggered by heat and pressure generated from gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into sunlight. In quantum mechanics , energy
14040-401: The universe over time are characterized by various kinds of potential energy, that has been available since the Big Bang , being "released" (transformed to more active types of energy such as kinetic or radiant energy) when a triggering mechanism is available. Familiar examples of such processes include nucleosynthesis , a process ultimately using the gravitational potential energy released from
14160-414: The upper limit of the available thermal energy produced by a complete combustion of fuel. It is measured as a unit of energy per unit mass or volume of substance. The HHV is determined by bringing all the products of combustion back to the original pre-combustion temperature, including condensing any vapor produced. Such measurements often use a standard temperature of 25 °C (77 °F; 298 K). This
14280-416: The value or convention should be clearly stated. Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen). AR, MF, and MAF are commonly used for indicating the heating values of coal: The International Energy Agency reports the following typical higher heating values per Standard cubic metre of gas: The lower heating value of natural gas
14400-440: Was conserved. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the motions of the constituent parts of matter, although it would be more than a century until this was generally accepted. The modern analog of this property, kinetic energy , differs from vis viva only by a factor of two. Writing in the early 18th century, Émilie du Châtelet proposed the concept of conservation of energy in
#595404