Misplaced Pages

#956043

50-436: The "gun" is a gland with a duct on the front of the head. It operates by a chemical reaction triggered when the termite contracts its mandibular muscles . The termite is "able to eject the frontal gland material over a distance of many centimeters". The gland material is forced out through the front of the internal nose and its pores, which covers much of the termite's head. Termites have been shown to be extremely accurate with

100-482: A cell . The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions , radioactive decays and reactions between elementary particles , as described by quantum field theory . Chemical reactions such as combustion in fire, fermentation and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as

150-470: A double displacement reaction , the anions and cations of two compounds switch places and form two entirely different compounds. These reactions are in the general form: AB + CD ⟶ AD + CB {\displaystyle {\ce {AB + CD->AD + CB}}} For example, when barium chloride (BaCl 2 ) and magnesium sulfate (MgSO 4 ) react, the SO 4 anion switches places with

200-434: A first-order reaction , which could be the disintegration of a substance A, is given by: Its integration yields: Here k is the first-order rate constant, having dimension 1/time, [A]( t ) is the concentration at a time t and [A] 0 is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with

250-736: A single displacement reaction , a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound These reactions come in the general form of: A + BC ⟶ AC + B {\displaystyle {\ce {A + BC->AC + B}}} One example of a single displacement reaction is when magnesium replaces hydrogen in water to make solid magnesium hydroxide and hydrogen gas: Mg + 2 H 2 O ⟶ Mg ( OH ) 2 ↓ + H 2 ↑ {\displaystyle {\ce {Mg + 2H2O->Mg(OH)2 (v) + H2 (^)}}} In

300-623: A "vital force" and distinguished from inorganic materials. This separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold , who, among many discoveries, established the mechanisms of substitution reactions . The general characteristics of chemical reactions are: Chemical equations are used to graphically illustrate chemical reactions. They consist of chemical or structural formulas of

350-536: A characteristic half-life . More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation : where E a is the activation energy and k B is the Boltzmann constant . One of the simplest models of reaction rate is the collision theory . More realistic models are tailored to a specific problem and include

400-500: A chemical reaction are called reactants or reagents . Chemical reactions are usually characterized by a chemical change , and they yield one or more products , which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions , and the information on the precise course of action is part of the reaction mechanism . Chemical reactions are described with chemical equations , which symbolically present

450-447: A complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes. In a unimolecular elementary reaction, a molecule A dissociates or isomerises to form the products(s) At constant temperature, the rate of such a reaction is proportional to the concentration of the species A In a bimolecular elementary reaction, two atoms , molecules, ions or radicals , A and B , react together to form

500-459: A distinct and not closely related lineage which convergently evolved a nasus that is superficially analogous but not functionally homologous to the nasus of true nasute termites. Unlike true nasutes, all Syntermitinae exhibit soldiers with highly developed mandibles which are physiologically incapable of expelling their frontal gland secretions from the nasus. No other known termite has a frontal apparatus as functionally and morphologically similar to

550-399: A few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time. The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or a dissociation into one or more other molecules. Such reactions require the addition of energy in

SECTION 10

#1732772396957

600-405: A fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion . This proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as a reaction with oxygen from the air. Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and

650-438: A neutral radical . In the second case, both electrons of the chemical bond remain with one of the products, resulting in charged ions . Dissociation plays an important role in triggering chain reactions , such as hydrogen–oxygen or polymerization reactions. For bimolecular reactions, two molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction . Another possibility

700-410: A number of termite soldiers will fire upon the enemy and the combined force of the "bullets" will kill the enemy along with covering it in the glue-like substance. It has also been observed that, for those that survive the force of the material, it also causes varying negative effects, likely because of the terpenes contained in the compound. Eventually, if not killed from other effects, the secretion of

750-416: Is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state . In practice, a reaction is assumed to be elementary if no reaction intermediates have been detected or need to be postulated to describe the reaction on a molecular scale. An apparently elementary reaction may be in fact a stepwise reaction , i.e.

800-444: Is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide : 8 Fe + S 8 ⟶ 8 FeS {\displaystyle {\ce {8Fe + S8->8FeS}}} Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water. A decomposition reaction

850-559: Is endothermic at low temperatures, becoming less so with increasing temperature. Δ H ° is zero at 1855  K , and the reaction becomes exothermic above that temperature. Changes in temperature can also reverse the direction tendency of a reaction. For example, the water gas shift reaction is favored by low temperatures, but its reverse is favored by high temperatures. The shift in reaction direction tendency occurs at 1100 K . Reactions can also be characterized by their internal energy change, which takes into account changes in

900-462: Is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. A reaction may be classified as redox in which oxidation and reduction occur or non-redox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as a combination, decomposition, or single displacement reaction. Different chemical reactions are used during chemical synthesis in order to obtain

950-427: Is released. Typical examples of exothermic reactions are combustion , precipitation and crystallization , in which ordered solids are formed from disordered gaseous or liquid phases. In contrast, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of gaseous or dissolved reaction products, which have higher entropy. Since

1000-537: Is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reaction is also called metathesis . for example Most chemical reactions are reversible; that is, they can and do run in both directions. The forward and reverse reactions are competing with each other and differ in reaction rates . These rates depend on

1050-413: Is used in retro reactions. The elementary reaction is the smallest division into which a chemical reaction can be decomposed, it has no intermediate products. Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as reaction mechanism . An elementary reaction involves

SECTION 20

#1732772396957

1100-532: Is when a more complex substance breaks down into its more simple parts. It is thus the opposite of a synthesis reaction and can be written as AB ⟶ A + B {\displaystyle {\ce {AB->A + B}}} One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas: 2 H 2 O ⟶ 2 H 2 + O 2 {\displaystyle {\ce {2H2O->2H2 + O2}}} In

1150-549: The Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water, air and earth. In the Middle Ages , chemical transformations were studied by alchemists . They attempted, in particular, to convert lead into gold , for which purpose they used reactions of lead and lead-copper alloys with sulfur . The artificial production of chemical substances already

1200-1343: The Gibbs free energy of the reaction. They require input of energy to proceed in the forward direction. Examples include: In a combustion reaction, an element or compound reacts with an oxidant, usually oxygen , often producing energy in the form of heat or light . Combustion reactions frequently involve a hydrocarbon . For instance, the combustion of 1 mole (114 g) of octane in oxygen C 8 H 18 ( l ) + 25 2 O 2 ( g ) ⟶ 8 CO 2 + 9 H 2 O ( l ) {\displaystyle {\ce {C8H18(l) + 25/2 O2(g)->8CO2 + 9H2O(l)}}} releases 5500 kJ. A combustion reaction can also result from carbon , magnesium or sulfur reacting with oxygen. 2 Mg ( s ) + O 2 ⟶ 2 MgO ( s ) {\displaystyle {\ce {2Mg(s) + O2->2MgO(s)}}} S ( s ) + O 2 ( g ) ⟶ SO 2 ( g ) {\displaystyle {\ce {S(s) + O2(g)->SO2(g)}}} Elementary reaction An elementary reaction

1250-497: The Le Chatelier's principle . For example, an increase in pressure due to decreasing volume causes the reaction to shift to the side with fewer moles of gas. The reaction yield stabilizes at equilibrium but can be increased by removing the product from the reaction mixture or changed by increasing the temperature or pressure. A change in the concentrations of the reactants does not affect the equilibrium constant but does affect

1300-458: The contact process in the 1880s, and the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont , Robert Boyle , and Isaac Newton tried to establish theories of experimentally observed chemical transformations. The phlogiston theory was proposed in 1667 by Johann Joachim Becher . It postulated the existence of

1350-471: The stoichiometry , the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (A, B, C and D in a schematic example below) by the appropriate integers a, b, c and d . More elaborate reactions are represented by reaction schemes, which in addition to starting materials and products show important intermediates or transition states . Also, some relatively minor additions to

1400-562: The transition state theory , the calculation of the potential energy surface , the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory . In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form: A + B ⟶ AB {\displaystyle {\ce {A + B->AB}}} Two or more reactants yielding one product

1450-486: The 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride . With the development of the lead chamber process in 1746 and the Leblanc process , allowing large-scale production of sulfuric acid and sodium carbonate , respectively, chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in

1500-555: The 2Cl anion, giving the compounds BaSO 4 and MgCl 2 . Another example of a double displacement reaction is the reaction of lead(II) nitrate with potassium iodide to form lead(II) iodide and potassium nitrate : Pb ( NO 3 ) 2 + 2 KI ⟶ PbI 2 ↓ + 2 KNO 3 {\displaystyle {\ce {Pb(NO3)2 + 2KI->PbI2(v) + 2KNO3}}} According to Le Chatelier's Principle , reactions may proceed in

1550-546: The Nasutitermitinae as the Syntermitinae. Chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated. Classically, chemical reactions encompass changes that only involve

Fontanellar gun - Misplaced Pages Continue

1600-408: The atomic theory of John Dalton , Joseph Proust had developed the law of definite proportions , which later resulted in the concepts of stoichiometry and chemical equations . Regarding the organic chemistry , it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically . According to the concept of vitalism , organic matter was endowed with

1650-451: The case of dilute fluids equivalent results have been obtained from simple probabilistic arguments. According to collision theory the probability of three chemical species reacting simultaneously with each other in a termolecular elementary reaction is negligible. Hence such termolecular reactions are commonly referred as non-elementary reactions and can be broken down into a more fundamental set of bimolecular reactions, in agreement with

1700-481: The concentration and therefore change with the time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium. The time to reach equilibrium depends on parameters such as temperature, pressure, and the materials involved, and is determined by the minimum free energy . In equilibrium, the Gibbs free energy of reaction must be zero. The pressure dependence can be explained with

1750-444: The desired product. In biochemistry , a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways . These reactions are often catalyzed by protein enzymes . Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperature and concentrations present within

1800-455: The entropy term in the free-energy change increases with temperature, many endothermic reactions preferably take place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur preferably at lower temperatures. A change in temperature can sometimes reverse the sign of the enthalpy of a reaction, as for the carbon monoxide reduction of molybdenum dioxide : This reaction to form carbon dioxide and molybdenum

1850-405: The entropy, volume and chemical potentials . The latter depends, among other things, on the activities of the involved substances. The speed at which reactions take place is studied by reaction kinetics . The rate depends on various parameters, such as: Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of

1900-406: The equilibrium position. Chemical reactions are determined by the laws of thermodynamics . Reactions can proceed by themselves if they are exergonic , that is if they release free energy. The associated free energy change of the reaction is composed of the changes of two different thermodynamic quantities, enthalpy and entropy : Reactions can be exothermic , where Δ H is negative and energy

1950-448: The form of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization , in which the cis-form of a compound converts to the trans-form or vice versa. In a typical dissociation reaction, a bond in a molecule splits ( ruptures ) resulting in two molecular fragments. The splitting can be homolytic or heterolytic . In the first case, the bond is divided so that each product retains an electron and becomes

2000-407: The forward or reverse direction until they end or reach equilibrium . Reactions that proceed in the forward direction (from left to right) to approach equilibrium are often called spontaneous reactions , that is, Δ G {\displaystyle \Delta G} is negative, which means that if they occur at constant temperature and pressure, they decrease the Gibbs free energy of

2050-399: The gun, even though they are blind. The exact nature of how termites retain such accuracy and are able to orient themselves toward their enemy is currently unknown, but it is believed that the ability is "olfactory or auditory" in nature. The fired material is glue-like and sticks to objects that it hits, causing the legs of an attacking enemy to become stuck to the ground. Most often, though,

Fontanellar gun - Misplaced Pages Continue

2100-454: The positions of electrons in the forming and breaking of chemical bonds between atoms , with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation . Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur. The substance (or substances) initially involved in

2150-500: The product(s) The rate of such a reaction, at constant temperature, is proportional to the product of the concentrations of the species A and B The rate expression for an elementary bimolecular reaction is sometimes referred to as the law of mass action as it was first proposed by Guldberg and Waage in 1864. An example of this type of reaction is a cycloaddition reaction. This rate expression can be derived from first principles by using collision theory for ideal gases . For

2200-404: The reactants on the left and those of the products on the right. They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields". The tip of the arrow points in the direction in which the reaction proceeds. A double arrow (⇌) pointing in opposite directions is used for equilibrium reactions . Equations should be balanced according to

2250-457: The reaction can be indicated above the reaction arrow; examples of such additions are water, heat, illumination, a catalyst , etc. Similarly, some minor products can be placed below the arrow, often with a minus sign. Retrosynthetic analysis can be applied to design a complex synthesis reaction. Here the analysis starts from the products, for example by splitting selected chemical bonds, to arrive at plausible initial reagents. A special arrow (⇒)

2300-477: The reaction. They require less energy to proceed in the forward direction. Reactions are usually written as forward reactions in the direction in which they are spontaneous. Examples: Reactions that proceed in the backward direction to approach equilibrium are often called non-spontaneous reactions , that is, Δ G {\displaystyle \Delta G} is positive, which means that if they occur at constant temperature and pressure, they increase

2350-464: The starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Some reactions produce heat and are called exothermic reactions , while others may require heat to enable the reaction to occur, which are called endothermic reactions . Typically, reaction rates increase with increasing temperature because there

2400-562: The termite will kill any enemy insects after 24–48 hours. In comparison, the effect of the secretion on termites of the same species was considerably worse, with the secretion causing death within 5–6 hours. The secretion from Tenuirostritermes tenuirostris consists of a mixture of three terpenes , namely 62% pinene , 27% myrcene and 11% limonene . These form a resinous glue resembling pine resin . The secretion contains an alarm pheromone that alerts other soldier termites of an enemy attack and causes them to fire their fontanellar gun. It

2450-456: Was a central goal for medieval alchemists. Examples include the synthesis of ammonium chloride from organic substances as described in the works (c. 850–950) attributed to Jābir ibn Ḥayyān , or the production of mineral acids such as sulfuric and nitric acids by later alchemists, starting from c. 1300. The production of mineral acids involved the heating of sulfate and nitrate minerals such as copper sulfate , alum and saltpeter . In

2500-440: Was discovered that the pinene was also acting as an alarm pheromone while it was forming the composition of the terpenes. Because later arriving termites did not also fire their gun, it is believed that the pinene pheromone lasts for only a brief period of time before dissipating. The Syntermitinae , which were once classified within the Nasutitermitinae in a clade known as the mandibulate nasute termites , are now recognized to be

#956043