CHEMISTRY GLOSSARY

collision theory    →   teorija sudara

Collision theory is theory that explains how chemical reactions take place and why rates of reaction alter. For a reaction to occur the reactant particles must collide. Only a certain fraction of the total collisions cause chemical change; these are called successful collisions. The successful collisions have sufficient energy (activation energy) at the moment of impact to break the existing bonds and form new bonds, resulting in the products of the reaction. Increasing the concentration of the reactants and raising the temperature bring about more collisions and therefore more successful collisions, increasing the rate of reaction.

law of chemical equilibrium    →   zakon o kemijskoj ravnoteži

Law of chemical equilibrium (also called the law of mass action) states that the rate at which a substance reacts is proportional to its active mass (i.e. to its molar concentration). Thus, the velocity of a chemical reaction is proportional to the product of the concentration of the reactants.

Einstein equation    →   Einsteinova jednadžba

Einstein equation is the mass-energy relationship introduced by Albert Einstein in 1905 in the form E = mc², where E is a quantity of energy, m its mass, and c is the speed of light. It presents the concept that energy possesses mass.

equilibrium constant    →   konstanta ravnoteže

The equilibrium constant (K) was originally introduced in 1863 by Norwegian chemists C.M. Guldberg and P. Waage using the law of mass action. For a reversible chemical reaction represented by the equation

chemical equilibrium occurs when the rate of the forward reaction equals the rate of the back reaction, so that the concentrations of products and reactants reach steady-state values.

The equilibrium constant is the ratio of chemical activities of the species A, B, C, and D at equilibrium.

To a certain approximation, the activities can be replaced by concentrations.

For gas reactions, partial pressures are used rather than concentrations

The units of K_p and K_c depend on the numbers of molecules appearing in the stoichiometric equation (a, b, c, and d).

The value equilibrium constant depends on the temperature. If the forward reaction is exothermic, the equilibrium constant decreases as the temperature rises. The equilibrium constant shows the position of equilibrium. A low value of K indicates that [C] and [D] are small compared to [A] and [B]; i.e. that the back reaction predominates.

The equilibrium constant is related to Δ_rG°, the standard Gibbs free energy change in the reaction, by

explosive    →   eksploziv

Explosive is a compound or mixture that, when ignited or detonated, undergoes rapid violent chemical reaction that produces large amount of gas and heat, accompanied by light, sound and a high pressure shock wave.

Haber process    →   Haberov proces

Haber process is an industrial process for producing ammonia by reaction of nitrogen with hydrogen:

The reaction is reversible and exothermic, so that a high yield of ammonia is favoured by low temperature. However, the rate of reaction would be too slow for equilibrium to be reached at normal temperatures, so an optimum temperature of about 450 °C is used, with a catalyst of iron containing potassium aluminium oxide promoters. The higher the pressure the greater the yield, although there are technical difficulties in using very high pressures. A pressure of about 250 atmospheres is commonly employed. The removal of ammonia from the batch as soon as it is formed ensures that an equilibrium favouring product formation is maintained. The nitrogen is obtained from air. Formerly, the hydrogen was from water gas and the water-gas shift reaction (the Bosch process) but now the raw material (called synthesis gas) is obtained by steam reforming natural gas.

The process is of immense importance for the fixation of nitrogen for fertilisers and explosives. It was developed in 1908 by German chemist Fritz Haber (1868-1934) and was developed for industrial use by Carl Bosch (1874-1940), hence the alternative name Haber-Bosch process.

theories of catalysis    →   teorije katalize

Theories of catalysis explain the influence of the catalysts upon the rate of a reaction by describing the detailed mechanism by which the catalyst is involved in the steps of the chemical reaction.

Heyrovsky-Ilkovic equation    →   Heyrovsky-Ilkovičeva jednadžba

The Heyrovsky-Ilkovic equation describes the entire current-potential curve (polarographic wave) of a reversible redox system in polarography

where R is the gas constant, T is the absolute temperature, F is the Faraday constant, n denotes the number of electrons taking part in the electrode reaction. E_1/2 is a unique potential (for a given reaction and supporting electrolyte) termed the half-wave potential.

In order to obtain E_1/2 from the above equation, we plot a graph of ln[(i_d-i)/i] against E. The intercept on the x-axis gives then an accurate value of E_1/2. The slope of the obtained straight line is equal to nF/RT from which n is determined.

acceleration    →   akceleracija

If a point-like object undergoes a change in velocity Δv=v_f-v_i in time Δt=t_f-t_i (indexes i and f stand for initial and final instant as well as for initial and final velocity) its average acceleration, a is defined as

The instantaneous acceleration, a, is obtained from the average acceleration by shrinking the time interval Δt towards zero. The average acceleration approaches a limiting value, which is the acceleration of a given instant:

Acceleration is a vector quantity. SI unit for acceleration is m s^-2.

Acheson process    →   Achesonov proces

Acheson process is an industrial process to synthesize graphite and silicon carbide (carborundum), named after its inventor the American chemist Edward Goodrich Acheson (1856-1931). In this process, a solid-state reaction between pure silica sand (SiO₂) and petroleum coke (C) at very high temperature (more than 2500 °C) leads to the formation of silicon carbide under the general reaction:

While studying the effects of high temperature on carborundum, Acheson had found that silicon vaporizes at about 4150 °C, leaving behind graphitic carbon.