CHEMISTRY GLOSSARY

activity coefficient    →   koeficijent aktiviteta

Activity coefficient (γ or f) is a fractional number which, when multiplied by the molar concentration of a substance in solution, yields the chemical activity. This term gives an idea of how much interaction exists between molecules at higher concentration.

In solutions of very low ionic strength, when m is less than 0.01, the Debye-Hückel limiting law can be used to calculate approximate activity coefficients

where γ_i = activity coefficient of the species i, z_i = charge on the species i and μ = ionic strength of the solution.

absorption coefficient    →   apsorpcijski koeficijent

Absorption coefficient (a) is the relative decrease in the intensity of a collimated beam of electromagnetic radiation, as a result of absorption by a medium, during traversal of an infinitesimal layer of the medium, divided by the length traversed.

molar absorption coefficient    →   molarni apsorpcijski koeficijent

Molar absorption coefficient (ε) is the absorption coefficient divided by amount-of-substance concentration of the absorbing material in the sample solution (ε = a/c). The SI unit is m²mol^-1. Also called extinction coefficient, but usually in units of dm³cm^-1mol^-1.

Joule-Thomson coefficient    →   Joule-Thompsonov koeficijent

Joule-Thomson coefficient (μ) is a parameter which describes the temperature change when a gas expands adiabatically through a nozzle from a high pressure to a low pressure region. It is defined by

where H is enthalpy.

stoichiometric coefficient    →   stehiometrijski koeficijent

Stoichiometric coefficient (ν) is the number appearing before the symbol for each compound in the equation for a chemical reaction. By convention, it is negative for reactants and positive for products.

Stoichiometric coefficients describe the stoichiometry of the chemical reaction.

In this equation, a, b, c and d are called as Stoichiometric coefficients of the A, B, C and D respectively.

activity    →   aktivitet

Activity (a) is a thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For the species i activity is defined as

where a_i is the activity of the species i, c_i is its molar concentration, and f_i is a dimensionless quantity called the activity coefficient.

Nernst’s electrode potential equation    →   Nernstova jednadžba za elektrodni potencijal

For general reaction of some redox system

dependence of electrode potential of redox system upon activity of oxidised and reduced form in solution is described in Nernst’s equation for electrode potential:

where E = to electrode potential of redox system

E° = standard electrode potential of redox system

R = universal gas constant

T = thermodymical temperature

F = Faraday’s constant

z = number of electrons exchanged in redox reaction

a_O = activity of oxidised form

a_R = activity of reduced form

n = stechiometrical coefficient of oxidised form

m = stechiometrical coefficient of reduced form

absorbance    →   apsorbancija

Absorbance (A) is a logarithm of the ratio of incident radiant power (P_o) to transmitted radiant power (P) through a sample (excluding the effects on cell walls).

The absorption of light by a substance in a solution can be described mathematically by the Beer-Lambert law

where A is the absorbance at a given wavelength of light, ε is the molar absorbtivity or extinction coefficient (L mol^-1 cm^-1), unique to each molecule and varying with wavelength, b is the length of light path through the sample (cm), and c is the concentration of the compound in solution (mol L^-1).

chemical equation equalization    →   izjednačavanje kemijske jednadžbe

Chemical equation equalization is determining values of stechiometric coefficients of reactants and products in a chemical equation in a way that the number of atoms of each element is equal before and after the reaction.

Fick’s law    →   Fickov zakon

Fick’s law is the statement that the flux J of a diffusing substance is proportional to the concentration gradient, i.e.,

where D is called the diffusion coefficient.