CHEMISTRY GLOSSARY

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

acid dissociation constant    →   konstanta disocijacije kiseline

Acid dissociation constant (K_a) is the equilibrium constant for the dissociation of an acid HA through the reaction

The quantity pK_a = -log K_a is often used to express the acid dissociation constant.

dissociation constant    →   konstanta disocijacije

Dissociation constant is a constant whose numerical value depends on the equilibrium between the undissociated and dissociated forms of a molecule. A higher value indicates greater dissociation.

The term dissociation is also applied to ionisation reactions of acids and bases in water. For example

which is often regarded as a straightforward dissociation into ions

The equilibrium constant of such a dissociation is called the acid dissociation constant or acidity constant, given by

The concentration of water [H₂O] can be taken as constant.

Similarly, for a base, the equilibrium

is also a dissociation; with the base dissociation constant or basicity constant, given by

K_a (K_b) is a measure of the strength of the acid (base).

solubility product constant    →   konstanta produkta topljivosti

Solubility product constant (K_sp) (or the solubility product) is the product of the molar concentrations of the constituent ions, each raised to the power of its stoichiometric coefficient in the equilibrium equation. For instance, if a compound A_aB_b is in equilibrium with its solution

the solubility product is given by

Avogadro constant    →   Avogadrova konstanta

Avogadro constant (N_A or L) is the number of elementary entities in one mole of a substance.

It has the value (6.022 045±0.000 031)×10²³ mol^-1.

constant    →   konstanta

Constant in a quantity that does not vary, e.g. in formula pV=RT, R is a universal constant and is equal to 8.314 J mol^-1 K^-1, while p, V and T are variables.

Boltzmann constant    →   Boltzmannova konstanta

The Boltzmann constant (k or k_B) is the physical constant describing the relationship between the thermodynamic temperature and the average kinetic energy of particles in a gas. It equals the molar gas constant R divided by the Avogadro constant N_A and has the value 1.380 648 52(79)×10^-23 J/K. It is named after the Austrian physicist Ludwig Eduard Boltzmann (1844-1906).

cryoscopic constant    →   krioskopska konstanta

Cryoscopic constant (E_f) is the constant that expresses the amount by which the freezing point T_f of a solvent is lowered by a non-dissociating solute, through the relation

where m is the molality of the solute.

ebullioscopic constant    →   ebulioskopska konstanta

Ebullioscopic constant (E_b) is the constant that expresses the amount by which the boiling point T_b of a solvent is raised by a nondissociating solute, through the relation

where b is the molality of the solute.

Faraday constant    →   Faradayeva konstanta

Faraday constant (F) is the electric charge of 1 mol of singly charged positive ions.

where N_A is Avogadro’s constant (6.022×10²³ mol^-1) and e is the elementary charge (1.602×10^-19 C).