Chemical and Physical Equilibrium

Vocabulary practice flashcards covering physical and chemical equilibrium, Henry's law, the law of mass action, equilibrium constants (Kc and Kp), active mass, and reaction kinetics.

Equilibrium

The state of a process in which measurable properties such as temperature, pressure, and concentration of the system do not show any change with the passage of time.

Dynamic Equilibrium

The condition of a system in which the rate of the forward reaction equals the rate of the backward reaction, represented as $a + b \rightleftharpoons c + d$.

Physical Equilibrium

An equilibrium state involving changes in physical state or size where two opposing processes or forces occur at equal rates, such as ice and water in equilibrium at $0^\circ\text{C}$.

Solid to Liquid Equilibrium

A physical phase equilibrium represented by $H_2O_{(s)} \rightleftharpoons H_2O_{(l)}$, where the rate of melting equals the rate of freezing.

Liquid to Vapour Equilibrium

A physical phase equilibrium represented by $H_2O_{(l)} \rightleftharpoons H_2O_{(g)}$, where the rate of vaporization equals the rate of condensation.

Solid to Vapour Equilibrium

A physical sublimation equilibrium, such as $\text{Camphor}_{(s)} \rightleftharpoons \text{Camphor}_{(g)}$ or $I_{2(s)} \rightleftharpoons I_{2(g)}$, where the rate of sublimation equals the rate of deposition of vapours.

Solid Solution Equilibrium

A dissolution equilibrium in a solution where the rate of sugar dissolution equals the rate of crystallisation.

Henry's Law

A gas law stating that the mass ($m$) of gas dissolved in a given mass of solvent at any temperature is directly proportional to the pressure ($P$) of the gas above the solution, expressed as $m = K_H P$, where $K_H$ is Henry's constant.

Reversible Reaction

A chemical reaction that proceeds in both the forward and backward directions under the same conditions, such as $N_2 + 3H_2 \rightleftharpoons 2NH_3$.

Irreversible Reaction

A chemical reaction that cannot take place in the reverse direction because the products do not react to regenerate the reactants under the same conditions, such as $C + O_2 \rightarrow CO_{2(g)}$.

Concentration

The number of moles of a substance per unit volume.

Factors Disturbing Equilibrium

External reaction parameters that can alter or disturb an established chemical equilibrium, specifically concentration, pressure, and temperature.

Role of a Catalyst in Equilibrium

A substance that equally increases or decreases the rate of both forward and backward reactions, helping achieve equilibrium earlier without shifting or impacting the equilibrium point.

Law of Mass Action

A law stating that the rate of a chemical reaction is directly proportional to the concentration of reactants raised to the power of their stoichiometric coefficients.

Equilibrium Constant ($K_c$)

The ratio of the forward rate constant ($k_f$) to the backward rate constant ($k_b$), equal to the concentration of products divided by the concentration of reactants raised to their respective stoichiometric coefficients: $K_c = \frac{k_f}{k_b} = \frac{[C]^c [D]^d}{[A]^a [B]^b}$.

Equilibrium Constant of Reverse Reaction ($K_c'$)

The equilibrium constant for the reverse reaction ($cC + dD \rightleftharpoons aA + bB$), which equals the reciprocal of the forward equilibrium constant ($K_c' = \frac{1}{K_c}$).

Active Mass of Pure Solids and Liquids

The effective concentration of a pure solid or liquid in an equilibrium system involving gases, which remains constant and is taken as unity ($1$).

Equilibrium Constant in Terms of Partial Pressure ($K_p$)

The equilibrium constant calculated using the partial pressures of gaseous species, derived from the ideal gas relationship $P = \frac{n_A}{V} RT$.

Relation Between $K_p$ and $K_c$

The thermodynamic equation linking pressure and concentration equilibrium constants: $K_p = K_c (RT)^{\Delta n_g}$, requiring at least one gaseous reactant or product, with gas constant $R = 0.082\,atm\,l\,mol^{-1}K^{-1}$.

$$\Delta n_g$$

The difference between the number of moles of gaseous products and gaseous reactants in a balanced chemical equation: $\Delta n_g = n_p - n_r$.

Equilibrium Condition where $K_p = K_c$

The specific state occurring when $\Delta n_g = 0$, such as in $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$, where $\Delta n_g = 2 - (1+1) = 0$, simplifying $K_p = K_c (RT)^0$ to $K_p = K_c$.

Equilibrium Condition where $K_p < K_c$

The specific state occurring when $\Delta n_g < 0$, such as in $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$ where $\Delta n_g = 2 - 4 = -2$, leading to $K_p = K_c (RT)^{-2}$.