Chemical Equilibrium Practice Flashcards

A set of vocabulary flashcards covering the fundamental concepts of chemical equilibrium, including the law of mass action, Kp calculations, reaction quotients, and Le Châtelier’s principle.

Chemical equilibrium

The state in which the concentrations of all reactants and products remain constant with time in a closed vessel.

Equilibrium position

A particular set of reactant and product concentrations that satisfies the equilibrium constant expression; there are an infinite number of these for a given system at a given temperature.

Law of mass action

A general description of the equilibrium condition proposed by Cato Maximilian Guldberg and Peter Waage in 1864, expressed by the ratio of product concentrations to reactant concentrations, each raised to the power of their coefficients in the balanced equation.

Equilibrium constant ($K$)

The value obtained when observed equilibrium concentrations are inserted into the equilibrium expression; it is constant at a given temperature.

Haber process

The commercial synthesis of ammonia from elemental nitrogen and hydrogen ($N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$), pioneered by German chemist Fritz Haber.

Equilibrium expression for reverse reaction

The reciprocal of the equilibrium expression for the original reaction ($K_r = \frac{1}{K}$).

Equilibrium expression for reaction multiplied by factor $n$

The original equilibrium expression raised to the $n\text{th}$ power ($K_{new} = (K_{original})^n$).

$$K_p$$

An equilibrium constant for gas-phase reactions expressed in terms of the equilibrium partial pressures of the gases.

Relationship between $K$ and $K_p$

Defined by the formula $K_p = K(RT)^{\triangle n}$, where $\triangle n$ is the sum of the coefficients of the gaseous products minus the sum of the coefficients of the gaseous reactants.

Activity

The ratio of the equilibrium pressure (or concentration) of a substance to a defined reference pressure (typically $1\text{ atm}$ or $1\text{ bar}$) or reference concentration ($1\text{ mol/L}$).

Homogeneous equilibria

Equilibrium systems in which all reactants and products are in the same phase, typically the gas phase.

Heterogeneous equilibria

Equilibrium systems involving more than one phase, such as reactions between solids and gases.

Activity of a pure solid or liquid

For the purposes of the equilibrium expression, this value is always $1$ and therefore does not affect the equilibrium position.

Reaction quotient ($Q$)

A value obtained by applying the law of mass action using initial concentrations instead of equilibrium concentrations to determine the direction of the shift to reach equilibrium.

$$Q < K$$

A condition where the ratio of initial concentrations of products to reactants is too small, causing the system to shift to the right to form more products.

$$Q > K$$

A condition where the ratio of initial concentrations of products to reactants is too large, causing the system to shift to the left to form more reactants.

ICE table

A shorthand representation (Initial, Change, Equilibrium) used to organize concentrations when solving chemical equilibrium problems.

Le Châtelier’s principle

The principle stating that if a change in conditions (stress) is imposed on a system at equilibrium, the equilibrium position will shift in a direction that tends to reduce that change.

Effect of volume decrease on gaseous equilibrium

The system shifts toward the side of the reaction involving the smaller number of gaseous molecules as defined by the balanced equation.

Effect of temperature on exothermic reaction equilibrium

In an exothermic process (where energy is a product), increasing temperature shifts the equilibrium to the left, decreasing the value of $K$.

Effect of temperature on endothermic reaction equilibrium

In an endothermic process (where energy is a reactant), increasing temperature shifts the equilibrium to the right, increasing the value of $K$.

Activity coefficient ($\text{γ}_i$)

A factor used to correct observed equilibrium pressures to ideal values to calculate the true equilibrium constant for real gases.