1.6 Equilibria and the Equilibrium Constant Kc

Vocabulary practice flashcards covering dynamic equilibrium, Le Chatelier's principle, industrial conditions, equilibrium constant Kc expressions and calculations, and related laboratory techniques.

Dynamic equilibrium

A state reached by all reversible reactions in which the forward and backward reactions occur at equal rates and the concentrations of reactants and products stay constant.

Dynamic (in chemical equilibrium)

Describes the condition where both forward and backward reactions are occurring simultaneously.

Position of equilibrium

An expression used to describe the composition of the equilibrium mixture, indicating whether reactants or products are favoured.

Le Chatelier's principle

A principle stating that if an external condition is changed, the equilibrium will shift to oppose the change and try to reverse it.

Compromise temperature

An operating temperature used in industrial reversible reactions that provides a balance between an acceptable yield and a sufficiently fast rate of reaction.

Catalyst (effect on equilibrium)

Has no effect on the position of equilibrium or the value of $K_c$ because it speeds up the rates of the forward and backward reactions by the same amount, but increases the rate at which equilibrium is reached.

Haber process conditions

An industrial process for producing ammonia using $T = 450^\top\text{C}$, $P = 200\text{ to }1000\text{ atm}$, and an iron catalyst for the reaction $N_2 + 3H_2 \rightleftharpoons 2NH_3$.

Contact process (Stage 2) conditions

Industrial reaction conditions of $T = 450^\top\text{C}$, $P = 1\text{ or }2\text{ atm}$, and a $V_2O_5$ catalyst for $SO_2(g) + \frac{1}{2}O_2(g) \rightleftharpoons SO_3(g)$ with $\triangle H = -98\text{ kJ mol}^{-1}$.

Hydration of ethene conditions

Industrial reaction conditions of $T = 300^\top\text{C}$, $P = 70\text{ atm}$, and concentrated $H_3PO_4$ catalyst to convert ethene and steam into liquid ethanol via $CH_2=CH_2(g) + H_2O(g) \rightleftharpoons CH_3CH_2OH(l)$.

Production of methanol from CO conditions

Industrial conditions of $T = 400^\top\text{C}$, $P = 50\text{ atm}$, and chromium and zinc oxides catalysts for the exothermic reaction $CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g)$.

Carbon neutral

An activity that results in no net annual carbon (greenhouse gas) emissions to the atmosphere.

Equilibrium constant $K_c$

A ratio representing the composition of an equilibrium mixture at a given temperature, expressed as $K_c = \frac{[C]^p [D]^q}{[A]^m [B]^n}$ for the generalised reaction $mA + nB \rightleftharpoons pC + qD$, where square brackets represent equilibrium concentrations in $mol\text{ dm}^{-3}$.

Moles of reactant at equilibrium (formula)

Calculated as: $\text{initial moles} - \text{moles reacted}$.

Moles of product at equilibrium (formula)

Calculated as: $\text{initial moles} + \text{moles formed}$.

Unit of $K_c$ for $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$

Given by substituting concentration units: $\frac{[mol\text{ dm}^{-3}]^2}{[mol\text{ dm}^{-3}][mol\text{ dm}^{-3}]^3} = [mol\text{ dm}^{-3}]^{-2} = mol^{-2}\text{ dm}^{+6}$.

Unit of $K_c$ for $H_2(g) + Cl_2(g) \rightleftharpoons 2HCl(g)$

Has no unit, because there are equal numbers of reactant and product moles, causing concentration units to cancel out completely.

Equimolar reaction (in $K_c$ calculations)

A reaction with the same number of moles of reactants as products, which allows the container volume ($V$) terms to cancel out completely in the $K_c$ expression.

Temperature dependence of $K_c$

The rule that the numerical value of $K_c$ only changes when temperature is altered; changes in pressure, concentration, or the addition of catalysts do not alter $K_c$.

Effect of temperature increase on an exothermic reaction

Causes the equilibrium to shift in the backward endothermic direction to oppose the change, shifting the position of equilibrium to the left and decreasing the value of $K_c$.

Effect of pressure increase on gas equilibria

Causes the equilibrium to shift towards the side with fewer moles of gas to reduce the pressure; the position of equilibrium shifts, but the value of $K_c$ stays constant.

Phenolphthalein fading (in esterification titrations)

The fading of the pink endpoint colour over time because adding sodium hydroxide removes carboxylic acid, causing the reversible esterification mixture to shift backward towards the reactants to restore equilibrium.