Chemical Equilibrium & Equilibrium Constant (Kc)

Question-and-answer flashcards covering definitions, calculations, and factors affecting chemical equilibrium and the equilibrium constant Kc.

What conditions define a system at chemical equilibrium?

The forward and reverse reactions proceed at the same rate, the concentrations of reactants and products remain constant, and the reaction occurs in a closed system.

What is the equilibrium constant Kc for a reversible reaction?

Kc is the ratio of the equilibrium concentrations of products to reactants, each raised to the power of their stoichiometric coefficients in the balanced chemical equation.

Write the general Kc expression for the reaction aA + bB ⇌ cC + dD.

Kc = [C]^c [D]^d / ([A]^a [B]^b).

How do you determine the units of Kc for a specific reaction?

Insert the concentration units (mol dm⁻³) into the Kc expression and cancel them algebraically; if all units cancel, Kc is unitless.

What does a Kc value much greater than 1 indicate about the position of equilibrium?

The equilibrium lies far to the right, meaning products are favoured.

What does a Kc value much less than 1 indicate?

The equilibrium lies far to the left, so reactants are favoured.

How does changing the concentration of reactants or products affect the value of Kc?

It does not change Kc; it only shifts the position of equilibrium in accordance with Le Chatelier’s Principle.

For an exothermic reaction, what happens to Kc when temperature is increased?

Kc decreases because heat is treated as a product, and the equilibrium shifts toward reactants.

For an endothermic reaction, how does increasing temperature affect Kc?

Kc increases because heat acts as a reactant and the equilibrium shifts toward products.

Do changes in total pressure affect the numerical value of Kc for gaseous equilibria?

No. Pressure changes can shift the equilibrium position but they do not alter the value of Kc.

What effect does a catalyst have on the equilibrium constant Kc?

None. A catalyst speeds up both the forward and reverse reactions equally, helping the system reach equilibrium faster without changing Kc or the equilibrium position.

List the four basic steps for calculating Kc from experimental data.

1) Write and balance the chemical equation, 2) determine equilibrium concentrations of all species, 3) substitute these values into the Kc expression, and 4) compute the value with appropriate units.

Calculate Kc for the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g) when [H₂] = 0.20 mol dm⁻³, [I₂] = 0.20 mol dm⁻³, and [HI] = 0.60 mol dm⁻³.

Kc = [HI]² / ([H₂][I₂]) = (0.60)² / (0.20 × 0.20) = 0.36 / 0.04 = 9.0 (unitless).

Which physical states are NOT included in a Kc expression?

Pure solids and pure liquids; only gaseous and aqueous species are included.

Why is it crucial to use a balanced chemical equation when constructing a Kc expression?

Because the stoichiometric coefficients from the balanced equation become the exponents (powers) of the concentration terms in the Kc expression.