Chemical Equilibrium Constant (Kc) – Key Concepts

Flashcards cover definition, mathematical form, temperature dependence, non-influencing factors (concentration, pressure, catalysts), illustrative example, and exam strategy tips for Kc.

What does the equilibrium constant Kc represent?

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of its stoichiometric coefficient.

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

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

Which single factor can change the numerical value of Kc?

Temperature; concentration, pressure, and catalysts do not alter Kc.

For an exothermic reaction (ΔH < 0), how does increasing temperature influence Kc?

Equilibrium shifts to the left (toward reactants), so Kc decreases.

For an endothermic reaction (ΔH > 0), how does increasing temperature influence Kc?

Equilibrium shifts to the right (toward products), so Kc increases.

Why does temperature affect the value of Kc?

Changing temperature alters the relative rates of the forward and reverse reactions, shifting equilibrium concentrations and therefore the Kc value.

Does changing the concentration of reactants or products alter Kc? Explain.

No. Concentration changes shift the equilibrium position temporarily, but the system readjusts to restore the original Kc.

In the reaction H₂(g) + I₂(g) ⇌ 2 HI(g), what happens if extra H₂ is added, and what happens to Kc?

Equilibrium shifts right to produce more HI, but the value of Kc remains unchanged after adjustment.

Does altering total pressure in a gaseous equilibrium change Kc? Why or why not?

No. Pressure changes may shift the position if gaseous mole numbers differ, but Kc stays the same once new equilibrium is reached.

Do catalysts affect the equilibrium constant Kc? Explain their role.

No. Catalysts speed up both forward and reverse reactions equally, helping the system reach equilibrium faster without changing Kc or the equilibrium position.

Exam tip: What two pieces of information should always be stated when discussing equilibrium shifts?

Indicate the direction of the shift (left or right) and whether Kc increases, decreases, or remains the same, citing Le Chatelier’s Principle and ΔH where relevant.

Final recap: list the four factors discussed and their effects on Kc.

Temperature: changes Kc; Concentration: no effect; Pressure: no effect; Catalyst: no effect.