Chemical equilibria, Le Chatelier's principle and Kc (AS Level) flashcards

Topic 6: Chemical equilibria, Le Chatelier's principle and Kc

Dynamic equilibrium

In a closed system, the forward and reverse reactions proceed at equal rates and the concentrations of reactants and products remain constant (not equal).

Le Chatelier’s principle

If a factor affecting a system at equilibrium is changed, the position of equilibrium shifts to oppose the change.

Effect of increasing pressure on an equilibrium

The position shifts to the side with fewer moles of gas, reducing the pressure. No shift if both sides have the same number of moles of gas.

Effect of increasing temperature on an equilibrium

The position shifts in the endothermic direction, absorbing heat.

Effect of a catalyst on an equilibrium

No shift in position and no change in yield. It speeds up the forward and reverse reactions equally, so equilibrium is reached faster.

Why are moderate temperatures used for an exothermic industrial equilibrium such as the Haber process?

Low temperature gives a higher yield but too slow a rate. A moderate temperature is a compromise: an acceptable yield at an acceptable rate.

Units of Kc for 2A(g) ⇌ B(g)

Kc = [B] ÷ [A]², so units are (mol dm⁻³) ÷ (mol dm⁻³)² = mol⁻¹ dm³.

What changes the value of Kc?

Only temperature. Concentration, pressure and catalysts change the position or rate, not Kc.

Forward reaction exothermic, temperature raised: effect on Kc

Equilibrium shifts left, so Kc decreases (and so does the yield).

2A ⇌ B: start with 1.00 mol A in 2.00 dm³; 0.200 mol B at equilibrium. Find Kc.

A used 0.400 mol, A left 0.600 mol. [A] = 0.300, [B] = 0.100 mol dm⁻³. Kc = 0.100 ÷ 0.300² = 1.11 mol⁻¹ dm³.