CHEM10007 · Fundamentals Of Chemistry
Chemical Equilibrium
Reversible reactions settle into a dynamic equilibrium where forward and reverse rates are equal and concentrations stay constant. You write the equilibrium constant Kc, compute the reaction quotient Q at any instant to predict the direction of change, and apply Le Chatelier's Principle to predict how a system responds to changes in concentration, pressure/volume or temperature.
What this chapter covers
- 01Dynamic equilibrium: forward rate = reverse rate; concentrations constant (not necessarily equal)
- 02Equilibrium constant Kc = [products]^coeff / [reactants]^coeff
- 03Pure solids and liquids omitted from K (heterogeneous equilibria)
- 04Magnitude of K and the extent of reaction (large K favours products)
- 05Reaction quotient Q: same form as K, evaluated at any instant
- 06Comparing Q to K: Q < K favours forward, Q > K favours reverse, Q = K is equilibrium
- 07Le Chatelier's Principle: the system shifts to oppose an imposed change
- 08Effects of adding/removing species, changing pressure/volume, and changing temperature (which changes K)
Reaction quotient Q versus K and the direction of shift
- 1 mark — correct Q expressionWrite Q in the same form as K: Q = [N2][C2H2] / [HCN]2.
- 1 mark — numerical value of QSubstitute: Q = (0.020)(0.030) / (0.050)2 = 6.0 × 10−4 / 2.5 × 10−3 = 0.24.
- 1 mark — direction with justificationCompare: Q = 0.24 < K = 0.25, so the forward reaction is favoured — the system makes more products until Q rises to K.
Key terms
- Dynamic equilibrium
- A state where the forward and reverse reactions proceed at equal rates, so concentrations remain constant even though both reactions continue.
- Equilibrium constant (Kc)
- The ratio of product to reactant concentrations, each raised to its coefficient, at equilibrium; pure solids and liquids are omitted.
- Reaction quotient (Q)
- The same concentration ratio as K but evaluated at any instant; comparing Q with K predicts the direction of net reaction.
- Le Chatelier's Principle
- When a system at equilibrium is disturbed, it shifts in the direction that partially opposes the change (in concentration, pressure/volume or temperature).
- Heterogeneous equilibrium
- An equilibrium involving more than one phase; pure solid and liquid phases are excluded from the K expression.
Chemical Equilibrium FAQ
What is the difference between Q and K?
They have the identical algebraic form, but K is the fixed value at equilibrium (at a given temperature) while Q is calculated for the actual concentrations at any instant. Comparing Q to K tells you which way the reaction must shift.
How does temperature differ from other stresses in Le Chatelier's analysis?
Changing concentration, pressure or volume shifts the position of equilibrium but leaves K unchanged. Changing temperature actually changes the value of K — raising temperature increases K for an endothermic reaction and decreases it for an exothermic one.
Why are pure solids and liquids left out of K?
Their concentration (effectively their density) does not change as the reaction proceeds, so they are assigned a value of 1 and omitted from the K and Q expressions.
Exam move
Practise writing K and Q correctly for both homogeneous and heterogeneous systems, remembering to drop pure solids and liquids. Make the Q-versus-K comparison a one-line habit for predicting direction. For Le Chatelier questions, decide the stress first, then reason which side absorbs it — and treat temperature changes separately because they alter K itself, a frequent multiple-choice trap.
