A-level Chemistry · Chemistry
Chemical equilibria: a clear study guide
Equilibrium is dynamic rather than static: forward and reverse reactions continue, but their rates are equal. Explain changes by identifying how the system responds, not by saying the equilibrium stops.
Dynamic equilibrium
In a closed system, a reversible reaction can reach dynamic equilibrium. The concentrations of reactants and products remain constant because the forward and reverse reaction rates are equal, even though particles continue to react.
The equilibrium position describes the relative amounts of reactants and products at equilibrium. It is not the same as the reaction rate.
- The system must be closed for the usual equilibrium model.
- Forward and reverse reactions continue.
- Equal rates do not mean equal concentrations.
Predicting a change
Le Chatelier's principle says that a system responds to a change in conditions in the direction that opposes that change. For a temperature change, treat heat as a reactant or product depending on whether the forward reaction is endothermic or exothermic.
Changing pressure affects a gaseous equilibrium when the two sides have different total numbers of gas molecules. A catalyst increases both rates and does not change the equilibrium position.
Equilibrium constants
The expression for Kc depends on the balanced equation and the convention in the relevant specification. Write the powers from the stoichiometric coefficients, omit pure solids and liquids where appropriate, and include units only when the question or specification requires them.
Quick check
What is equal at dynamic equilibrium?
Answer: The rates of the forward and reverse reactions.
Does a catalyst change the equilibrium position?
Answer: No. It speeds up both directions so equilibrium is reached faster.
Common questions
Does equilibrium mean the reaction has stopped?
No. Forward and reverse reactions continue at equal rates in a dynamic equilibrium.
How does pressure affect a gaseous equilibrium?
If the sides have different total gas moles, increasing pressure favours the side with fewer gas molecules; if the totals are equal, there is no equilibrium-position change from pressure alone.