Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Definition
A reversible reaction in a closed system is at equilibrium when the forward and reverse reaction rates are equal and reactant and product concentrations no longer change. Both reactions continue, so the equilibrium is dynamic.
Rates over time
Initially, reactant concentration is high and the forward rate is greater. As products form, the reverse rate increases. Eventually the rates become equal. They remain equal until a condition changes.
Concentrations
At equilibrium, concentrations are constant because each substance is formed and used at equal rates. Constant does not mean equal: one side may be strongly favoured and present at much higher concentration.
Macroscopic and microscopic views
There is no visible overall change, yet particles continue reacting. Dynamic equilibrium connects stable bulk observations with continual microscopic reaction.
Practical or data skill
Use coloured counters to model forward and reverse conversion each round. Adjust transfer probabilities until equal rates and constant totals emerge.
Examination tip
State both defining conditions: equal forward/reverse rates and constant concentrations.
Review questions and suggested answers
Question 1
Why is equilibrium dynamic?
Suggested answer
Forward and reverse reactions continue even though there is no net change.
Question 2
Are the two rates zero?
Suggested answer
No. They are equal and usually non-zero.
Question 3
Why are concentrations constant?
Suggested answer
Each substance is produced and consumed at the same rate.
