Equilibrium Means Equal Rates, Not Equal Amounts

What students typically write

“At equilibrium, the amounts of reactants and products are the same.” Or: “Equilibrium means there are equal concentrations on both sides.”

Students interpret “equilibrium” in its everyday sense of “balance” and assume this means equal quantities. In chemistry, the balance is between rates, not amounts.

Why it is wrong

Dynamic equilibrium has three defining features:

1. The rate of the forward reaction equals the rate of the reverse reaction
2. The concentrations of reactants and products remain constant (but not necessarily equal)
3. The system is closed (nothing enters or leaves)

The word “equal” applies to the rates, not to the concentrations. At equilibrium, the forward reaction converts reactants to products at exactly the same rate as the reverse reaction converts products back to reactants. The net effect is no change in concentration, but the actual concentrations can be vastly different on each side.

For example, in the decomposition of calcium carbonate, the equilibrium strongly favours the solid calcium carbonate at room temperature. At high temperatures, the equilibrium shifts to favour the products. At no point does “equilibrium” guarantee 50:50.

What the mark scheme actually wants

A typical question: “Explain what is meant by dynamic equilibrium.” [3]

Marking points:

1. The forward and reverse reactions both continue to occur / it is dynamic (1 mark)
2. The rate of the forward reaction equals the rate of the reverse reaction (1 mark)
3. The concentrations of reactants and products remain constant / do not change (1 mark)

An answer that says “the amounts of products and reactants are equal” does not score mark 3 and may lose a mark if it contradicts the idea of constant but not equal concentrations.

Worked example: wrong vs right

Question: In the Haber process, N2 + 3H2 is in equilibrium with 2NH3. At 450 degrees C and 200 atm, the equilibrium mixture contains about 15% ammonia. Explain what is meant by equilibrium in this context. [2]

Wrong answer: “Equilibrium means there are equal amounts of nitrogen, hydrogen and ammonia in the mixture.” This scores 0/2. The question itself tells you there is only 15% ammonia, so equal amounts is clearly wrong.

Right answer: “The rate of the forward reaction (forming NH3) equals the rate of the reverse reaction (decomposing NH3) (1). The concentrations of N2, H2 and NH3 remain constant, though they are not equal (1).” This scores 2/2.

How to avoid this mistake

Whenever you write about equilibrium, use the word “rates” immediately. This anchors your answer to the correct concept. “Equal rates” is right. “Equal amounts” is wrong.

A helpful mental image: two escalators side by side, one going up and one going down, both running at the same speed. The number of people on each floor stays constant, but the upper and lower floors do not need the same number of people.

For the full treatment of reversible reactions and Le Chatelier’s principle, see reversible reactions and equilibrium. The rate of reaction page explains what determines reaction rates.