Equilibrium: The Dynamic Balance Revision Notes | A Level AQA Chemistry

1. The Puzzle of the Reaction That Never Finishes

Most reactions you met at GCSE ran to completion — reactants disappeared and products built up until one reagent ran out. But a large number of industrially and biologically important reactions refuse to behave that way. They reach a point where both reactants and products coexist indefinitely, and no further net change appears to happen.

This is not a reaction grinding to a halt. At the molecular level, chemistry is still happening at full speed in both directions simultaneously. Understanding that distinction — between apparent stillness and genuine dynamic activity — is the foundation of this entire topic.

2. Reversible Reactions: The Two-Way Street

A reversible reaction is one in which the products can react together to regenerate the original reactants. The reaction can proceed in both the forward direction (reactants $\rightarrow$ products) and the reverse direction (products $\rightarrow$ reactants). The symbol used to show this is the equilibrium sign: $\rightleftharpoons$.

A classic example from the AQA specification is the thermal decomposition of ammonium chloride. When heated, solid ammonium chloride dissociates into ammonia gas and hydrogen chloride gas. On cooling, those gases recombine to reform the white solid.

Written as an equation: $\text{NH}_4\text{Cl}(s) \rightleftharpoons \text{NH}_3(g) + \text{HCl}(g)$. The $\rightleftharpoons$ sign signals that the reaction can proceed in both directions, and the equilibrium position depends on the conditions.

Notice that reversibility alone does not mean equilibrium has been reached. A reversible reaction only reaches equilibrium when it is carried out in a closed system — one where no substances can enter or escape. If products are continuously removed, the reverse reaction rate cannot build up to match the forward rate, so equilibrium is never established.

3. What Actually Happens at Equilibrium

Picture a sealed flask containing nitrogen and hydrogen gas at high temperature. Initially, only the forward reaction occurs: $\text{N}_2$ and $\text{H}_2$ collide and form $\text{NH}_3$. As ammonia builds up, molecules of $\text{NH}_3$ begin to decompose back into $\text{N}_2$ and $\text{H}_2$ — the reverse reaction starts.

Over time, the rate of the forward reaction decreases (because reactant concentrations are falling) while the rate of the reverse reaction increases (because product concentration is rising). Eventually, these two rates become equal. That is the moment of dynamic equilibrium.

The chart above captures the key idea: the forward rate starts high and falls; the reverse rate starts at zero and rises. They converge at a common value — and from that point on, both rates remain equal and constant. This is the hallmark of dynamic equilibrium.

4. Why Concentrations Stay Constant

This is the part that trips students up most often. At equilibrium, concentrations are constant — but they are not necessarily equal. The concentration of ammonia at equilibrium might be very different from the concentration of nitrogen or hydrogen. What matters is that none of those values is changing over time.

The reason concentrations stay constant is a direct consequence of the equal rates. Consider any product molecule: it is being formed by the forward reaction at exactly the same rate as it is being consumed by the reverse reaction. The net change in its concentration is therefore zero. The same logic applies to every reactant.

This is fundamentally different from a reaction that has run out of one reactant. In that case, the reaction has stopped because there is nothing left to react. At equilibrium, all species are still present and both reactions are still occurring — the constancy of concentration arises from a balance of rates, not from exhaustion of reagents.

5. Static vs Dynamic: A Critical Distinction

A common source of confusion is the difference between a static equilibrium and a dynamic one. In a static equilibrium — like a book resting on a table — nothing is moving at all. Forces balance, but there is no ongoing process.

A dynamic equilibrium is fundamentally different. Molecules are constantly reacting in both directions. If you could label individual molecules and track them, you would see reactant molecules continuously converting to products and product molecules continuously converting back to reactants — at the same rate. The macroscopic observation (constant concentration) masks the microscopic reality (constant molecular activity).

AQA exam questions sometimes describe a scenario and ask whether equilibrium has been reached, or ask you to justify why a system is or is not at dynamic equilibrium. The table above gives you the framework to answer those questions precisely.

6. The Equilibrium Position

Once equilibrium is established, the relative amounts of reactants and products present are described by the equilibrium position. This can lie anywhere between two extremes: entirely to the left (almost all reactants, very little product) or entirely to the right (almost all products, very little reactant).

For the Haber process, $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$, the equilibrium position at typical industrial conditions lies to the left — meaning the mixture at equilibrium contains more unreacted nitrogen and hydrogen than ammonia. This is why industrial chemists work hard to optimise conditions and remove ammonia as it forms.

The equilibrium constant Kc is fixed at a given temperature. Changing concentration or pressure shifts the equilibrium position to a new set of equilibrium concentrations, but Kc remains unchanged. Only a change in temperature alters Kc, and therefore permanently changes the equilibrium position (this connects to Le Chatelier's principle, which you will study in the next lesson).

7. Exam Technique and Mark-Winning Precision

Equilibrium questions at A Level are heavily language-dependent. The mark scheme rewards precise scientific vocabulary, and a single imprecise word can cost you a mark even when your underlying understanding is correct. The guidance below consolidates the most important exam-technique points for this topic.

When a question asks you to describe dynamic equilibrium, give a two-part answer: (1) the rates of the forward and reverse reactions are equal, and (2) the concentrations of reactants and products remain constant. Both parts are needed for full marks. When a question asks you to explain why concentrations are constant, you must link the constant concentrations back to the equal rates — that causal link is the explanation.

Worked example: Constructing a Full-Mark Equilibrium Explanation

Explain what is meant by a dynamic equilibrium. [3 marks]

1. Command word: 'Explain' — requires reasons, not just description. — An 'explain' answer must include cause-and-effect language. stating facts without linking them will not score full marks.
2. The rate of the forward reaction equals the rate of the reverse reaction. — This is the first independent mark. Use the word 'equal' and name both directions explicitly.
3. The concentrations of reactants and products remain constant. — Use 'constant', not 'equal'. This is the second independent mark. Avoid saying the reaction has stopped.
4. Both reactions continue to occur simultaneously in a closed system, so there is no net change in composition. — The word 'dynamic' must be justified by stating that both reactions are still proceeding. This earns the third mark and distinguishes a dynamic equilibrium from a static one.

Answer: At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. Both reactions continue to occur simultaneously in a closed system, so the concentrations of all reactants and products remain constant. [3 marks]

Notice how the model answer above uses three distinct, examinable statements. Each one maps to a separate mark. In an exam, writing a long paragraph that repeats the same idea in different words will not score more marks than a concise answer that hits each point once, clearly.

The long-answer question above models the kind of evaluative reasoning AQA expects at A Level. Notice that a strong answer does not stop at confirming what the evidence supports — it also identifies what the evidence cannot prove, and suggests how the gap could be addressed. That analytical structure is what separates a Level 3 response from a Level 2 one.

To bring everything together: a reversible reaction in a closed system will always tend towards dynamic equilibrium. At that point, both forward and reverse reactions proceed at equal rates, and the concentrations of all species remain constant — not because nothing is happening, but because formation and consumption of every species are perfectly balanced. That balance, and the precise language used to describe it, is the foundation for everything that follows in this topic.