Equilibrium [H]

This lesson covers dynamic equilibrium, a Higher tier topic in the AQA GCSE Chemistry specification. When a reversible reaction takes place in a closed system, it eventually reaches a state of equilibrium. Understanding what equilibrium means, how it is established, and what it looks like on a graph is essential for achieving top grades at GCSE.

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What Is Dynamic Equilibrium?

When a reversible reaction occurs in a closed system, it reaches a point where the rate of the forward reaction equals the rate of the reverse reaction. This is called dynamic equilibrium.

At equilibrium:

• The forward and reverse reactions are both still occurring — the reaction has not stopped
• The rates of the forward and reverse reactions are equal
• The concentrations of reactants and products remain constant (but not necessarily equal)

Feature	At Equilibrium
Forward reaction	Still occurring
Reverse reaction	Still occurring
Rate of forward reaction	Equal to rate of reverse reaction
Concentrations of reactants	Constant (not changing)
Concentrations of products	Constant (not changing)
Reaction appears to have	Stopped (but it has not)

The word dynamic means "moving" — at equilibrium, the reaction is still happening in both directions. It only appears to have stopped because there is no net change in the amounts of reactants and products.

Exam Tip: [H] The most common mistake with equilibrium is to say "the reaction has stopped." It has NOT stopped. Both the forward and reverse reactions are still occurring, but at the same rate, so there is no overall change. Always use the phrase "dynamic equilibrium" to show you understand this.

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How Equilibrium Is Established

When a reversible reaction begins in a closed system:

1. Initially, only reactants are present
2. The forward reaction starts, producing products
3. As product concentration increases, the reverse reaction begins and speeds up
4. As reactants are used up, the forward reaction slows down
5. Eventually, the rate of the forward reaction equals the rate of the reverse reaction
6. Equilibrium is reached

graph TD
    A[Start: Only reactants present] --> B[Forward reaction begins]
    B --> C[Products start to form]
    C --> D[Reverse reaction begins and speeds up]
    D --> E[Forward reaction slows as reactants used up]
    E --> F[Rate forward = Rate reverse]
    F --> G[Dynamic Equilibrium reached]
    G --> H[Concentrations remain constant]

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Equilibrium on a Concentration-Time Graph

A graph of concentration against time for a reversible reaction reaching equilibrium shows:

Time Period	Reactant Concentration	Product Concentration
Start	High (decreasing)	Zero (increasing)
Approaching equilibrium	Decreasing more slowly	Increasing more slowly
At equilibrium	Constant	Constant

Key features:

• The reactant concentration curve decreases and then levels off
• The product concentration curve increases and then levels off
• Both curves become flat (horizontal) at the same time — this is when equilibrium has been reached
• The flat lines are at different heights because the concentrations of reactants and products at equilibrium are usually different

Exam Tip: [H] If the graph shows the concentration of a reactant decreasing and levelling off, and the concentration of a product increasing and levelling off, the system has reached equilibrium. The concentrations at equilibrium do NOT have to be equal — they just have to be constant.

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Equilibrium on a Rate-Time Graph

A rate-time graph for reaching equilibrium shows:

Time Period	Rate of Forward Reaction	Rate of Reverse Reaction
Start	High	Zero
Approaching equilibrium	Decreasing	Increasing
At equilibrium	Equal to reverse rate	Equal to forward rate

The two rate lines converge and become equal at the point of equilibrium.

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The Position of Equilibrium

The position of equilibrium describes the relative amounts of reactants and products at equilibrium.

Position	Meaning	Relative Amounts
Equilibrium lies to the right	More products than reactants at equilibrium	High yield of products
Equilibrium lies to the left	More reactants than products at equilibrium	Low yield of products
Equilibrium lies in the middle	Roughly equal amounts of reactants and products	Moderate yield

The position of equilibrium can be changed by altering the conditions. This is described by Le Chatelier's principle, which you will study in the next lesson.

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Conditions for Equilibrium

For dynamic equilibrium to be established, the following conditions must be met:

Condition	Why It Is Necessary
Closed system	No reactants or products can escape, so both forward and reverse reactions can occur
Reversible reaction	The reaction must be able to go in both directions
Constant conditions	Temperature and pressure must remain constant for the equilibrium position to remain stable

If the system is open (e.g. a gas can escape), equilibrium cannot be established because the products leave the system and the reverse reaction cannot occur.

graph TD
    A[Requirements for Equilibrium] --> B[Closed system]
    A --> C[Reversible reaction]
    A --> D[Constant conditions]
    B --> B1[No matter enters or leaves]
    C --> C1[Reaction can go forward and backward]
    D --> D1[Temperature and pressure unchanged]

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Equilibrium Does NOT Mean Equal Amounts

A very important point: at equilibrium, the concentrations of reactants and products are constant, but they are usually not equal.

Scenario	Example
Equilibrium lies to the right	90% products, 10% reactants — the equilibrium favours product formation
Equilibrium lies to the left	10% products, 90% reactants — the equilibrium favours the reactants
Equilibrium in the middle	50% products, 50% reactants — roughly equal (this is rare)

The key word is constant, not equal.

Exam Tip: [H] Never write "at equilibrium, the amounts of reactants and products are equal." This is almost always wrong. Instead, write "at equilibrium, the concentrations of reactants and products are constant." The word "constant" is what earns the mark.

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The Effect of a Catalyst on Equilibrium

A catalyst does not change the position of equilibrium. It increases the rate of both the forward and reverse reactions by the same amount, so equilibrium is reached faster but the final proportions of reactants and products are the same.