AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    If the concentration of one of the reactants or products is changed, the…

    Full specification wording

    The effect of changing concentration (HT only): If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.

    When the concentration of a reactant or product in a dynamic equilibrium is changed, the system is temporarily no longer at equilibrium.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 1 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    When the concentration of a reactant or product in a dynamic equilibrium is changed, the system is temporarily no longer at equilibrium. The rates of the forward and reverse reactions become unequal, so the concentrations of all substances change until a new equilibrium is established. For example, in the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), adding more H₂ increases the forward reaction rate, so H₂ and I₂ are used up and more HI is formed until equilibrium is restored. Removing a product has a similar effect, shifting equilibrium to replace it. The new equilibrium may have different concentrations from the original, but the system remains dynamic: both forward and reverse reactions continue at equal rates.

    What earns the marks

    • State that changing the concentration of a reactant or product disturbs the equilibrium so the system is temporarily not at equilibrium.
    • Explain that the rates of the forward and reverse reactions become unequal immediately after the change.
    • Describe that the concentrations of all substances change until a new equilibrium is reached.
    • Use a balanced equation to show how adding a reactant increases the forward reaction and leads to more product formation.
    • Recognise that at the new equilibrium both forward and reverse reactions continue at equal rates, but the concentrations may differ from the original equilibrium.

    Common mistakes and how to put them right

    • Thinking that only the concentration of the added substance changes.

      Put it right: In fact the concentrations of all substances change as the system re-equilibrates.

    • Believing that equilibrium stops when a concentration is changed.

      Put it right: The system is temporarily not at equilibrium but re-establishes equilibrium.

    • Assuming the new equilibrium has the same concentrations as before.

      Put it right: The position of equilibrium shifts, so concentrations differ.

    Exam tips

    • Use the phrase the system is no longer at equilibrium to show understanding of the disturbance.
    • Link the change in concentration to the relative rates of the forward and reverse reactions.
    • State that a new equilibrium is reached and that concentrations of all substances change, not just the one added or removed.

    Try the lesson

    The start of the The effect of changing concentration (HT only): If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A system is at equilibrium when extra reactant is added. What happens to the system immediately after the addition?

    Listen

    When the concentration of a reactant or product in a dynamic equilibrium is changed, the system is temporarily no longer at equilibrium. The rates of the forward and reverse reactions become unequal, so the concentrations of all substances change until a new equilibrium is established. For example, in the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), adding more H₂ increases the forward reaction rate, so H₂ and I₂ are used up and more HI is formed until equilibrium is restored. Removing a product has a similar effect, shifting equilibrium to replace it. The new equilibrium may have different concentrations from the original, but the system remains dynamic: both forward and reverse reactions continue at equal rates. A strong answer earns marks for these points. State that changing the concentration of a reactant or product disturbs the equilibrium so the system is temporarily not at equilibrium. Explain that the rates of the forward and reverse reactions become unequal immediately after the change. Describe that the concentrations of all substances change until a new equilibrium is reached. Use a balanced equation to show how adding a reactant increases the forward reaction and leads to more product formation. Exam tip: Use the phrase the system is no longer at equilibrium to show understanding of the disturbance.

    See it

    A vertical flowchart with six boxes connected by downward arrows. The first box says the concentration of a reactant or product is changed. An arrow leads to the second box, which says the system is no longer at equilibrium. An arrow leads to the third box, which says the rates of forward and reverse reactions become unequal. An arrow leads to the fourth box, which says the concentrations of all substances change. An arrow leads to the fifth box, which says a new equilibrium is reached. An arrow leads to the sixth box, which says forward and reverse reactions continue at equal rates.
    It shows the sequence from changing the concentration of a reactant or product, through the system no longer being at equilibrium and the rates becoming unequal, to a new equilibrium being reached where both reactions continue at equal rates.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Rates of forward and reverse reactions become unequal
    2. 2. System is no longer at equilibrium
    3. 3. Concentration of a reactant or product is changed
    4. 4. Forward and reverse reactions continue at equal rates
    5. 5. New equilibrium is reached
    6. 6. Concentrations of all substances change

    Key words

    Say what each word means before you turn the card.

    Dynamic equilibrium

    Try answering aloud or in your head before turning the card.

    Disturbance and re-equilibration

    Try answering aloud or in your head before turning the card.

    Concentration changes

    Try answering aloud or in your head before turning the card.

    Rates of forward and reverse reactions

    Try answering aloud or in your head before turning the card.

    See every definition
    Dynamic equilibrium
    A state in which both forward and reverse reactions continue at equal rates.
    Disturbance and re-equilibration
    Changing the concentration of a reactant or product means the system is no longer at equilibrium, and concentrations of all substances change until equilibrium is reached again.
    Concentration changes
    When the concentration of one reactant or product is changed, the concentrations of all the substances will change until equilibrium is reached again.
    Rates of forward and reverse reactions
    Immediately after a concentration change the rates of the forward and reverse reactions become unequal, until a new equilibrium is established.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart with six boxes connected by downward arrows. The first box says the concentration of a reactant or product is changed. An arrow leads to the second box, which says the system is no longer at equilibrium. An arrow leads to the third box, which says the rates of forward and reverse reactions become unequal. An arrow leads to the fourth box, which says the concentrations of all substances change. An arrow leads to the fifth box, which says a new equilibrium is reached. An arrow leads to the sixth box, which says forward and reverse reactions continue at equal rates.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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