AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    The importance of the conditions when measuring the electrode potential, E…

    Full specification wording

    Electrode potentials and cells (A-level only): The importance of the conditions when measuring the electrode potential, E (Nernst equation not required).

    An electrode potential is the position of an equilibrium, so anything that shifts that equilibrium changes E. Standard values are quoted at 298 K with every solution at 1.00 mol dm⁻³ and every gas…

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 4 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    An electrode potential is the position of an equilibrium, so anything that shifts that equilibrium changes E. Standard values are quoted at 298 K with every solution at 1.00 mol dm⁻³ and every gas at 100 kPa. Lowering [Cu²⁺] in Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) pulls the equilibrium to the left, so the electrode accepts electrons less readily and E becomes less positive. Changing the temperature or the pressure of a gaseous reactant moves the equilibrium too. What does not change E is the surface area of the electrode: a larger platinum surface lets the equilibrium be reached faster but does not alter where that equilibrium lies. Rate and position are separate ideas, and only the position sets the potential.

    What earns the marks

    • Stating the standard conditions of 298 K, 100 kPa and 1.00 mol dm⁻³ solutions.
    • Explaining the effect of a concentration change using the position of the half-cell equilibrium.
    • Stating that the surface area of the electrode has no effect on the electrode potential.
    • Deducing whether E becomes more or less positive from the direction the equilibrium shifts.

    Common mistakes and how to put them right

    • Claiming that increasing the surface area of the platinum electrode makes the potential more positive, when it only changes how fast equilibrium is reached.

    • Saying temperature has no effect because E⦵ is a constant.

    • Treating E as a fixed property of the half-cell whatever the concentrations are.

    • Quoting 273 K or 100 °C as the standard temperature instead of 298 K.

    Exam tips

    • Treat every half-equation as an equilibrium and apply Le Chatelier's principle to it; that answers almost every 'effect of' question here.
    • Watch for distractors about electrode size, shape or surface area; these change rate, not potential.

    Try the lesson

    The start of the Electrode potentials and cells (A-level only): The importance of the conditions when measuring the electrode potential, E (Nernst equation not required). lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which set of conditions is used when measuring a standard electrode potential?

    Listen

    An electrode potential is the position of an equilibrium, so anything that shifts that equilibrium changes E. Standard values are quoted at 298 K with every solution at 1.00 mol dm⁻³ and every gas at 100 kPa. Lowering [Cu²⁺] in Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) pulls the equilibrium to the left, so the electrode accepts electrons less readily and E becomes less positive. Changing the temperature or the pressure of a gaseous reactant moves the equilibrium too. What does not change E is the surface area of the electrode: a larger platinum surface lets the equilibrium be reached faster but does not alter where that equilibrium lies. Rate and position are separate ideas, and only the position sets the potential. A strong answer earns marks for these points. stating the standard conditions of 298 K, 100 kPa and 1.00 mol dm⁻³ solutions. explaining the effect of a concentration change using the position of the half-cell equilibrium. stating that the surface area of the electrode has no effect on the electrode potential. deducing whether E becomes more or less positive from the direction the equilibrium shifts. Exam tip: Treat every half-equation as an equilibrium and apply Le Chatelier's principle to it; that answers almost every 'effect of' question here.

    See it

    A top-down flowchart. The top node reads 'Equilibrium position sets E'. From it, three branches. The first branch leads to 'Standard conditions: 298 K, 100 kPa, 1.00 mol dm⁻³'. The second branch leads to 'Change in conditions', which splits into two: one path goes to 'Lower [Cu²⁺]: equilibrium shifts left', then to 'Electrode accepts electrons less readily', then to 'E becomes less positive'; the other path goes to 'Change in temperature or gas pressure moves equilibrium'. The third branch leads to 'Surface area of electrode', then to 'Changes how fast equilibrium is reached', then to 'No effect on E'.
    It shows that the position of the half-cell equilibrium sets E, how changing conditions shift it, and that electrode surface area affects only rate, not potential.

    Key words

    Say what each word means before you turn the card.

    standard conditions

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    equilibrium position

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    effect of concentration

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    rate versus position

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    See every definition
    standard conditions
    Values are quoted at 298 K with every solution at 1.00 mol dm⁻³ and every gas at 100 kPa.
    equilibrium position
    An electrode potential is the position of an equilibrium, so anything that shifts that equilibrium changes E.
    effect of concentration
    Lowering [Cu²⁺] pulls the equilibrium to the left, so the electrode accepts electrons less readily and E becomes less positive.
    rate versus position
    Rate and position are separate ideas, and only the position sets the potential.

    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 top-down flowchart. The top node reads 'Equilibrium position sets E'. From it, three branches. The first branch leads to 'Standard conditions: 298 K, 100 kPa, 1.00 mol dm⁻³'. The second branch leads to 'Change in conditions', which splits into two: one path goes to 'Lower [Cu²⁺]: equilibrium shifts left', then to 'Electrode accepts electrons less readily', then to 'E becomes less positive'; the other path goes to 'Change in temperature or gas pressure moves equilibrium'. The third branch leads to 'Surface area of electrode', then to 'Changes how fast equilibrium is reached', then to 'No effect on E'.

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