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Energetics

Standard enthalpy of neutralisation

  • 3 things to know
  • Unit AS 2

Overview

Standard enthalpy of neutralisation and how the enthalpy of neutralisation is measured in the laboratory.

What you need to know

Answer each one before you reveal it.

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Key term

Describe how the enthalpy of neutralisation is measured in the laboratory.

Key term

Why is the enthalpy of neutralisation defined per mole of water rather than per mole of acid?

Key term

State the approximate value of the enthalpy of neutralisation for a strong acid and a strong alkali.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define the standard enthalpy of neutralisation.

    Model answer

    The standard enthalpy of neutralisation is the enthalpy change when an acid and a base react under standard conditions to form one mole of water.

  2. 2

    Why is the enthalpy of neutralisation defined per mole of water rather than per mole of acid?

    Model answer

    Acids differ in the number of moles of hydrogen ions they can supply, so only the water produced gives a basis common to all of them.

  3. 3

    State the approximate value of the enthalpy of neutralisation for a strong acid and a strong alkali.

    Model answer

    The value is close to −57 kJ mol⁻¹.

  4. 4

    Why is almost the same value obtained for every strong acid and strong alkali pair?

    Model answer

    Both are fully ionised in solution, so in every case the only reaction taking place is the combination of hydrogen ions with hydroxide ions.

  5. 5

    Write the ionic equation for the neutralisation of a strong acid by a strong alkali.

    Model answer

    The ionic equation is H⁺(aq) + OH⁻(aq) → H₂O(l).

  6. 6

    Why is the enthalpy of neutralisation of ethanoic acid by sodium hydroxide less exothermic than that of hydrochloric acid?

    Model answer

    Ethanoic acid is only partly ionised, so energy has to be absorbed to complete its dissociation and less heat is released overall.

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