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Thermochemistry

Calculating enthalpy changes from experimental data

  • 3 things to know
  • Components 1 and 2

Overview

Recall questions on q = mcΔT and what each symbol means, which mass is used in a solution reaction, converting the heat change to a molar enthalpy change, the sign convention, and worked combustion, neutralisation and dissolving calculations checked in both directions.

What you need to know

Answer each one before you reveal it.

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Numbers

Give the relationship used to calculate the heat energy transferred to a measured mass of water.

Key term

State the value of the specific heat capacity of water normally used in calorimetry calculations, with its units

Key term

Why can a temperature change be substituted in either degrees Celsius or kelvin?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Give the relationship used to calculate the heat energy transferred to a measured mass of water.

    Model answer

    The heat transferred is q = mcΔT, where m is the mass of water, c is its specific heat capacity and ΔT is the temperature change.

  2. 2

    What is meant by the specific heat capacity of a substance?

    Model answer

    It is the energy needed to raise the temperature of one gram of the substance by one kelvin.

  3. 3

    State the value of the specific heat capacity of water normally used in these calculations, with its units.

    Model answer

    The value used is 4.18 J g⁻¹ K⁻¹.

  4. 4

    Why can a temperature change be substituted in either degrees Celsius or kelvin?

    Model answer

    The two scales have divisions of the same size, so a difference of one degree Celsius equals a difference of one kelvin.

  5. 5

    How is an enthalpy change per mole obtained from a measured heat energy?

    Model answer

    The heat energy in kilojoules is divided by the number of moles of the substance that reacted, and a negative sign is added if the temperature rose.

  6. 6

    Burning 0.92 g of ethanol raised the temperature of 100 g of water by 30.0 °C. Calculate the heat energy transferred to the water.

    Model answer

    q = 100 × 4.18 × 30.0 = 12540 J, which is 12.5 kJ.

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