5 (a) Balance the chemical equation for the combustion of methane - AQA - GCSE Chemistry - Question 5 - 2016 - Paper 3

To calculate the heat energy released, we first find the temperature change:

$\Delta T = 38.4 °C - 22.0 °C = 16.4 °C$

Now, using the formula:

$Q = m \times c \times \Delta T$

Where:

• m = 0.8 g (mass of alcohol)
• c = 4.2 J/g/°C (specific heat capacity)
• \Delta T = 16.4 °C

Substituting the values in:

$Q = 0.8 \times 4.2 \times 16.4 = 3444 J$

Using the bond energies provided:

• Energy for breaking bonds:

  • C–H: 5 bonds × 413 kJ/mol = 2065 kJ
  • C–C: 1 bond × 347 kJ/mol = 347 kJ
  • O=O: 3 bonds × 467 kJ/mol = 1401 kJ
  • Total energy absorbed = 2065 + 347 + 1401 = 3813 kJ

• Energy for forming bonds:

  • C=O: 2 bonds × 799 kJ/mol = 1598 kJ
  • O–H: 6 bonds × 495 kJ/mol = 2970 kJ
  • Total energy released = 1598 + 2970 = 4568 kJ

• Overall energy change:

$\text{Energy change} = \text{Energy absorbed} - \text{Energy released} = 3813 - 4568 = -1276 kJ/mol$

The reaction is exothermic because the energy released when bonds form is greater than the energy used when bonds break. This means overall, the reaction gives off heat, resulting in a lower energy state for the products compared to the reactants.

On the energy level diagram, label:

• Activation energy: the peak height from the reactants' level to the top of the curve.
• Overall energy change: the difference in energy from the reactants' level to the products' level, indicating that the products are at a lower energy level than the reactants.