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

To calculate Q, we first determine ( \Delta T ):

( \Delta T = 38.4 °C - 22.0 °C = 16.4 °C )

Now, using the equation: $Q = m \times c \times \Delta T$

Where:

• ( m = 0.8 , g )
• ( c = 4.2 , J/g/°C )
• ( \Delta T = 16.4 , °C )

Substituting the values: $Q = 0.8 \, g \times 4.2 \, J/g/°C \times 16.4 \, °C = 3444.8 \, J$

Thus, the heat energy released is approximately 3444 J.

To calculate the overall energy change, we need to find the total bond energies of the bonds broken and the bonds formed.

Bonds broken:

• 5 C-H bonds: ( 5 \times 413 , kJ/mol = 2065 , kJ/mol )
• 3 O=O bonds: ( 3 \times 498 , kJ/mol = 1494 , kJ/mol )

Total energy of bonds broken: ( 2065 + 1494 = 3559 , kJ/mol )

Bonds formed:

• 4 C=O bonds: ( 4 \times 799 , kJ/mol = 3196 , kJ/mol )
• 6 H-O bonds: ( 6 \times 467 , kJ/mol = 2802 , kJ/mol )

Total energy of bonds formed: ( 3196 + 2802 = 5998 , kJ/mol )

Overall energy change: ( \Delta H = \text{Energy of bonds broken} - \text{Energy of bonds formed} = 3559 - 5998 = -1439 , kJ/mol )

Therefore, the overall energy change is -1276 kJ/mol.

The reaction is exothermic because the energy released when bonds form is greater than the energy used when bonds break. Thus, more energy is released during the formation of products than is consumed during the breaking of reactant bonds.

On the energy level diagram, the activation energy is represented as the energy barrier that reactants need to overcome to convert into products. The overall energy change is shown as the difference in energy between reactants and products, with products lying lower than the reactants.