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

We can calculate the heat energy using the following values:

• Mass of water (m) = 50 g
• Specific heat capacity (c) = 4.2 J/g/°C
• Temperature change (ΔT) = 38.4 °C - 22.0 °C = 16.4 °C

Using the formula:

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

Substituting the values:

$Q = 50 g \times 4.2 \frac{J}{g/°C} \times 16.4 °C = 3444 J$

To find the overall energy change, we first need to calculate the total bond energy of bonds broken and formed. For ethanol combustion:

Bonds broken:

• 5 C–H bonds = 5 x 413 kJ/mol = 2065 kJ/mol
• 3 O=O bonds = 3 x 498 kJ/mol = 1494 kJ/mol Total energy of bonds broken = 2065 + 1494 = 3560 kJ/mol

Bonds formed:

• 4 C=O bonds = 4 x 799 kJ/mol = 3196 kJ/mol
• 6 O–H bonds = 6 x 467 kJ/mol = 2802 kJ/mol Total energy of bonds formed = 3196 + 2802 = 5998 kJ/mol

Overall energy change:

Overall energy change = Energy of bonds broken - Energy of bonds formed = 3560 - 5998 = -2438 kJ/mol.

The reaction is exothermic because the energy released when bonds form is greater than the energy required to break bonds. This means that the overall energy of the system decreases as the reaction proceeds.

On the energy level diagram, label the following:

• Activation energy: The energy required to initiate the reaction, which is typically the difference in energy from the reactants to the peak of the energy hill.
• Overall energy change: This will be represented as a downward slope from the energy level of reactants to the energy level of products, indicating that energy is released during the reaction.