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

To calculate the heat energy (Q) released, we can use the equation:

where:

• m = mass of water = 50 g
• c = specific heat capacity = 4.2 J/g/°C
• ΔT = temperature change = 38.4 °C - 22.0 °C = 16.4 °C

Substituting the values into the equation:

To calculate the overall energy change for the combustion of ethanol, we first identify the bonds broken and formed:

• Bonds broken:

  • 5 C—H bonds
  • 3 O=O bonds

• Bonds formed:

  • 4 C=O bonds
  • 6 O—H bonds

Using the bond energies provided:

• C—H: 413 kJ/mol
• O=O: 497 kJ/mol
• C=O: 799 kJ/mol
• O—H: 467 kJ/mol

Calculating the total energy:

Bonds broken:

• Total energy for C—H: $5 imes 413 = 2065 ext{ kJ}$
• Total energy for O=O: $3 imes 497 = 1491 ext{ kJ}$
• Total broken energy: $2065 + 1491 = 3556 ext{ kJ}$

Bonds formed:

• Total energy for C=O: $4 imes 799 = 3196 ext{ kJ}$
• Total energy for O—H: $6 imes 467 = 2802 ext{ kJ}$
• Total formed energy: $3196 + 2802 = 5998 ext{ kJ}$

Overall energy change:

• Overall energy change = Energy required to break bonds - Energy released from formed bonds
• Overall energy change: $3556 - 5998 = -1442 ext{ kJ}$

The overall energy change is -1442 kJ.

The reaction is exothermic because the energy released when bonds form is greater than the energy used when bonds are broken. In this reaction, the bonds formed in the products (CO₂ and H₂O) release more energy than the bonds broken in the reactants (C₂H₅OH and O₂). Therefore, the net energy change is negative, indicating that the reaction releases energy to the surroundings.

On the energy level diagram for the combustion of ethanol:

• The activation energy is represented by the energy barrier to the peak.
• The overall energy change is represented by the difference in energy between the reactants (C₂H₅OH + 3O₂) and the products (2CO₂ + 3H₂O). The products should be at a lower energy level than the reactants, indicating an exothermic reaction.