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5.1 Exothermic and endothermic reactions

5.1 Exothermic and endothermic reactions

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Question 6

Methane burns in oxygen to produce carbon dioxide and water according to the following chemical equation:

CH4+2O2⟶CO2+2H2O \text{CH}_4 + 2\text{O}_2 \longrightarrow \text{CO}_2 + 2 \text{H}_2\text{O} CH4​+2O2​⟶CO2​+2H2​O

The bonds broken and made during this reaction are as follows:

  • Reactants: one mole of CH4\text{CH}_4CH4​ contains 4 moles of C−H\text{C}-\text{H}C−H bonds; two moles of O2\text{O}_2O2​ contain 2 moles of O=O\text{O}=\text{O}O=O bonds.
  • Products: one mole of CO2\text{CO}_2CO2​ contains 2 moles of C=O\text{C}=\text{O}C=O bonds; two moles of H2O\text{H}_2\text{O}H2​O contain 4 moles of O−H\text{O}-\text{H}O−H bonds.

Table 1 shows the bond energies for these bonds.

Table 1

BondBond energy (kJ/mol)
C−H\text{C}-\text{H}C−H413
O=O\text{O}=\text{O}O=O495
C=O\text{C}=\text{O}C=O799
O−H\text{O}-\text{H}O−H463
a.

Calculate the overall energy change for the reaction using the bond energies in Table 1.

[3]
b.

Explain why this reaction is exothermic, using values from your calculation.

[2]
Markscheme

5.1 Exothermic and endothermic reactions Questions

  1. GCSE
  2. /Chemistry
  3. /5.1 Exothermic and endothermic reactions

13 exam-style questions on AQA GCSE Chemistry 5.1 Exothermic and endothermic reactions, covering 5.1.1 Energy transfer during exothermic and endothermic reactions, 5.1.2 Reaction profiles, and 5.1.3 The energy change of reactions (HT only). Each one has a worked solution and a mark scheme showing where the marks go.

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