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5.1 Exothermic and endothermic reactions

5.1 Exothermic and endothermic reactions

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Question 3

Hydrazine (N2H4\text{N}_2\text{H}_4N2​H4​) is a highly reactive liquid used as a rocket propellant. It undergoes combustion with oxygen (O2\text{O}_2O2​) to produce nitrogen gas (N2\text{N}_2N2​) and water vapor (H2O\text{H}_2\text{O}H2​O) according to the following chemical equation:

N2H4+O2⟶N2+2H2O \text{N}_2\text{H}_4 + \text{O}_2 \longrightarrow \text{N}_2 + 2\text{H}_2\text{O} N2​H4​+O2​⟶N2​+2H2​O

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

  • Reactants: one mole of N2H4\text{N}_2\text{H}_4N2​H4​ contains 4 moles of N−H\text{N}-\text{H}N−H bonds and 1 mole of N−N\text{N}-\text{N}N−N bonds; one mole of O2\text{O}_2O2​ contains 1 mole of O=O\text{O}=\text{O}O=O bonds.
  • Products: one mole of N2\text{N}_2N2​ contains 1 mole of N≡N\text{N}\equiv\text{N}N≡N 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)
N−H\text{N}-\text{H}N−H391
N−N\text{N}-\text{N}N−N163
O=O\text{O}=\text{O}O=O495
N≡N\text{N}\equiv\text{N}N≡N945
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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