Exothermic and endothermic reactions

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Question 5
Medium

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]

Exothermic and endothermic reactions Questions

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