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Measuring Enthalpy Change · Paper 2 — Free Chemistry HL Practice Questions

1ChallengeSAQ-LExothermic vs endothermic reactions9 marksPaper 2~14 min

Data

| Bond enthalpy /

\text{kJ mol}^{-1}

\text{N} \equiv \text{N}

—

945

\text{H} - \text{H}

—

436

\text{N} - \text{H}

—

391

The Haber process produces ammonia:

\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}

. The forward reaction is exothermic. Bond enthalpy

(a)

State what is meant by the term average bond enthalpy. [1 mark]

(b)

Determine the enthalpy change,

\Delta H

, for the forward reaction using the bond enthalpy data. State whether the reaction is exothermic or endothermic. [3 marks]

(c)

Explain why the value of

\Delta H

calculated using bond enthalpies differs from the value measured experimentally. [2 marks]

(d)

The Haber process is operated at approximately

450\,^\circ\text{C}

and

200\,\text{atm}

, using an iron catalyst. Evaluate the choice of

450\,^\circ\text{C}

, referring to reaction rate, equilibrium yield, and the sign of

\Delta H

. [3 marks]

Solutions

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