Margarine is made by reacting vegetable oils with hydrogen. The hydrogen adds across the C=C bonds in the oil molecules. A nickel catalyst is used and the mixture is heated to about 150 °C.
A chemist uses the reaction of ethene with hydrogen as a simple model of this process.
C2H4(g)+H2(g)→C2H6(g)\mathrm{C_{2}H_{4}(g)} + \mathrm{H_{2}(g)} \rightarrow \mathrm{C_{2}H_{6}(g)}C2H4(g)+H2(g)→C2H6(g)
The energy change for this reaction is −125kJ mol−1-125 \mathrm{kJ\,mol^{-1}}−125kJmol−1.
The table shows some bond energies.
| Bond | Bond energy / kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1 |
|---|---|
| C–H | 413 |
| H–H | 436 |
| C–C | 347 |
| C–Br | 290 |
| Br–Br | 193 |
State why nickel is not included in the equation.
Calculate the bond energy of the C=C\mathrm{C{=}C}C=C bond.
Explain why the mixture is heated to 150 °C even though the reaction is exothermic.
Ethene also reacts with bromine.
C2H4(g)+Br2(g)→C2H4Br2(g)\mathrm{C_{2}H_{4}(g)} + \mathrm{Br_{2}(g)} \rightarrow \mathrm{C_{2}H_{4}Br_{2}(g)}C2H4(g)+Br2(g)→C2H4Br2(g)
Calculate the energy change for this reaction. Use your answer to (b) and data from the table.
53 exam-style questions on Edexcel GCSE Chemistry 8.2 Heat energy changes in chemical reactions, covering 8.2.1 Exothermic and endothermic changes, 8.2.2 Bond breaking, bond making and overall energy change, 8.2.3 Calculating energy change from bond energies, and 8.2.4 Activation energy and reaction profiles. Each one has a worked solution and a mark scheme showing where the marks go.