Hydrogen reacts with chlorine to make hydrogen chloride.
H2+Cl2→2HCl \mathrm{H_{2} + Cl_{2} \rightarrow 2HCl} H2+Cl2→2HClThe table shows some bond energies.
| Bond | Bond energy (kJ/mol) |
|---|---|
| H–H | 436 |
| Cl–Cl | 243 |
| H–Cl | 432 |
| Br–Br | 193 |
| H–Br | 366 |
Explain why breaking bonds is an endothermic process.
Calculate the energy change for the reaction of hydrogen with chlorine.
Use your answer to (b) to explain whether the reaction is exothermic or endothermic.
Hydrogen also reacts with bromine.
H2+Br2→2HBr \mathrm{H_{2} + Br_{2} \rightarrow 2HBr} H2+Br2→2HBrThe energy change for this reaction is −103 kJ/mol.
Explain, using the bond energies, why less energy is given out in this reaction than in the reaction of hydrogen with chlorine.
51 exam-style questions on OCR GCSE Chemistry 3.2 Energetics, covering 3.2.1 Exothermic and endothermic reactions and reaction profiles and 3.2.2 Calculating energy change from bond energies. Each one has a worked solution and a mark scheme showing where the marks go.