A student reads about two reactions of gases.
Observation 1: A mixture of hydrogen and oxygen can be kept at room temperature for years without reacting. When a spark is used, the mixture explodes and gives out a lot of energy.
2H₂ + O₂ → 2H₂O energy change = −490 kJ/mol
Observation 2: Nitrogen and oxygen are both in the air but do not react at room temperature. They only react to make nitrogen monoxide at very high temperatures, such as in lightning or inside a car engine. The reaction stops when the gases cool down.
N2+O2→2NO \mathrm{N_{2} + O_{2} \rightarrow 2NO} N2+O2→2NOEach nitrogen molecule has one N≡N bond. Each oxygen molecule has one O=O bond. Each nitrogen monoxide molecule has one N=O bond.
The table shows some bond energies.
| Bond | Bond energy (kJ/mol) |
|---|---|
| H–H | 436 |
| O=O | 498 |
| O–H | 465 |
| N≡N | 945 |
| N=O | 631 |
Calculate the energy change for the reaction of nitrogen with oxygen.
Explain both observations.
Use the bond energies, your answer to (a) and ideas about activation energy and reaction profiles.
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.