Ammonia is made by reacting nitrogen with hydrogen.
N2(g)+3H2(g)⇌2NH3(g)\mathrm{N_{2}(g)} + \mathrm{3H_{2}(g)} \rightleftharpoons \mathrm{2NH_{3}(g)}N2(g)+3H2(g)⇌2NH3(g)
Each nitrogen molecule contains one N≡N bond. Each hydrogen molecule contains one H–H bond. Each ammonia molecule contains three N–H bonds.
State whether breaking bonds is exothermic or endothermic.
State the number of N–H bonds made when two molecules of ammonia form.
The energy needed to break all the bonds in the reactants is 2253 kJ. The energy released when all the bonds in the products are made is 2346 kJ.
Explain, using this information, whether the reaction is exothermic or endothermic.
The thermal decomposition of calcium carbonate is endothermic.
CaCO3(s)→CaO(s)+CO2(g)\mathrm{CaCO_{3}(s)} \rightarrow \mathrm{CaO(s)} + \mathrm{CO_{2}(g)}CaCO3(s)→CaO(s)+CO2(g)
Explain, in terms of bond breaking and bond making, why this reaction is endothermic.
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.