Baking powder contains sodium hydrogencarbonate. When baking powder is added to cake mixture and placed in a hot oven, the sodium hydrogencarbonate decomposes to release carbon dioxide gas, causing the cake to rise. A student investigates this reaction by heating sodium hydrogencarbonate in a test tube and recording the temperature of the surroundings. She notices the temperature of the surroundings decreases during the reaction. Explain why the temperature of the surroundings decreases, and describe what this tells us about the bond breaking and bond making processes occurring during this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium hydrogencarbonate decomposes on heating to produce sodium carbonate, water and carbon dioxide.
Model answer (5 marks)
The decomposition of sodium hydrogencarbonate is an endothermic reaction. Energy is absorbed from the surroundings, so the temperature of the surroundings falls.
During the reaction, the energy required to break the bonds in the reactants (NaHCO₃) is greater than the energy released when the bonds in the products (Na₂CO₃, H₂O, CO₂) are formed. Thus, more energy is taken in to break bonds than is given back by making bonds, making the overall process endothermic.
Bond breaking is an endothermic process – it requires an input of energy. Bond making is exothermic – it releases energy. In this reaction the endothermic bond‑breaking step dominates, so the net effect is a loss of heat from the surroundings.
During the reaction, the energy required to break the bonds in the reactants (NaHCO₃) is greater than the energy released when the bonds in the products (Na₂CO₃, H₂O, CO₂) are formed. Thus, more energy is taken in to break bonds than is given back by making bonds, making the overall process endothermic.
Bond breaking is an endothermic process – it requires an input of energy. Bond making is exothermic – it releases energy. In this reaction the endothermic bond‑breaking step dominates, so the net effect is a loss of heat from the surroundings.
Examiner tips
- Use the word ‘endothermic’ to link the temperature drop to energy absorption.
- Show that bond breaking needs more energy than bond making releases.
- Mention that the overall ΔH is positive.
- Keep the explanation concise and use the exact terminology from the mark scheme.
Common mistakes
- Saying the reaction is exothermic or that it releases heat into the surroundings.
- Confusing the temperature change of the test tube with that of the surroundings.
- Failing to explain the relative energies of bond breaking and making.
Mark scheme (5 marks)
- The reaction is endothermic
- Energy is taken in from the surroundings (causing the temperature of the surroundings to decrease)
- More energy is required to break bonds in the reactants than is released when bonds are made in the products
- Bond breaking is an endothermic process (requires energy input)
- Bond making is an exothermic process (releases energy)
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Physical chemistry questions
- Hydrogen gas can be produced by reacting methane with steam at high temperatures…
- Cold packs used in sports medicine contain ammonium nitrate and water in separat…
- A student dissolves citric acid in water and measures the pH. She then dissolves…
- Hand warmers contain iron powder. When the pack is opened and exposed to air, th…