Bond Energy Concepts
Explain energy changes in terms of bond breaking and bond making • Cambridge IGCSE Chemistry 0620
🔑 The Two Golden Rules — Memorise These
⬆️
Bond BREAKING
= ENDOTHERMIC
Energy IN
⬇️
Bond MAKING
= EXOTHERMIC
Energy OUT
Section A: The Golden Rules
Is bond breaking endothermic or exothermic? State the reason.
[1 mark]Is bond making endothermic or exothermic? State the reason.
[1 mark]A reaction is described as overall exothermic. What does this tell you about the energy of bonds broken versus bonds made?
[1 mark]A reaction is described as overall endothermic. What does this tell you about the energy changes?
[1 mark]Which statement correctly describes what happens during a chemical reaction in terms of bond energy?
[1 mark]Section B: The 3-Mark Explanation
🔥 SCENARIO: You are a forensic chemist analysing the explosion of a natural gas leak. Your report must explain the energy changes at the molecular level.
Explain, in terms of bond breaking and bond making, why the combustion of methane (CH₄) is exothermic. [3]
[3 marks]🚀 SCENARIO: A rocket scientist is explaining why burning hydrogen fuel releases so much energy.
Explain, in terms of bond breaking and bond making, why the reaction of hydrogen with oxygen to form water is exothermic. [3] 2H₂(g) + O₂(g) → 2H₂O(l)
[3 marks]🏭 SCENARIO: A chemical engineer is optimising the Haber Process for ammonia production.
The formation of ammonia from nitrogen and hydrogen is exothermic: N₂(g) + 3H₂(g) → 2NH₃(g) Explain, in terms of bond breaking and bond making, why this reaction is exothermic. [3]
[3 marks]🌿 SCENARIO: A biochemist is explaining cellular respiration to a medical student.
Explain, in terms of bond breaking and bond making, why the oxidation of glucose (C₆H₁₂O₆) during respiration releases energy. [3]
[3 marks]🕯️ SCENARIO: A candle is burning. A student says 'the wax is releasing energy stored in its bonds.' Critically assess this statement.
Explain, in terms of bond breaking and bond making, why the combustion of wax (a hydrocarbon) is exothermic. [3]
[3 marks]Section C: Reverse Logic — Endothermic Reactions
🏔️ SCENARIO: Limestone (calcium carbonate) is heated in a kiln to make quicklime (calcium oxide). The kiln must run continuously — the moment you stop heating, the reaction stops.
Explain, in terms of bond breaking and bond making, why the thermal decomposition of calcium carbonate is endothermic. [3] CaCO₃(s) → CaO(s) + CO₂(g)
[3 marks]🥶 SCENARIO: Mixing citric acid and sodium hydrogen carbonate creates a cold pack used by sports teams. The mixture feels cold to the touch.
A student mixes two chemicals and the mixture feels cold. Explain, in terms of bond breaking and bond making, why this reaction is endothermic. [3]
[3 marks]🌡️ SCENARIO: Dissolving ammonium nitrate in water is used in instant cold packs for medical emergencies.
Dissolving ammonium nitrate in water is endothermic. Explain this observation in terms of bond breaking and bond making. [3]
[3 marks]☀️ SCENARIO: Plants use sunlight to power photosynthesis. Without a continuous energy input, the reaction cannot proceed.
Explain, in terms of bond breaking and bond making, why photosynthesis is endothermic. [3] 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
[3 marks]⚡ SCENARIO: Electrolysis of water splits water molecules into hydrogen and oxygen gases. Electricity must be continuously supplied.
The electrolysis of water requires a continuous supply of electrical energy. Explain, in terms of bond breaking and bond making, why this process is endothermic. [3] 2H₂O(l) → 2H₂(g) + O₂(g)
[3 marks]🎯 Examiner's Summary: The Bond Energy Checklist
Bond breaking = endothermic (energy required)
Bond making = exothermic (energy released)
Exothermic reaction: energy released > energy required
Endothermic reaction: energy required > energy released
Always state BOTH bond breaking AND bond making for full marks
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