Topic 5
2 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Exothermic reactions release energy to the surroundings (temperature rises). Endothermic reactions absorb energy from the surroundings (temperature falls).
Explanation
EXOTHERMIC reactions: - Energy is released to the surroundings. - Temperature of surroundings INCREASES. - Products have less energy than reactants. - deltaH is NEGATIVE. - Examples: combustion, neutralisation, respiration, rusting, hand warmers. ENDOTHERMIC reactions: - Energy is absorbed from the surroundings. - Temperature of surroundings DECREASES. - Products have more energy than reactants. - deltaH is POSITIVE. - Examples: photosynthesis, thermal decomposition, dissolving ammonium nitrate, sports cold packs.
Practice Questions
Which of the following is an endothermic process?
Key Idea
Breaking bonds requires energy (endothermic). Making bonds releases energy (exothermic). deltaH = energy in (bonds broken) minus energy out (bonds made).
Explanation
Bond energy is the energy needed to break 1 mole of a particular covalent bond in the gaseous state. Formula: deltaH = sum of bond energies broken - sum of bond energies made If deltaH is negative: more energy released making bonds than breaking, so exothermic. If deltaH is positive: more energy needed to break bonds than released, so endothermic.
Worked Examples
Practice Questions
Calculate deltaH for: N2(g) + 3H2(g) to 2NH3(g). Bond energies: N-N triple bond = 945 kJ/mol, H-H = 436 kJ/mol, N-H = 391 kJ/mol. [3 marks]