Physical Chemistry (Section 3) covers energy changes in reactions, factors affecting rates of reaction, reversible reactions and equilibrium, and electrolysis — key concepts linking thermodynamics and kinetics.
Exothermic: releases energy (ΔH negative) — combustion, neutralisation. Products are lower energy than reactants. Endothermic: absorbs energy (ΔH positive) — thermal decomposition, photosynthesis. Products are higher energy. Energy profile diagrams show activation energy (minimum energy for reaction). Bond breaking = endothermic; bond making = exothermic. Overall: if energy to break bonds > energy released making bonds → endothermic. Rate of reaction = change in amount / time. Factors: concentration (more collisions), temperature (faster movement + more energy), surface area (more exposed), catalyst (lower activation energy). Collision theory: particles must collide with sufficient energy and correct orientation.
Reversible reactions: proceed in both directions (⇌). Dynamic equilibrium: forward rate = reverse rate, concentrations constant. Le Chatelier\'s principle: system opposes change. Increase temperature → favours endothermic direction. Increase pressure → favours fewer moles of gas. Increase concentration → favours reaction that removes it. Catalyst: speeds up both directions equally, doesn\'t change position. Electrolysis: splitting ionic compounds using electricity. Cathode (−) attracts cations (metals/hydrogen deposited). Anode (+) attracts anions (non-metals released). Molten NaCl: Na at cathode, Cl₂ at anode. Brine: H₂ at cathode, Cl₂ at anode, NaOH solution remains. Electroplating: object is cathode, plating metal is anode.
The Haber process: N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol, exothermic). By Le Chatelier\'s: (1) Pressure: 4 moles of gas on left → 2 moles on right; high pressure favours the right (fewer moles) → more NH₃. Uses ~200 atm. (2) Temperature: forward reaction is exothermic; low temperature favours forward → more NH₃. However, low temperature = slow rate. Compromise: ~450°C (gives reasonable rate and yield ≈ 15%). (3) Iron catalyst: lowers activation energy, speeds up reaching equilibrium but doesn\'t change the position. Unreacted N₂ and H₂ are recycled. The conditions are an industrial compromise between yield and rate.
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