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Chemistry  ·  AFCAT

CA07 — Thermodynamics, Equilibrium & Kinetics

✈ Chemistry – CA07  ·  AFCAT General Awareness AFCAT Level

This chapter covers the energy of chemical reactions and how they reach equilibrium. For AFCAT, the focus is on recognising exothermic vs endothermic reactions with real examples, and understanding Le Chatelier's principle qualitatively — no complex calculations needed.

📌 AFCAT Focus: Exothermic = releases heat (combustion, neutralisation, rusting); endothermic = absorbs heat (photosynthesis, dissolving ammonium chloride); Le Chatelier's principle (pressure ↑ → reaction shifts to fewer moles of gas; temp ↑ → shifts toward endothermic direction); catalyst speeds up reaction without being consumed.

1. Exothermic vs Endothermic Reactions

Fig. 1 — Exothermic and Endothermic: Energy Flow and Common Examples
EXOTHERMIC vs ENDOTHERMIC REACTIONS EXOTHERMIC Releases heat to surroundings Reactants Products Energy ↓ Heat released; ΔH < 0 Surroundings warm up. Examples: ● Combustion (burning coal, wood) ● Neutralisation (acid + base) ● Respiration (glucose + O₂) ● Rusting of iron ● NaOH dissolved in water ENDOTHERMIC Absorbs heat from surroundings Reactants Products Energy ↑ Heat absorbed; ΔH > 0 Surroundings cool down. Examples: ● Photosynthesis (CO₂+H₂O→glucose) ● Dissolving NH₄Cl in water (cooling) ● Thermal decomposition of CaCO₃ ● Evaporation of water ● Electrolysis of water

2. Chemical Equilibrium & Le Chatelier's Principle

Le Chatelier's Principle (AFCAT Level):

When a system at equilibrium is disturbed, it responds to partially counteract the disturbance and re-establish equilibrium.

Effect of changes on equilibrium (A + B ⇌ C + D):
● Increase concentration of A or B: Equilibrium shifts forward (toward products) → more C and D formed
● Increase pressure: Equilibrium shifts toward the side with fewer moles of gas
● Increase temperature: Equilibrium shifts toward the endothermic direction (absorbs excess heat)
● Catalyst: Speeds up both forward and reverse reactions equally; does NOT shift equilibrium

3. Rate of Reaction & Catalysts

▶ Factors Affecting Reaction Rate

  • Nature of reactants: Ionic reactions (NaCl + AgNO₃) are instantaneous; covalent bond-breaking reactions are slower
  • Concentration: More molecules per unit volume → more collisions → faster reaction
  • Temperature: Every 10°C rise roughly doubles the reaction rate (approx)
  • Surface area: Powder reacts faster than a lump (more surface exposed)
  • Catalyst: Provides alternate reaction pathway with lower activation energy

▶ Catalysts — Key Facts

  • Catalyst speeds up a reaction without being consumed
  • Lowers activation energy (energy needed to start reaction)
  • Positive catalyst: increases rate (Fe in Haber process; MnO₂ in O₂ from H₂O₂)
  • Negative catalyst (inhibitor): slows reaction
  • Enzyme: biological catalyst (e.g., amylase, zymase)
  • Zymase (in yeast): glucose → ethanol + CO₂

📝 AFCAT PYQs — Thermodynamics & Equilibrium

Q1. Photosynthesis is an example of: AFCAT PYQ
(a) Exothermic reaction(b) Endothermic reaction(c) Neutralisation reaction(d) Displacement reaction
✔ Answer: (b) Endothermic reaction
Photosynthesis: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. It absorbs energy (sunlight) to build glucose from CO₂ and H₂O — making it endothermic. Respiration (reverse process) releases that energy — it is exothermic. This pairing of photosynthesis (endo) and respiration (exo) is a direct AFCAT question.
Q2. Which of the following is correct about a catalyst? AFCAT PYQ
(a) It is consumed in the reaction(b) It shifts the equilibrium position(c) It provides an alternative path with lower activation energy(d) It only works at high temperature
✔ Answer: (c) Lower activation energy
A catalyst speeds up a reaction by providing an alternative reaction pathway with lower activation energy. It is NOT consumed (regenerated at the end), does NOT shift equilibrium position (speeds both forward and reverse equally), and does NOT appear in the overall equation. Example: MnO₂ in H₂O₂ → H₂O + O₂; MnO₂ is recovered unchanged.
Q3. The neutralisation reaction (acid + base) is: AFCAT PYQ
(a) Endothermic(b) Exothermic(c) Neither(d) Thermally neutral
✔ Answer: (b) Exothermic
Neutralisation (HCl + NaOH → NaCl + H₂O) is exothermic — heat is released. You can feel the flask warm up. The heat released is called the enthalpy of neutralisation (about 57 kJ/mol for strong acid + strong base). This is a direct AFCAT question — neutralisation always releases heat (exothermic, ΔH < 0).

🧠 Quick Memory Chart — CA07

🔥 Exothermic
  • Releases heat; ΔH < 0
  • Surroundings warm up
  • Combustion, neutralisation
  • Respiration, rusting
  • NaOH in water (heats up)
⛴ Endothermic
  • Absorbs heat; ΔH > 0
  • Surroundings cool down
  • Photosynthesis (needs light)
  • NH₄Cl in water (cools down)
  • Electrolysis, evaporation
⚛ Catalyst & Rate
  • Catalyst: lowers activation energy
  • NOT consumed; NOT in equation
  • Conc ↑, Temp ↑, Surface area ↑ = rate ↑
  • Le Chatelier: system opposes disturbance
  • Enzyme = biological catalyst

📝 Practice Exercise

E1. Which of the following is exothermic?
(a) Evaporation of water(b) Photosynthesis(c) Burning of coal(d) Dissolving NH₄Cl in water
E2. Increasing the temperature of a reaction at equilibrium shifts the equilibrium in the:
(a) Exothermic direction always(b) Endothermic direction(c) No effect(d) Forward direction always
E3. MnO₂ speeds up the decomposition of H₂O₂ but is not consumed. MnO₂ is acting as:
(a) Reactant(b) Product(c) Catalyst(d) Solvent
Answers:
E1 → (c) Burning of coal [combustion; releases heat; ΔH < 0]  |  E2 → (b) Endothermic direction [Le Chatelier: system absorbs excess heat by favouring the endothermic direction]  |  E3 → (c) Catalyst [speeds reaction; not consumed; provides lower energy pathway]
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