A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Theory and principles
CSCA Chemistry · Lesson 15 of 21Chemical equilibrium
Équilibre chimique · التوازن الكيميائي
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Many reactions go both ways. At equilibrium the forward and reverse rates are equal and the concentrations stop changing.
Key ideas
- Equilibrium is dynamic: v(forward) = v(reverse) ≠ 0.
- For aA + bB ⇌ cC + dD: K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ).
- Le Chatelier: a system shifts to oppose a change in concentration, pressure or temperature.
- A catalyst does not shift the equilibrium; it is only reached faster.
Formulas to know
- K = [products] / [reactants], powers = coefficients
- Pressure up → shift to the side with fewer gas moles
- Exothermic reaction: heating shifts it backwards
Worked example
H₂ + I₂ ⇌ 2HI. At equilibrium [H₂] = [I₂] = 0.1 mol/L and [HI] = 0.8 mol/L. Find K.
- Write: K = [HI]² / ([H₂][I₂])
- Replace: K = 0.8² / (0.1 × 0.1)
- Compute: K = 0.64 / 0.01 = 64
Answer: K = 64
Practice questions
1. N₂ + 3H₂ ⇌ 2NH₃: raising the pressure shifts the equilibrium:
to the left · to the right ✓ · nowhere · it stops the reaction
4 gas moles → 2 gas moles.
2. Adding a catalyst to a mixture at equilibrium:
increases K · shifts it right · does not shift it ✓ · shifts it left
It speeds both directions equally.
3. 2NO₂ (brown) ⇌ N₂O₄ (colourless) is exothermic. Cooling makes the mixture:
darker · paler ✓ · unchanged · green
Cooling favours the exothermic direction: more N₂O₄.