Chemical Equilibrium
In a reversible reaction, equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction — concentrations remain constant but not equal.
A. The Equilibrium Concept
What is Chemical Equilibrium?
- Occurs in reversible reactions (⇌) — reactions that can proceed in both directions
- At equilibrium: rate of forward reaction = rate of reverse reaction
- Concentrations of reactants and products remain constant (but not necessarily equal)
- Equilibrium is a dynamic state — both reactions continue, but at equal rates
- The position of equilibrium describes whether products or reactants are favoured
Reversible Reaction Notation
aA + bB ⇌ cC + dD
⇌ symbol indicates the reaction is reversible and can reach equilibrium
B. Le Chatelier's Principle
Stress & Equilibrium Shift
Statement: If a stress is applied to a system at equilibrium, the system will shift in the direction that opposes (relieves) the stress.
Increase [Reactants]
→ Shifts FORWARD
More reactant → system makes more product to reduce reactant concentration
Increase [Products]
← Shifts BACKWARD
More product → system converts product back to reactants
Increase Temperature
→ Shifts ENDOTHERMIC direction
System absorbs the extra heat energy — shifts toward the heat-absorbing side
Decrease Temperature
→ Shifts EXOTHERMIC direction
System produces heat to compensate — shifts toward heat-releasing side
Increase Pressure (gas)
→ Side with FEWER gas moles
Reducing total moles of gas reduces pressure — relieves the stress
Decrease Pressure (gas)
→ Side with MORE gas moles
More gas moles increases pressure to compensate
Add a Catalyst
NO shift in equilibrium
Catalyst speeds up BOTH forward and reverse reactions equally — reaches equilibrium faster but does NOT change position
⚡ MCQ Tip A catalyst does NOT shift equilibrium — it only speeds up reaching it. Temperature change is the only stress that changes the value of Kc. Pressure only affects reactions with gas-phase reactants/products.
C. Equilibrium Constant (Kc)
Kc Expression & Interpretation
Kc Expression
Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
For: aA + bB ⇌ cC + dD · [ ] = molar concentration at equilibrium · Only gases and aqueous species included
| Kc Value | Meaning | Equilibrium position |
|---|---|---|
| Kc > 1 | Products favoured — more products than reactants at equilibrium | Lies to the right (→) |
| Kc < 1 | Reactants favoured — more reactants than products at equilibrium | Lies to the left (←) |
| Kc = 1 | Roughly equal amounts of reactants and products | Lies in the middle |
| Very large Kc | Reaction goes essentially to completion | Far to the right |
| Very small Kc | Barely any product forms | Far to the left |
⚡ MCQ Tip Kc > 1 = more products. Kc < 1 = more reactants. Temperature change DOES change Kc. Adding catalyst or changing pressure/concentration does NOT change Kc — only changes the speed of reaching equilibrium.
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Equilibrium is reached when | Rate of forward = rate of reverse reaction |
| Le Chatelier's Principle | System shifts to oppose the applied stress |
| Increase temperature → shifts toward | Endothermic direction |
| Increase pressure → shifts toward | Side with fewer moles of gas |
| Adding a catalyst | Does NOT shift equilibrium — only speeds it up |
| Kc > 1 | Products favoured (equilibrium right) |
| Kc < 1 | Reactants favoured (equilibrium left) |
| Only stress that changes Kc | Temperature change |