Equilibrium Cheatsheet
Cheat Codes & Shortcuts
- Chemical Equilibrium: State where forward and reverse reaction rates are equal.
- Dynamic Nature: Both reactions continue at equal rates; no net change.
- Equilibrium Constant (\(K_c\)): \(K_c = \frac{[Products]}{[Reactants]}\) at equilibrium.
- Le Chatelier’s Principle: System shifts to oppose changes in concentration, pressure, temperature.
- Reaction Quotient (\(Q\)): Same form as \(K\), but for non-equilibrium.
- Relationship: \(Q < K\) forward favoured; \(Q > K\) reverse favoured.
- Effect of Temperature: Endothermic reaction: increase shifts right, exothermic: shifts left.
- Effect of Pressure: Increasing pressure favours side with fewer moles gas.
- Homogeneous Equilibrium: Equilibrium species in same phase.
- Heterogeneous Equilibrium: Different phases; solids/liquids omitted in \(K_c\) expression.
Quick Reference Table
| Term | Definition / Formula | Example / Note |
|---|---|---|
| Equilibrium Constant, \(K_c\) | \(K_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}\) for \(\mathrm{aA + bB \rightleftharpoons cC + dD}\) | Only gases and aqueous species included |
| Reaction Quotient, \(Q\) | Calculated same as \(K_c\), but at any point | Compare with \(K_c\) to predict direction |
| Le Chatelier's Principle | System shifts to counteract disturbance | Add reactant → shift right |
| Effect of Pressure | Increased pressure favors side with fewer moles gas | \( \mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3} \) - Pressure increase shifts right |
| Effect of Temperature | Endothermic favor forward on heating; exothermic favor reverse | Heat added shifts equilibrium |
| Heterogeneous Equilibrium | Phases differ; solids/liquids omit from \(K_c\) | \( \mathrm{CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)} \) |
Advice
Step 1: Write balanced chemical equation and expression for \(K_c\).
Step 2: Calculate or use given initial concentrations to find \(Q\).
Step 3: Compare \(Q\) and \(K_c\) to predict reaction direction.
Step 4: Apply Le Chatelier’s principle for changes in concentration, pressure, temperature.
Step 5: For heterogeneous equilibria, omit pure solids/liquids in expressions.
Equilibrium Quick Tips
- Calculate \(K_c\): Concentrations raised to stoichiometric powers.
- Reaction Quotient \(Q\): Use initial concentrations to decide direction.
- Temperature effects: Consider endothermic/exothermic nature carefully.
- Pressure effects: Count moles of gases on each side.
- Solid and liquid phases: Not included in equilibrium expressions.
Equilibrium Speed Quiz
Test your speed with 5 equilibrium questions! You have 30 seconds per question.
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