Short on time? This page is the recap. For the proofs, a step-by-step diagram and worked examples, use the full study guide.
Recap
- A value of K belongs to one equation written one way. Rewrite the equation and K changes.
- Coefficients become powers in the expression; that is why the rules work the way they do.
- To find K for an overall multistep process, add the steps and multiply their K values. Intermediates cancel.
- Q has the same mathematical form as K, so every rule also applies to Q.
Key relationships
| Change to the equation | New K | Example (K = 100) |
|---|---|---|
| Reverse | 1/K | 0.010 |
| Multiply all coefficients by 2 | K² | 1.0 × 10⁴ |
| Multiply all coefficients by ½ | √K | 10 |
| Multiply by c (general) | Kᶜ | 100ᶜ |
| Add equation 1 and equation 2 | K₁ × K₂ | 100 × K₂ |
| Reverse and multiply by c | K⁻ᶜ = 1/Kᶜ | for c = ½: 0.10 |
Parallel with Hess’s law (Topic 6.9): ΔH changes sign, is multiplied by c, and adds. K inverts, is raised to the power c, and multiplies.
Assumptions behind the rules
- Every K is at the same temperature.
- Every K is the same type (all Kc or all Kp). Do not convert between them.
- Pure solids and liquids are left out of every expression, so they never affect the result.
Mistakes to avoid
- Doubling K when the equation is doubled. Square it.
- Adding K values when equations are added. Multiply them.
- Negative K after reversing. Take the reciprocal; K is always positive.
- Halving K when the equation is halved. Take the square root.
- Forgetting the power on a step you multiplied before adding.
- Leaving an intermediate uncancelled. Check that the summed equation matches the target exactly.
Quick self-check
An equation has K = 2.0 × 10⁻³.
- What is K for the reverse equation? (5.0 × 10²)
- What is K if all coefficients are tripled? (8.0 × 10⁻⁹)
- What is K if all coefficients are halved? (0.045)
- Two steps with K = 4.0 and K = 0.50 add to give an overall equation. What is the overall K? (2.0)
Next: practice questions.