Short on time? This page is the recap. For explanations, a particle diagram and worked examples, use the full study guide.
Recap
- K has products on top and reactants on the bottom, so its size shows which side the equilibrium mixture favours.
- Very large K: the mixture at equilibrium is mostly products. The reaction goes essentially to completion.
- Very small K: the mixture is mostly reactants. The reaction barely proceeds.
- K near 1: both reactants and products are present in significant amounts.
- A rough guide (not a rule): above about 10³ is product-favoured, below about 10⁻³ is reactant-favoured.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ) | products over reactants, at equilibrium | decide which side is favoured |
| For A ⇌ B: [B] / [A] = K | volume cancels, so also number of B ÷ number of A | read a particle diagram |
| For A ⇌ B: fraction as B = K / (1 + K) | e.g. K = 10² gives 99.0 % B | turn K into a percentage |
| Small K: substitute the reactant concentration into the expression | reactant barely changes | estimate the trace of product |
Assumptions behind the reasoning
- The system has actually reached equilibrium. K says nothing about a mixture that is still changing.
- The temperature is fixed. K has one value per temperature.
- You are comparing K values for expressions of the same shape. For P ⇌ 2R, K = 1 does not mean equal concentrations.
Mistakes to avoid
- “Large K, so fast.” K is about position, not rate. A catalyst changes the rate, not K.
- “Small K, so no product.” A zero product concentration would make K = 0.
- “Complete” instead of “essentially complete”. A trace of reactant always remains.
- Changing K by adding a substance. Adding reactant changes Q; only temperature changes K.
- Forgetting the powers. Square the concentrations that have a coefficient of 2.
- Mixing Kc and Kp. Use whichever the question gives; do not convert.
Quick self-check
- For A ⇌ B, K = 1 × 10⁻⁵. What percentage of the material is B at equilibrium? (about 0.001 %)
- For X ⇌ Y, K = 4.0, and a box holds 25 particles at equilibrium. How many are Y? (20 Y and 5 X)
- A reaction has K = 10³⁰ but no change is seen after a week. Give one reason. (The rate is very slow, for example because of a high activation energy; K does not describe rate.)
Next: practice questions.