Short on time? This page is the recap. For explanations, the graph and worked examples, use the full study guide.
Recap
- Kinetics is about how fast reactions go. The rate is the change in concentration (or amount) of a species per unit time.
- Reactants disappear and products form. Rates are always reported as positive numbers.
- The coefficients in the balanced equation fix the ratio of the rates of the species.
- Rates usually fall as the reaction goes on, because reactant concentrations fall.
- Average rate = slope of the line joining two points on a concentration–time graph. Instantaneous rate = slope of the tangent at one time. Initial rate = instantaneous rate at t = 0.
- A rate is measured by following something that changes: gas volume, mass, colour (absorbance), pressure, conductivity or pH.
- Faster: higher concentration (or gas pressure), higher temperature, larger surface area of a solid, a catalyst.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| rate of disappearance = −Δ[A]/Δt | A is a reactant; Δ[A] is negative | report a reactant’s rate as a positive number |
| rate of formation = +Δ[C]/Δt | C is a product | report a product’s rate |
| For a A + b B → c C: rate of reaction = −(1/a)Δ[A]/Δt = −(1/b)Δ[B]/Δt = (1/c)Δ[C]/Δt | one value for the whole reaction | convert the rate of one species into another |
| moles per second ÷ volume (L) = M s⁻¹ | convert an amount rate to a concentration rate | use gas or mass data for a solution |
Assumptions behind the numbers
- The rates of different species are linked only through the balanced equation; there are no side reactions.
- The solution volume stays constant, so a change in moles gives a change in concentration directly.
- For mass-loss data, all the mass lost is the gas product (no splashing or evaporation of water).
- The temperature is constant during the measurement.
Mistakes to avoid
- Forgetting the minus sign for a reactant, giving a negative rate.
- Inverting a coefficient ratio. In 2 A → 3 B, B forms 3/2 as fast as A disappears, not 2/3.
- Mixing “rate of A” with “rate of reaction”. Read the question.
- Treating the rate as constant. An average rate belongs to one interval.
- Saying more solid raises its concentration. For solids, surface area matters.
- Explaining temperature only by more collisions. Say that more collisions have enough energy.
Quick self-check
- For 2 SO₂ + O₂ → 2 SO₃, SO₃ forms at 0.040 M s⁻¹. How fast is O₂ used, and what is the rate of reaction? (0.020 M s⁻¹; 0.020 M s⁻¹)
- [A] falls from 0.500 M to 0.350 M in 25 s. What is the average rate of disappearance of A? (0.0060 M s⁻¹)
- Why does powdered zinc react with acid faster than one lump of the same mass? (More surface is exposed, so acid particles collide with zinc atoms more often.)
Next: practice questions.