5.2.1Additional lessons
Rate of Reaction — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 66 of 98
Part 1 of 6Defining Rate
Rate tells you how fast a reaction proceeds.
Definition
- Rate of reaction is the change in concentration of a reactant or product per unit time.
Units
- The usual unit is mol dm⁻³ s⁻¹, though other units such as cm³ s⁻¹ or g s⁻¹ are used when other quantities are measured.
Reactants and products
- Reactant concentration falls with time, whereas product concentration rises, so the sign differs but the rate is positive.
Changing rate
- The rate is greatest at the start, when reactant concentration is highest, and decreases as reactants are used up.
Part 2 of 6Average and Instantaneous Rates
Average and Instantaneous Rates
Average rate
- The change in concentration divided by the time interval, which gives the mean rate over that period.
Instantaneous rate
- The rate at a specific moment, found as the gradient of the tangent to a concentration–time curve at that point.
Initial rate
- The instantaneous rate at time zero, found from the gradient of the tangent at the start of the curve.
Curve shape
- For a reactant the curve falls steeply and then flattens, and for a product it rises steeply and then levels off.
Part 3 of 6Calculating from a Graph
1
Plot concentration (vertical axis) against time (horizontal axis) and draw a smooth best-fit curve through the points.
2
To find the rate at a chosen time, draw a tangent that just touches the curve at that point.
3
Choose two points far apart on the tangent and read their coordinates.
4
Divide the change in concentration by the change in time to get the gradient.
5
Take the magnitude of the gradient as the rate, with the correct unit.
| Quantity | Expression |
|---|---|
| Average rate | Δ[concentration] ÷ Δtime |
| Rate from a tangent | Gradient of the tangent at that time |
| Unit | mol dm⁻³ s⁻¹ |
Part 4 of 6Link to Stoichiometry
Link to Stoichiometry
Different species
- In a reaction such as 2A → B, A disappears twice as fast as B appears.
Which species
- Always state which reactant or product the rate refers to.
Why rates change
- Rate depends on how often particles collide, which falls as reactants are consumed.
Importance
- Rates determine how long processes take in industry and how long food and medicines last.
Part 5 of 6Worked Example
Question
- The concentration of a reactant falls from 0.800 mol dm⁻³ to 0.500 mol dm⁻³ in 30.0 s.
- Calculate the average rate.
- Change in concentration = 0.800 − 0.500 = 0.300 mol dm⁻³.
- Time taken = 30.0 s.
- Rate = 0.300 ÷ 30.0 = 0.0100 mol dm⁻³ s⁻¹.
Answer Average rate = 1.00 × 10⁻² mol dm⁻³ s⁻¹
Part 6 of 6Exam Checklist & Tips
5.2.1 – Key Points for the Exam
- Rate of reaction The change in concentration of a reactant or product per unit time.
- Average rate The overall change in concentration divided by the time taken.
- Instantaneous rate The rate at one moment in time, given by the gradient of a tangent.
- Initial rate The rate at the start of the reaction.
Common Mistakes
- Calculating a gradient between two points on the curve and calling it the rate at one point.
- Giving the rate a negative sign for a reactant.
- Forgetting to state the species to which the rate refers.
- Using units of s instead of s⁻¹ for the time part of the rate.
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