1.7.2Physical Chemistry
Measuring Rates of Reaction — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 41 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Definition and Units
Rate
- Rate The rate of reaction is the change in concentration of a reactant or product per unit time.
Equation
- Equation Rate = change in concentration ÷ time taken, so the unit is mol dm⁻³ s⁻¹.
Mean rate
- Mean rate Dividing the overall change in concentration by the total time gives only an average rate over that period.
Rate at a time
- Rate at a time The rate at a particular moment is the gradient of a tangent drawn to the concentration–time curve at that point.
Initial rate
- Initial rate The gradient of the curve at time zero is the initial rate, when reactant concentrations are known most accurately.
Part 2 of 5Methods
Methods
Gas volume
- Gas volume Collect gas given off in a gas syringe and record volume at regular intervals, as when acid reacts with carbonate.
Mass loss
- Mass loss Measure the loss in mass of an open flask as gas escapes, which works best for heavy gases like carbon dioxide.
Colorimetry
- Colorimetry Measure how the absorbance of a coloured solution changes with time to follow concentration directly.
Clock reaction
- Clock reaction Measure the time to reach a fixed change, such as a cross disappearing, and use 1 ÷ time as a measure of rate.
| Method | Measures | Suitable when |
|---|---|---|
| Gas syringe | Volume of gas | A gas is produced |
| Balance | Mass lost | A heavy gas escapes |
| Colorimeter | Absorbance | A colour change occurs |
Part 3 of 5Graph Skills
1
Plot concentration (or volume) on the y-axis against time on the x-axis, with a smooth best-fit curve.
2
The curve for a reactant slopes downwards and flattens as the reactant is used up.
3
To find the rate at a time, draw a tangent so that it touches the curve and matches its slope at one point.
4
Calculate the gradient using a large triangle on the tangent, with units from the axes.
5
Compare gradients at different times to show that the rate falls as the concentration falls.
Part 4 of 5Worked Example
Question
- The concentration of a reactant falls from 0.80 mol dm⁻³ to 0.20 mol dm⁻³ in 60 s.
- Calculate the mean rate of reaction.
- Change in concentration = 0.80 − 0.20 = 0.60 mol dm⁻³.
- Time taken = 60 s.
- Mean rate = 0.60 ÷ 60 = 0.010 mol dm⁻³ s⁻¹.
Answer Mean rate = 0.010 mol dm⁻³ s⁻¹ (1.0 × 10⁻² mol dm⁻³ s⁻¹).
Part 5 of 5Exam Checklist & Tips
1.7.2 – Key Points for the Exam
- Rate of reaction The change in concentration of a reactant or product per unit time.
- Tangent A straight line that touches a curve at one point and has the same gradient there.
- Initial rate The rate at the start of the reaction, when time is zero.
- Clock reaction An experiment timing a visible change to compare the rates of reaction under different conditions.
Common Mistakes
- Using the gradient of a chord rather than a tangent to find the rate at a point.
- Giving the rate in units that do not match the axes of the graph.
- Drawing a tangent that crosses the curve instead of just touching it.
- Taking time as a direct measure of rate; rate is inversely related to the time taken.
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