1.9.2Physical Chemistry
Calculating Rates of Reaction — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 39 of 42
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.
Section 1 of 6Defining Rate
Definition
- Definition The rate of reaction is the change in amount or concentration of a substance per unit time.
Units
- Units Usually mol dm⁻³ s⁻¹ for concentration or cm³ s⁻¹ and g s⁻¹ for other measures.
Average rate
- Average rate Total change in a quantity divided by the total time taken.
Initial rate
- Initial rate The rate at the start of the reaction, found from the gradient of the tangent at time zero.
Section 2 of 6Methods of Measuring Rate
Methods of Measuring Rate
Gas volume
- Gas volume Collect the gas in a syringe and record its volume at regular intervals.
Mass loss
- Mass loss Place the reaction on a balance and record how mass falls as gas escapes.
Colour change
- Colour change Use a colorimeter to follow changes in absorbance as coloured species form or are used up.
Clock reactions
- Clock reactions Time how long it takes for a fixed amount of product to appear, for example a cloudy precipitate hiding a cross.
| Method | Suitable when |
|---|---|
| Gas syringe | A gas is produced |
| Balance | A dense gas escapes |
| Colorimeter | A coloured species changes concentration |
Section 3 of 6Using Graphs
1
Plot the measured quantity on the y-axis and time on the x-axis.
2
Draw a smooth best-fit curve through the points.
3
At the chosen time, draw a tangent to the curve so that it just touches at one point.
4
Choose two points far apart on the tangent and calculate the gradient (Δy ÷ Δx).
5
Ignore the sign and quote the rate with the correct units.
Section 4 of 6Reading the Curve
Steep start
- Steep start A high gradient at the start shows the reaction is fastest when reactant concentrations are greatest.
Decreasing gradient
- Decreasing gradient As reactants are used up the curve flattens and the rate falls.
Flat line
- Flat line A horizontal line shows the reaction has finished because a reactant is used up.
Comparing curves
- Comparing curves A steeper curve reaching the same final value shows a faster reaction with the same amount of product.
Section 5 of 6Worked Example
Question
- The concentration of a reactant falls from 0.50 mol dm⁻³ to 0.20 mol dm⁻³ in 120 s.
- Calculate the average rate of reaction.
- Change in concentration = 0.50 − 0.20 = 0.30 mol dm⁻³.
- Average rate = 0.30 ÷ 120 s.
- = 0.0025 mol dm⁻³ s⁻¹ (2.5 × 10⁻³).
Answer 2.5 × 10⁻³ mol dm⁻³ s⁻¹
Section 6 of 6Exam Checklist & Tips
1.9.2 – Key Points for the Exam
- Rate of reaction Change in concentration per unit time.
- Tangent A straight line touching a curve at a single point.
- Initial rate Rate at time zero.
- Gradient Change in y divided by change in x.
Common Mistakes
- Drawing a tangent that crosses the curve instead of touching it.
- Taking points too close together on the tangent when calculating the gradient.
- Giving the rate without the correct units.
- Assuming the rate stays constant throughout the reaction.
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