4.1.5Unit 4. Rates, Equilibria and Further Organic Chemistry
Initial-Rate Method — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 14 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 6Principle
Initial rates make it possible to find each order separately.
Initial rate
- Initial rate This is the rate at the very start of the reaction, when the concentrations are known and equal the starting values.
Finding it
- Finding it Draw the tangent at t = 0 on a concentration–time or product–time graph and take its gradient.
Why use it
- Why use it At the start no reactants have been used and no products have built up, so side effects are minimal.
Experiments
- Experiments Run a series of experiments in which only one concentration changes and compare the rates.
Part 2 of 6Deducing Orders
Deducing Orders
Compare pairs
- Compare pairs Pick two experiments where only one concentration changes and compare the change in rate.
Rate unchanged
- Rate unchanged If rate stays the same when a concentration doubles, the order is zero in that species.
Rate doubles
- Rate doubles If rate doubles when a concentration doubles, the order is one.
Rate quadruples
- Rate quadruples If rate increases by a factor of four when the concentration doubles, the order is two.
| Exp | [A] / mol dm⁻³ | [B] / mol dm⁻³ | Rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|
| 1 | 0.10 | 0.10 | 2.0 × 10⁻³ |
| 2 | 0.20 | 0.10 | 8.0 × 10⁻³ |
| 3 | 0.10 | 0.20 | 4.0 × 10⁻³ |
Part 3 of 6Process
1
Compare experiments 1 and 2: [A] doubles with [B] fixed and the rate quadruples, so the reaction is second order in A.
2
Compare experiments 1 and 3: [B] doubles with [A] fixed and the rate doubles, so it is first order in B.
3
Write the rate equation as rate = k[A]²[B], with an overall order of three.
4
Substitute any one experiment to find k, and use its concentrations.
5
Work out the units of k from the rate equation.
Part 4 of 6Clock Reactions
Idea
- Idea The time for a fixed small amount of product to appear, shown by a sudden colour change, is measured.
Approximation
- Approximation Because only a small fraction reacts, the rate is approximately proportional to 1 ÷ time.
Example
- Example The iodine clock reaction uses starch to give a sudden blue-black colour when iodine appears.
Limitation
- Limitation The approximation holds only when the extent of reaction at the end point is small.
Part 5 of 6Worked Example
Question Use the data in the table (experiments 1 to 3) to find the rate equation and k.
- Experiments 1 and 2: [A] × 2 gives rate × 4, so order in A is 2.
- Experiments 1 and 3: [B] × 2 gives rate × 2, so order in B is 1.
- Rate = k[A]²[B]. Using experiment 1: k = 2.0 × 10⁻³ ÷ (0.10² × 0.10) = 2.0 × 10⁻³ ÷ 1.0 × 10⁻³ = 2.0.
- Unitsmol dm⁻³ s⁻¹ ÷ (mol dm⁻³)³ = dm⁶ mol⁻² s⁻¹.
Answer Rate = k[A]²[B]; k = 2.0 dm⁶ mol⁻² s⁻¹
Part 6 of 6Exam Checklist & Tips
4.1.5 – Key Points for the Exam
- Initial rate The rate of reaction at time zero, found from the gradient of the tangent at t = 0.
- Clock reaction A reaction in which the time for a sudden visible change is used to estimate the rate.
- Order The power of a concentration in the rate equation.
- Overall order The sum of the orders of all reactants.
- Tangent at t = 0 The line giving the gradient of the curve at the start of the reaction.
Common Mistakes
- Changing two concentrations at once when comparing experiments, so the effect cannot be separated.
- Misreading a rate that quadruples when a concentration doubles as first order.
- Using the final concentration instead of the initial concentration in an initial-rate calculation.
- Forgetting to give the units of k.
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