4.1Unit 4. Rates, Equilibria and Further Organic Chemistry
Kinetics — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
25 min read 39 parts Lesson 1 of 56 1 quiz
Lesson summary
4.1 Kinetics: rates of reaction and obtaining rate data — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Define rate of reaction, relate rate to concentration, and describe the methods used to obtain rate data from experiments.Rate of reaction — the change in the amount or concentration o…
- What the symbols mean
- Disappearing cross
- Arrhenius plot
- Rate-determining step
- Graph shapes
- Rate equation
Part 1 of 9Rate of reaction
Rate of reaction
- Rate of reaction the change in the amount or concentration of a reactant or product per unit time.
Units
- Units mol dm−3 s−1.
Rate = change in concentration of reactant or product (mol dm−3) ÷ time (s).
The rate at any moment is the gradient of the tangent to a concentration–time graph.
- The initial rate is the tangent at t = 0.
Part 2 of 9Rate and concentration
Rate and concentration
For D(aq) → E(aq) + F(g), rate is directly proportional to [D]: rate ∝ [D], so rate = k[D].
- Doubling [D] doubles the rate.
- Halving [D] halves the rate.
A plot of rate against concentration is a straight line through the origin for a first-order reactant.
Part 3 of 9The rate equation
What the symbols mean:
For A + B → C + D, rate = k[A]m[B]n.
What the symbols mean: rate (mol dm−3 s−1) · k = rate constant (units depend on the overall order) · [ ] = concentration (mol dm−3) · each power is the order with respect to that reactant
[A] and [B] are concentrations; m and n are the orders with respect to each reactant.
Rate equations can only be found by experiment, not from the stoichiometric equation.
Products and catalysts can appear in rate equations; intermediates cannot.
A catalyst can appear in the rate equation but not in the overall equation.
Part 4 of 9Properties used to follow a reaction
- Any property that changes during the reaction and is proportional to concentration can be measured.
| Property | Method |
|---|---|
| Mass lost | Balance |
| Volume of gas | Gas syringe or collection over water |
| Colour change | Colorimeter |
| pH | pH probe |
| Electrical conductivity | Conductivity meter |
| Concentration | Titration of quenched samples |
- Some reactions are followed as they proceed,
- giving more data; faster reactions are easier to measure by averaging a measurement over the whole reaction.
Part 5 of 9Colorimetry
1
A colorimeter measures the amount of light passing through a solution.
2
If a solution changes colour, the intensity of light reaching the detector is recorded every few seconds.
3
Light intensity is related to concentration, so the data give concentration against time.
4
Colorimetry cannot be used for coloured precipitates because they scatter or block the light.
Part 6 of 9Mass loss and gas volume
Mass loss and gas volume
Mass loss
- A gas that escapes lowers the mass, e.g. CaCO3 + 2HCl → CaCl2 + H2O + CO2.
- The gas must be dense enough to see on a 2 or 3 d.p. balance: CO2 (Mr = 44) is suitable, H2 (Mr = 2) is not.
Gas volume
- Collect the gas in a gas syringe, or over water in an inverted measuring cylinder if it is not water soluble, e.g. Mg + 2HCl → MgCl2 + H2.
- Plot volume (amount of product) against time.
Part 7 of 9Sampling, quenching and stop-the-clock methods
Taking a sample and titrating it can change the rate, unless the reaction is stopped by quenching.
Quenching methods include adding sodium hydrogencarbonate to remove an acid catalyst, or cooling and diluting.
A convenient alternative is to stop the clock at a visible point, e.g. a piece of magnesium dissolving completely.
Disappearing cross
- Na2S2O3 + 2HCl → 2NaCl + SO2 + H2O + S(s); sulfur obscures a cross viewed through the solution.
- It gives only one piece of data per experiment.
Part 8 of 9More from the specification
1
Polarimetric chiral purity assessment
- Polarimetric chiral purity assessment measuring the optical rotation of monochromatic light provides an analytical means to assess enantiomeric excess.
2
Halogenoalkane
- An alkane in which hydrogen atoms are replaced by halogen atoms.
3
Activation energy
- The minimum energy colliding particles need to react.
4
Nucleophilic substitution
- Replacement of an atom or group by a nucleophile.
5
Continuous monitoring
- Collecting data throughout a reaction.
6
Elementary step
- A single stage in a reaction mechanism.
7
Arrhenius plot
- Arrhenius plot a graph of ln k against 1/T is a straight line with gradient −Ea/R, so Ea can be calculated.
8
Rate-determining step
- Rate-determining step the slowest step in a mechanism, and the species in it appear in the rate equation.
9
Mechanism
- Mechanism the steps must add up to the overall equation and be consistent with the experimental rate equation.
10
Graph shapes
- Graph shapes a zero-order concentration–time graph is a straight line, while a first-order one is a falling curve with constant half-life.
Part 9 of 9More on this topic
- A catalyst provides an alternative route with a lower activation energy.
Key terms
| Term | Meaning |
|---|---|
| Rate of reaction | The change in concentration of a reactant or product per unit time, in mol dm−3 s−1. |
| Rate equation | An expression rate = k[A]m[B]n found by experiment. |
| Colorimeter | An instrument that measures how much light a coloured solution absorbs. |
| Quenching | Stopping a reaction in a sample so it can be analysed. |
| Initial rate | The rate at time zero, from the tangent at the start of the curve. |
Avoid these mistakes
1
Writing the rate equation from the balanced equation — orders must be found by experiment.
2
Using colorimetry to follow a precipitate — the solid scatters the light.
3
Using hydrogen for mass loss — its mass is too small to measure.
4
Forgetting to quench a sample before titrating — the reaction continues and the result is wrong.
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