Edexcel International A Level Chemistry

Topic 9: Introduction to Kinetics and Equilibria

Two questions about any reaction. How fast does it go, and how far? Kinetics answers the first with collisions and activation energy, and equilibrium answers the second with Le Chatelier. Industry has to balance both.

The one idea

Petrol and air can sit in the same tank for years without reacting. A spark and the same mixture explodes in a fraction of a second. The energy released is exactly the same in both cases. What changed was the rate. This topic asks two separate questions about every reaction:

1. How fast? Reactions happen when particles collide with enough energy (the activation energy) and in the right way. Every factor that changes the rate does so by changing the number of successful collisions per second.
2. How far? Many reactions can run both ways. In a closed system they reach dynamic equilibrium, and Le Chatelier's principle predicts how the position shifts when you change the conditions.

The two questions are independent, and they often pull against each other. A high temperature makes an exothermic reaction faster but lowers the yield. The last pages of the topic are about choosing conditions anyway.

Almost every question here is one of four moves:

1. Measure a rate. Use 1 ÷ time, or the gradient of a tangent.
2. Explain a rate change. Say what happens to the frequency of collisions, to the proportion of molecules with energy of at least $E_\text{a}$, or to both.
3. Predict a shift. Name the change, say the position shifts to oppose it, give the direction and the result.
4. Evaluate conditions. Quote data, explain it, spot the conflict, and recommend a compromise.

Where to find things

How to use these notes

• Read the pages in order the first time. Pages 1 to 3 are measuring a rate. Pages 4 to 10 explain it with collisions, the distribution and catalysts. Pages 11 to 15 are equilibrium and its industrial use.
• Each page opens with its one idea in a sentence. If you can explain it to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, practical steps to describe, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

The specification lists a suggested practical for this topic without a code: experiments to show the factors that change a rate, including the decomposition of hydrogen peroxide, marble chips with acid, and thiosulfate ions with acid. These are on page 1 and page 3. All times, volumes and yields in the worked examples are made up to show the method and are marked illustrative. Reference values, such as the conditions of industrial processes, are standard rounded values.

Constants used in this topic: relative atomic masses are the Edexcel IAL values to one decimal place: H 1.0, C 12.0, N 14.0, O 16.0, Ca 40.1. The gas constant is $R = 8.31$ J K⁻¹ mol⁻¹, temperatures in kelvin are °C + 273, and the molar volume of a gas at room temperature and pressure is 24.0 dm³ mol⁻¹.

The skills that earn most marks here are saying "frequency of successful collisions" and not just "more collisions", drawing the Maxwell–Boltzmann curve with the area unchanged and the peak lower and to the right, putting the catalyst on the activation-energy line and not on the curve, counting only gas moles for pressure, and never saying a catalyst changes the yield.

Pages

1. Measuring rate with time

   Rate is how fast an amount changes. When the same change is timed in every run, the rate is proportional to 1 ÷ time, and the thiosulfate and acid experiment gives you the numbers.

   11 min 9.39.3(i)

2. Rate from the gradient of a tangent

   On a curve the rate changes from moment to moment. Draw a tangent at the point you want, find its gradient, and you have the rate at that point, including the initial rate at t = 0.

   13 min 9.39.3(ii)

3. Rate practicals

   Two ways to follow a reaction over time, collecting a gas and measuring mass lost, and how to change one factor at a time so that the curves you draw can be compared fairly.

   13 min 9.19.39.3(ii)

4. Collisions and activation energy

   Particles must collide to react, but most collisions do nothing. Only a collision with at least the activation energy is successful, and that splits "rate" into how often collisions happen and how many are successful.

   11 min 9.19.2

5. Concentration, pressure and surface area

   Concentration, gas pressure and the surface area of a solid all raise the rate in the same way. They put more reactant particles where they can collide, so collisions are more frequent, while the energy of the particles is unchanged.

   12 min 9.1

6. The Maxwell–Boltzmann distribution

   The molecules in a sample do not all have the same energy. The Maxwell–Boltzmann curve shows how many have each energy, and the area beyond the activation energy shows how many can react.

   11 min 9.49.2

7. Temperature and the curve

   Raising the temperature flattens the Maxwell–Boltzmann curve and moves its peak to the right, but the area and the activation energy stay put. The share of molecules beyond the activation energy rises sharply, and that is why rate rises so fast.

   12 min 9.49.1

8. Catalysts and reaction profiles

   A catalyst gives the reaction an alternative route with a lower activation energy. On a reaction profile that is two lower humps with a dip between them, while the reactants, products and ΔH stay exactly where they were.

   14 min 9.59.6

9. Catalysts and the Maxwell–Boltzmann curve

   A catalyst does not change the temperature, so the curve stays exactly the same. It moves the activation energy to the left, so more molecules lie beyond it. Temperature moves the curve; a catalyst moves the line.

   9 min 9.8

10. Catalysts in industry

    Industry uses catalysts to make processes more sustainable in two ways. A lower activation energy means less energy is needed, and a better route can mean fewer steps, less waste and a higher atom economy.

    11 min 9.7

11. Reversible reactions and dynamic equilibrium

    Many reactions go both ways. In a closed system a reversible reaction settles into dynamic equilibrium, where the forward and backward rates are equal and the concentrations stay constant, though not equal.

    14 min 9.99.9(i)9.9(ii)

12. Le Chatelier — concentration and pressure

    If you change the conditions of a system at equilibrium, the position shifts to oppose the change. Adding a substance shifts the position away from it, and raising the pressure shifts it towards the side with fewer moles of gas.

    13 min 9.10

13. Temperature, catalysts and writing the justification

    Raising the temperature shifts the position in the endothermic direction, and lowering it shifts it in the exothermic direction. A catalyst does not shift the position at all. Then bring all the changes together in a full justification.

    13 min 9.10

14. Yield, rate and the industrial compromise

    For an exothermic reaction, a low temperature gives a high yield but a slow reaction, and a high temperature gives a fast reaction but a low yield. Industry settles on a compromise, and adds a catalyst, a pressure and recycling to make the best of it.

    14 min 9.119.10

15. Evaluating data on yield and rate

    How to read a table or graph of yield and rate against temperature and pressure, and turn it into a recommended set of conditions. The method is describe, explain, spot the conflict, recommend.

    12 min 9.11

Also in this topic: Topic 9 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.