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You have now worked through the whole of Topic C5 for OCR Gateway Combined Science — rates of reaction and collision theory, the factors that affect a rate, measuring and calculating rates, catalysts and enzymes, and reversible reactions and equilibrium. This final lesson pulls it all together. It shows how the C5 ideas connect, gathers the reasoning and the one calculation you need in one place, revisits the required practical, drills the misconceptions that catch students out, and finishes with a synoptic model answer. Treat it as a revision and exam-technique session rather than new content.
By the end of this lesson you should be able to link the C5 ideas together, use the mean-rate calculation confidently, describe the required practical, reason clearly about rates and equilibrium, avoid the common C5 errors, and structure a top-band answer.
This synoptic lesson exercises all three objectives together: AO1 recall of the C5 facts, AO2 application in the mean-rate calculation and the required practical, and AO3 analysis when you interpret rate graphs and evaluate equilibrium changes.
The theme that runs through all of C5 is control: chemists control reactions by changing the rate and by shifting the position of equilibrium. Keeping that theme in mind turns a list of separate facts into one connected story, which is exactly the kind of thinking that lifts an exam answer.
It helps to see C5 as one connected story about controlling reactions. Everything comes back to collision theory on the rate side, and to dynamic equilibrium and Le Chatelier's principle on the equilibrium side — with the catalyst appearing in both.
flowchart TD
A["Monitoring and controlling reactions"] --> B["Controlling the RATE"]
A --> C["Controlling the EQUILIBRIUM"]
B --> D["Collision theory<br/>collide + activation energy"]
D --> E["Factors: temperature, concentration,<br/>pressure, surface area"]
D --> F["Catalysts and enzymes<br/>lower the activation energy"]
B --> G["Measuring rate<br/>gas volume / mass loss / cross"]
C --> H["Reversible reactions<br/>dynamic equilibrium"]
H -.->|"Le Chatelier"| I["Change T, pressure, concentration"]
F -.->|"catalyst speeds both directions equally"| H
Notice the key link: a catalyst appears on both sides of the topic. On the rate side it lowers the activation energy so more collisions succeed; on the equilibrium side it speeds up the forward and backward reactions equally, so equilibrium is reached faster without its position changing. Seeing connections like this is the synoptic thinking that earns the highest marks.
Combined Science C5 has a single calculation to master — the mean rate of a reaction:
mean rate=timequantity of reactant used or product formed
The unit depends on what you measured: cm³/s for a gas volume, g/s for a mass change, mol/s for an amount in moles.
A reaction gives off 75 cm3 of gas in 30 s. Calculate the mean rate.
mean rate=3075=2.5 cm3/s.
A flask loses 0.9 g of mass in 45 s as carbon dioxide escapes. Calculate the mean rate.
mean rate=450.9=0.02 g/s.
A tangent to a gas-volume curve rises from 12 cm3 to 30 cm3 between 10 s and 40 s. Find the rate at that point.
rate=40−1030−12=3018=0.6 cm3/s.
Exam Tip: Show three lines in every rate calculation — equation, substitution, answer with unit. The commonest lost mark is a missing unit: a rate is never just a number, it is cm³/s, g/s or mol/s.
Combined Science C5 has one required practical to know well: investigating the rate of a reaction.
| Practical | What you do | Key techniques | Top marks come from |
|---|---|---|---|
| Rate of reaction | Follow a quantity over time (gas volume / mass loss / disappearing cross) | Gas syringe, balance, or "disappearing cross" over a drawn mark | Choosing the correct method for the reaction; controlling variables; reading the gradient; a shorter time = a faster rate |
The variables that score marks are the independent variable (the factor you change — e.g. concentration or temperature), the dependent variable (the rate you measure), and the control variables you keep the same to make it a fair test — typically the volume and temperature of the solutions and the mass and surface area of any solid.
Exam Tip: For the rate practical, examiners reward the three kinds of variable and the fair-test idea: change only the factor under study, keep everything else the same. For the disappearing-cross method, always state that a shorter time means a faster rate.
OCR uses specific command words that tell you exactly what kind of answer to give. Matching your answer to the command word is one of the easiest ways to gain — or lose — marks.
| Command word | What it asks for |
|---|---|
| State / Name / Give | A short fact, no explanation (e.g. name a factor that increases the rate) |
| Describe | Say what happens (e.g. describe how to measure a rate) |
| Explain | Give reasons why — use "because", "so that" (e.g. why a powder reacts faster) |
| Calculate | Work out a number — show working and give a unit (e.g. a mean rate) |
| Predict | Use the rules to say what will happen (e.g. the effect of pressure on an equilibrium) |
| Compare | Give the similarities and differences (e.g. two rate curves on the same axes) |
Exam Tip: The difference between describe and explain decides many marks. "The rate increases" describes; "…because collisions are more frequent" explains. If the command word is explain, you must give the reason through collision theory or Le Chatelier.
A reliable routine for any data question is: (1) read the headings and units of the table or the axes; (2) describe the pattern in words (rises, falls, levels off); (3) quote figures to support what you say; and (4) if asked to explain, give the chemistry behind the pattern — for example, a rate–time curve is steepest at the start because the concentration is highest then, and levels off because a reactant has run out. Quoting figures turns a vague description into an evidenced one, which examiners reward.
Use this as a final recall list. Cover the right-hand column and test yourself.
| Prompt | Answer |
|---|---|
| Rate of reaction | How fast reactants are used up or products formed |
| Collision theory | Particles must collide with at least the activation energy |
| Activation energy | The minimum energy needed for a collision to react |
| Factors increasing rate | Temperature, concentration, pressure, surface area, catalyst |
| Temperature effect | More frequent and more energetic collisions (more reach the activation energy) |
| Concentration/pressure effect | Particles closer together → more frequent collisions |
| Surface area effect | More particles exposed → more frequent collisions |
| Catalyst | Lowers the activation energy; not used up |
| Enzyme | A biological catalyst (protein) made by cells |
| Mean rate | timequantity changed (cm³/s, g/s, mol/s) |
| Graph shape | Steepest at the start; levels off when a reactant runs out |
| Reversible reaction | Shown by ⇌; products can re-form reactants |
| Dynamic equilibrium | Forward = backward rate; concentrations constant (closed system) |
| Le Chatelier (H) | Equilibrium shifts to oppose the change |
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