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You have now worked through the whole of Topic C3 of OCR Gateway Science A — conservation of mass and equations, the mole, energy changes, reaction profiles, redox, acids and the pH scale, the reactions of acids, and electrolysis. This final lesson pulls it all together. It shows how the C3 ideas connect, revisits the three required practicals as a set, drills the calculations that earn reliable marks, gathers 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 recall the C3 required practicals and their variables, perform every C3 calculation confidently, avoid the common C3 errors, and structure a top-band synoptic answer.
It helps to see C3 as one connected story: every chemical reaction conserves mass and transfers energy, and the reactions we meet can be classified — as redox, as neutralisation, or as electrolysis.
flowchart TD
A["Chemical reaction<br/>(atoms rearranged)"] --> B["Mass is conserved<br/>balanced equations"]
A --> C["Energy is transferred<br/>exothermic / endothermic"]
A --> D["Reactions can be classified"]
C --> E["Reaction profiles<br/>+ activation energy"]
D --> F["Redox<br/>(oxidation + reduction)"]
D --> G["Neutralisation<br/>(acids + bases)"]
D --> H["Electrolysis<br/>(splitting ionic compounds)"]
F -.->|"electron transfer"| H
G -.->|"making salts"| F
Notice the links at the bottom: electrolysis is a redox process (ions gain or lose electrons), and the reactions of acids that make salts include redox (acid + metal) and neutralisation (acid + base). Mass conservation and energy transfer underpin every one of them. Seeing these connections is exactly the kind of synoptic thinking that lifts an answer.
| Practical | What you change / measure | Key technique | Top marks come from |
|---|---|---|---|
| Temperature change | Change the volume (or reactant); measure ΔT | Insulated polystyrene cup with lid | Controlling concentration & volume; reducing heat loss; ΔT = highest − start |
| Making a pure salt | Make a soluble salt from acid + insoluble base | Add base in excess, filter, crystallise | Excess base so all acid reacts; filter off excess; crystallise (not boil dry) |
| Electrolysis of solutions | Identify the product at each electrode | Inert graphite electrodes, d.c. supply | Correct electrode rules; correct gas tests |
Exam Tip: For every practical, examiners reward identifying the independent variable (what you change), the dependent variable (what you measure) and the control variables (what you keep the same). For the temperature-change practical the control variables that score most often are concentration and volume.
Several calculation types recur in C3. Here is each one again, with a fresh worked example so you can check your method.
Balance using big numbers in front of formulae — never change subscripts.
Worked example: Balance Al+O2→Al2O3.
Aluminium: put 4 in front of Al; oxygen: put 3 in front of O2 and 2 in front of Al2O3:
4Al+3O2→2Al2O3
Check: Al 4 = 4; O 6 = 6. Balanced.
Mr=sum of all the Ar values
Worked example: Find Mr of sodium carbonate, Na2CO3 (Na = 23, C = 12, O = 16).
Mr=(2×23)+12+(3×16)=46+12+48=106
moles=Mrmass
Worked example: What mass of magnesium oxide forms when 12 g of magnesium burns? 2Mg+O2→2MgO (Mg = 24, Mr(MgO)=40).
Moles of Mg =2412=0.5 mol; ratio Mg : MgO is 1:1, so moles of MgO =0.5 mol; mass =0.5×40=20 g.
energy change=(bonds broken)−(bonds made)
Worked example: For H2+Cl2→2HCl (H–H = 436, Cl–Cl = 242, H–Cl = 431 kJ/mol): broken =436+242=678; made =2×431=862; change =678−862=−184 kJ/mol — negative, so exothermic.
Exam Tip: Always show three lines — equation, substitution, answer with unit. For reacting masses follow moles → ratio → moles → mass, and for bond energies use broken − made and read the sign (negative = exothermic).
OCR uses specific command words that tell you exactly what kind of answer to give. Reading them correctly is worth easy marks.
| Command word | What it asks for |
|---|---|
| State / Name / Give | A short fact, no explanation (e.g. "Name the gas produced when a carbonate reacts with acid") |
| Describe | Say what happens, with no need for reasons (e.g. describe the colour change of an indicator) |
| Explain | Give reasons why — use "because", "so that", "this means that" (e.g. why a mass appears to decrease) |
| Calculate | Work out a number — show working and give a unit (e.g. a reacting mass or Mr) |
| Balance | Make the numbers of each atom equal on both sides using front multipliers |
| Predict | Use the rules to say what will happen (e.g. the products at each electrode) |
Exam Tip: The difference between describe and explain decides many marks. "The temperature rises" describes; "...because energy is transferred to the surroundings as the reaction is exothermic" explains. If the command word is explain, you must give the reason.
Several C3 ideas are tested through data — temperature-change tables, pH values, or rates. A reliable routine for any data question is: (1) read the headings and units of the table or axes; (2) describe the pattern in words (rises, falls, levels off); (3) quote figures to support what you say (e.g. "ΔT rises from 3.5 to 8.0 °C"); and (4) if asked to explain, give the chemistry behind the pattern (for example, the temperature change stops rising once all the limiting reactant has been used up). 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.
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