Chemical Reactions: IGCSE Chemistry 0620

Topic 6 is where 0620 tests thinking rather than recall: rate graphs to interpret, practical methods to design, equilibrium shifts to predict. It is worth 6-10 marks across Papers 2 and 4 in a typical series, anchors rate experiments on Paper 6, and supplies more 6-mark extended responses than any other topic. The examiner complaint repeats every year: students say a reaction is “faster” without saying why in collision terms, and they say equilibrium “moves” without naming a direction and a consequence.

Physical and chemical change (Core)

A physical change makes no new substance and is easily reversed: melting ice, dissolving salt. A chemical change makes one or more new substances, is usually difficult to reverse, and comes with observable evidence: a colour change, a gas given off, a precipitate, or an energy change (the link to chemical energetics). The 1-mark question is classification; the trap option is dissolving, which feels dramatic but is physical.

Rate of reaction and the factors that affect it (Core)

Rate is measured by how quickly a product forms or a reactant is used up. Paper 6 expects you to name a practical method to match the reaction:

• Gas produced: collect in a gas syringe and record volume against time, or follow the mass loss on a balance.
• Precipitate forming (sodium thiosulfate + acid): time how long a cross under the flask takes to disappear.

Four factors increase rate: higher concentration (or pressure, for gases), higher temperature, smaller particle size of a solid (larger surface area), and adding a catalyst. A rate graph starts steepest, flattens as reactants are used up, and goes horizontal when the reaction finishes. Two curves on one grid is the standard question: the faster condition gives a steeper initial slope reaching the same final volume, unless the amount of reactant changed, in which case the plateau moves too. Read which variable changed before describing the curve.

A catalyst increases rate and is unchanged at the end of the reaction. Enzymes are biological catalysts.

Collision theory (S)

Extended candidates explain every rate factor with the same machine: reacting particles must collide, and they must collide with energy greater than or equal to the activation energy. Rate depends on the frequency of these successful collisions.

Factor	Collision-theory explanation
Concentration / pressure ↑	More particles per unit volume, so collisions are more frequent
Surface area ↑	More solid particles exposed, so collisions with the solid are more frequent
Temperature ↑	Particles move faster: collisions are more frequent AND a greater proportion have energy ≥ activation energy
Catalyst	Alternative pathway with lower activation energy, so a greater proportion of collisions are successful

Temperature is the discriminator: it is the only factor with two marking points, and answers that stop at “particles collide more often” leave the second mark behind. Note the language: collisions become more frequent, not “there are more collisions” (a longer experiment also has more collisions in total; frequency is the scoring idea).

Reversible reactions and equilibrium (Core, with S equilibrium)

Core candidates know the ⇌ symbol and the set-piece example: hydrated copper(II) sulfate (blue) loses its water of crystallisation on heating to form white anhydrous copper(II) sulfate; adding water reverses the change and turns it blue again, the chemical test for water. Ammonium chloride does the same trick thermally, splitting into ammonia and hydrogen chloride on heating and recombining on cooling.

(S) Equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products stay constant. Both halves (equal rates, constant concentrations) appear as separate marks.

(S) Changing conditions shifts the position of equilibrium, and 0620 wants the prediction phrased as direction plus consequence:

• Increase temperature: equilibrium shifts in the endothermic direction. For an exothermic forward reaction, that means towards the reactants, so yield falls.
• Increase pressure: equilibrium shifts towards the side with fewer moles of gas.
• Increase concentration of a reactant: equilibrium shifts towards the products.
• A catalyst does not shift the position of equilibrium: it speeds up both directions equally, so equilibrium is reached sooner.

The Haber process (N2 + 3H2 ⇌ 2NH3, exothermic forward) is the worked example: high pressure favours ammonia (2 gas moles vs 4); low temperature favours yield but makes the rate too slow, so 450 °C is the compromise; an iron catalyst speeds up reaching equilibrium. “Compromise between rate and yield” is the exact phrase mark schemes credit, and the question is a recurring 6-marker. The structure for it is in our 6-mark extended response technique guide.

Redox and oxidation numbers (Core definitions, S extensions)

Core: oxidation is gain of oxygen, reduction is loss of oxygen, and a redox reaction has both happening at once. In CuO + H2 → Cu + H2O, copper oxide is reduced and hydrogen is oxidised.

(S) Extended adds two more powerful definitions. Electron transfer: oxidation is loss of electrons, reduction is gain (OILRIG). Oxidation numbers: oxidation is an increase in oxidation number, reduction is a decrease, with Roman numerals naming the states, as in iron(III) oxide. An oxidising agent oxidises something else and is itself reduced; a reducing agent is itself oxidised. Get the mirror right: in CuO + H2 → Cu + H2O, CuO is the oxidising agent.

(S) Two colour tests identify the agents: acidified potassium manganate(VII) turns from purple to colourless with a reducing agent; potassium iodide turns brown (iodine forms) with an oxidising agent.

Electron-transfer redox is the engine behind every half-equation in electrochemistry (reduction at the cathode, oxidation at the anode), so the OILRIG habit pays twice.

Worked exam question

Excess magnesium ribbon is added to 50 cm³ of dilute hydrochloric acid and the volume of hydrogen is recorded every 10 seconds. The experiment is repeated at a temperature 20 °C higher, with all other conditions the same. Explain, using collision theory, why the reaction is faster at the higher temperature. [3]

Model answer: At the higher temperature the particles have more kinetic energy and move faster (1), so they collide more frequently (1), and a greater proportion of the collisions have energy greater than or equal to the activation energy, so more collisions are successful (1).

Mark-by-mark: mark 1 is the energy/speed statement about particles: “the acid is hotter” scores nothing. Mark 2 is collision frequency. Mark 3 is the activation-energy idea, and it needs “proportion” or “more collisions have enough energy”; this is the mark most scripts miss because students stop after frequency. Three sentences, three separate ideas.

The mistakes that cost marks

1. Stopping at “more collisions” for temperature questions. Frequency is one mark; the proportion of collisions exceeding activation energy is the other. Temperature answers need both.
2. Saying a catalyst “increases the yield” at equilibrium. It does not. It only shortens the time taken to reach equilibrium. Yield is set by temperature, pressure and concentration.
3. Equilibrium answers with no direction. “The equilibrium shifts” earns nothing; “shifts to the right/towards the products, so the yield of ammonia increases” earns both marks.
4. Mixing up the agents: the oxidising agent is the substance that is reduced. Label what each species gains or loses before naming agents.
5. Rate-graph descriptions that ignore the plateau. If only temperature changed, both curves finish at the same final volume; drawing or describing a higher plateau contradicts the chemistry.

How to phrase it for full marks

Student wording	Mark-scheme wording
”Heating makes it react faster"	"Particles gain energy, collide more frequently, and a greater proportion have energy ≥ activation energy"
"Crushing the solid speeds it up"	"Smaller pieces have a larger surface area, so collisions are more frequent"
"The equilibrium balances out"	"Forward and reverse rates are equal, so concentrations remain constant"
"High pressure pushes it to ammonia"	"Equilibrium shifts to the side with fewer moles of gas (2 vs 4), increasing the yield of NH3"
"Hydrogen takes the oxygen away"	"Hydrogen is oxidised (gains oxygen); copper(II) oxide is reduced. Hydrogen is the reducing agent”

Every full-mark answer in this topic is a chain: condition → particle-level cause → measurable consequence. Write all three links and the marking points fall out in order.

The Malaysia note

Rates and equilibrium dominate the Extended papers Malaysian international-school students sit in May/June, and they are also the topics where school mocks are most generous: teachers accept “more collisions” answers that Cambridge will not. Students who transfer from KSSM Kimia know “teori perlanggaran” but rarely the activation-energy proportion idea, because SPM phrases it differently. The fastest fix we see is pure phrasing: in a free 1-hour trial lesson, a Chemistry specialist will mark one of your past-paper rate answers against the real mark scheme, line by line, so you can see which of the three chain links your answers are dropping before you spend a ringgit.

Every sub-topic in Chemical Reactions

• Physical and Chemical Change
• Rate of Reaction and the Factors That Affect It
• Collision Theory (Supplement)
• Reversible Reactions and Equilibrium (Supplement)
• Redox and Oxidation Numbers (Supplement)