Predicting and Identifying Reactions: Synthesis and Exam Skills

You have now met every big idea in Topic C4: the reactivity series, displacement and redox, the reactions and trends of Group 1 and Group 7, and how to predict reactions and deduce a reactivity order. This final lesson does not add new chemistry — instead it stitches these ideas together, shows how the exam links them, and drills the technique that turns knowledge into marks. Almost every C4 exam question is, at heart, one instruction: use reactivity to make a prediction and justify it. The aim here is to make that instinctive.

By the end of this lesson you should be able to connect the ideas of C4 into a single framework, choose the right rule for any prediction question, avoid the most common mistakes, write clear balanced and ionic equations under exam conditions, and structure an extended answer that reaches the top band.

This lesson builds AO1 recall across the whole of C4, AO2 application when you write clear balanced and ionic equations under exam conditions, and AO3 evaluation when you choose the right rule for a prediction and justify an extended answer that reaches the top band.

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The Big Picture: One Idea, Many Questions

Everything in C4 flows from a single organising idea: reactivity determines what reacts, how vigorously, and what is formed. The map below shows how the parts connect.

flowchart TD
  A["Reactivity<br/>(how readily an element reacts)"] --> B["Reactivity series of metals"]
  A --> C["Group trends<br/>(Group 1 up, Group 7 down)"]
  B --> D["Metal + water / acid / oxygen"]
  B --> E["Metal displacement"]
  C --> F["Group 1 reactions"]
  C --> G["Halogen displacement"]
  D --> H["PREDICT reactions<br/>and products"]
  E --> H
  F --> H
  G --> H
  E --> I["Redox: OIL RIG<br/>(oxidation and reduction)"]
  G --> I

Read the map and a pattern jumps out: every branch feeds into prediction. Whether a question is about a metal in acid, an alkali metal in water, or one halogen added to another's salt, you are always doing the same thing — comparing reactivities and applying a rule.

Exam Tip: When a C4 question looks unfamiliar, ask yourself the single question at the top of the map: which is more reactive? Nine times out of ten the answer to the whole problem follows from getting that comparison right.

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Choosing the Right Rule

The trick in the exam is matching each question to the correct rule quickly. This decision table is worth learning:

The question is about…	Ask yourself…	The rule
Metal + dilute acid	Is the metal above hydrogen?	Above → salt + hydrogen; below (Cu, Ag, Au) → no reaction
Metal + water	How reactive is the metal?	K, Na, Li, Ca → cold water (hydroxide + H₂); Mg, Zn, Fe → steam (oxide + H₂); Cu, Ag, Au → none
Metal + metal salt	Is the added metal more reactive?	More reactive → displacement; less reactive → no reaction
Halogen + halide salt	Is the added halogen more reactive (higher in Group 7)?	More reactive → displacement; less reactive → no reaction
A trend down a group	Group 1 or Group 7?	Group 1 reactivity up; Group 7 reactivity down
Electron transfer (Higher)	Which species loses/gains electrons?	Loses → oxidised; gains → reduced (OIL RIG)

Exam Tip: Keep this decision table in your head as a checklist. Reading a question, decide first which row it belongs to, then apply that row's rule. Choosing the wrong rule (e.g. using the displacement rule for a metal-and-acid question) is a common way to lose easy marks.

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Writing Equations Cleanly Under Pressure

A large share of C4 marks come from balanced equations, so a reliable routine matters. Work in this order every time:

1. Word equation first — get the names of reactants and products right from the reaction family.
2. Formulae next — write correct formulae, using ion charges for any salt (e.g. $\text{MgCl}_2$MgCl2​, not $\text{MgCl}$MgCl).
3. Balance — adjust the big numbers in front until every element balances; never change a formula to balance.
4. Check — count each type of atom on both sides.

For ionic equations (Higher), cancel the spectator ions — the ions that appear unchanged on both sides. In displacement, the spectators are the ions that stay in solution throughout (such as the sulfate in $\text{CuSO}_4$CuSO4​), leaving only the species that actually change.

Exam Tip: The commonest equation error is changing a formula to make an equation balance. You may only change the big numbers in front of formulae, never the small subscripts inside them — $\text{H}_2$H2​ never becomes $\text{H}_3$H3​.

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Reading the Command Word

The way a C4 question is marked depends heavily on its command word, and answering the wrong "type" of question wastes marks. The three you meet most often in this topic are:

• State / Give / Name — a short factual answer, no explanation needed. "Name the products when zinc reacts with dilute acid" wants only "zinc chloride and hydrogen".
• Describe — say what happens or what you would see, in order, without needing the reason. "Describe what happens when sodium is added to water" wants the observations (floats, fizzes, melts into a ball, moves about).
• Explain — give the reason why. "Explain why potassium is more reactive than lithium" needs the electronic-structure argument (further from the nucleus, more shielded, electron lost more easily), not just the observation.

A frequent slip is to describe when the question says explain, or to pour in explanation when only a name is wanted. Match the depth of your answer to the command word: a one-mark "state" question does not need a paragraph, and a "explain" question will not score for observations alone.

Exam Tip: Underline the command word before you write. Describe = what happens; Explain = why it happens. For "explain" questions in C4, the "why" is almost always reactivity or electronic structure — make sure that reasoning is on the page.

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The Highest-Value Facts to Memorise

If you learn nothing else, learn these — they underpin the majority of C4 marks:

• The reactivity series: K, Na, Li, Ca, Mg, Al, (C), Zn, Fe, (H), Cu, Ag, Au.
• Metal + cold water → metal hydroxide + hydrogen; metal + steam → metal oxide + hydrogen.
• Metal + acid → salt + hydrogen (only if above hydrogen).
• More reactive displaces less reactive — for both metals and halogens.
• Group 1 reactivity increases down the group; Group 7 reactivity decreases down the group; both explained by the outer shell being further out and more shielded.
• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

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Worked Examples

Worked Example 1 — A mixed prediction

Predict whether a reaction occurs when magnesium is added to iron(II) sulfate solution. If it does, write the balanced symbol equation and name the type of reaction.

Step 1 — choose the rule: this is metal + metal salt → use the displacement rule.

Step 2 — compare reactivity: magnesium is above iron, so magnesium is more reactive → a reaction occurs.

Step 3 — write and balance: $\text{Mg} + \text{FeSO}_4 \rightarrow \text{MgSO}_4 + \text{Fe}$Mg+FeSO4​→MgSO4​+Fe. Check: Mg 1 = 1; Fe 1 = 1; S 1 = 1; O 4 = 4. Balanced.

Answer: yes, a displacement reaction; $\text{Mg} + \text{FeSO}_4 \rightarrow \text{MgSO}_4 + \text{Fe}$Mg+FeSO4​→MgSO4​+Fe.

Worked Example 2 — Linking a metal and a halogen prediction

(a) Predict whether zinc reacts with dilute hydrochloric acid. (b) Predict whether bromine displaces chlorine from sodium chloride solution. Justify each.

Step 1 — (a) rule = metal + acid. Zinc is above hydrogen, so it reacts: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$Zn+2HCl→ZnCl2​+H2​.

Step 2 — (b) rule = halogen displacement. Bromine is below chlorine in Group 7, so bromine is less reactive → no reaction.

Answer: (a) reacts, giving zinc chloride and hydrogen; (b) no reaction, because bromine is less reactive than chlorine.

Worked Example 3 — Redox in a displacement (Higher)