Predicting Chemical Reactions

Chemistry would be impossible to learn if every reaction had to be memorised one by one. The good news is that a small number of patterns — built on the reactivity series and the group trends from earlier in this topic — let you predict what will happen and what will be made for whole families of reactions at once. Give a chemist a metal and an acid, or two metals and a salt solution, and they can usually tell you the products, and even whether a reaction happens at all, without ever having seen that exact combination. This lesson is the capstone of Topic C4 of OCR Gateway Combined Science A: it draws the patterns together and shows how to turn a prediction into a balanced equation, and how to work backwards from experimental results to deduce an order of reactivity.

By the end of this lesson you should be able to predict the products of metal-with-acid, metal-with-water and displacement reactions (metals and halogens), decide whether a given reaction will occur, name and give the formula of the salt formed, write balanced equations for your predictions, and deduce a reactivity order from a set of results.

This lesson builds AO1 recall of the key reaction patterns and rules, AO2 application when you name and give the formula of the salt and write balanced equations for your predictions, and AO3 analysis when you decide whether a reaction occurs and deduce a reactivity order from a set of results.

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The Key Reaction Patterns

Most of the reactions you are asked to predict in C4 fall into a few families. Learn the general pattern for each, and predicting the products becomes routine.

Reactants	Products	Example
Metal + acid	salt + hydrogen	$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$Mg+2HCl→MgCl2​+H2​
Metal + water (cold)	metal hydroxide + hydrogen	$2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$2Na+2H2​O→2NaOH+H2​
Metal + steam	metal oxide + hydrogen	$\text{Zn} + \text{H}_2\text{O} \rightarrow \text{ZnO} + \text{H}_2$Zn+H2​O→ZnO+H2​
Metal + oxygen	metal oxide	$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$2Mg+O2​→2MgO
More reactive metal + salt of less reactive metal	displacement (new salt + displaced metal)	$\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$Fe+CuSO4​→FeSO4​+Cu
More reactive halogen + halide salt	displacement (new salt + displaced halogen)	$\text{Cl}_2 + 2\text{NaBr} \rightarrow 2\text{NaCl} + \text{Br}_2$Cl2​+2NaBr→2NaCl+Br2​

Exam Tip: Identify the family first — metal+acid, metal+water, metal+oxygen, or displacement. The family fixes the product pattern; then all you do is fill in the names and balance. Working in that order stops careless mistakes.

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Deciding Whether a Reaction Will Happen

Just as important as predicting the products is deciding whether a reaction occurs at all — because sometimes the correct answer is "no reaction", and that itself earns the mark. Three rules from earlier in C4 do almost all of the work:

1. Metal + acid: the metal reacts only if it is above hydrogen in the reactivity series. Copper, silver and gold (below hydrogen) do not react with dilute acid.
2. Metal displacement: a metal displaces another only if it is more reactive (higher in the reactivity series).
3. Halogen displacement: a halogen displaces another only if it is more reactive (higher in Group 7).

For metal + water, remember that only the most reactive metals (potassium, sodium, lithium, calcium) react with cold water; less reactive metals (magnesium, zinc, iron) react only with steam; and unreactive metals (copper, silver, gold) do not react with water at all.

flowchart TD
  A["A metal is added to<br/>a dilute acid"] --> B{"Is the metal above<br/>hydrogen in the series?"}
  B -->|"Yes"| C["Reaction: salt + hydrogen"]
  B -->|"No (Cu, Ag, Au)"| D["No reaction"]
  E["A metal is added to<br/>a salt solution"] --> F{"Is the added metal more<br/>reactive than the metal in the salt?"}
  F -->|"Yes"| G["Displacement occurs"]
  F -->|"No"| H["No reaction"]

Exam Tip: Before predicting products, first decide if there is a reaction. For acids ask "is the metal above hydrogen?"; for displacement ask "is the added element more reactive?". If the answer is no, the correct prediction is "no reaction" — do not force a reaction that cannot happen.

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Naming the Salt and Getting Its Formula

When a metal reacts with an acid, a salt is formed. The first part of the salt's name comes from the metal; the second part comes from the acid:

Acid	Salt ending	Example salt
Hydrochloric acid ($\text{HCl}$HCl)	chloride	sodium chloride, $\text{NaCl}$NaCl
Sulfuric acid ($\text{H}_2\text{SO}_4$H2​SO4​)	sulfate	magnesium sulfate, $\text{MgSO}_4$MgSO4​
Nitric acid ($\text{HNO}_3$HNO3​)	nitrate	calcium nitrate, $\text{Ca(NO}_3)_2$Ca(NO3​)2​

So magnesium with hydrochloric acid gives magnesium chloride; zinc with sulfuric acid gives zinc sulfate.

Naming the salt is only half the job — you also need its formula to write a balanced equation, and that depends on the charges of the ions. The positive and negative charges must cancel so the compound is neutral overall. A few common ions and their charges:

Positive ions (cations)	Negative ions (anions)
$\text{Na}^+$Na+, $\text{K}^+$K+, $\text{Li}^+$Li+ ($+1$+1)	$\text{Cl}^-$Cl−, $\text{NO}_3^-$NO3−​, $\text{OH}^-$OH− ($-1$−1)
$\text{Mg}^{2+}$Mg2+, $\text{Ca}^{2+}$Ca2+, $\text{Zn}^{2+}$Zn2+, $\text{Cu}^{2+}$Cu2+, $\text{Fe}^{2+}$Fe2+ ($+2$+2)	$\text{SO}_4^{2-}$SO42−​, $\text{O}^{2-}$O2− ($-2$−2)
$\text{Al}^{3+}$Al3+, $\text{Fe}^{3+}$Fe3+ ($+3$+3)	—

So magnesium chloride is $\text{MgCl}_2$MgCl2​ (one $\text{Mg}^{2+}$Mg2+ needs two $\text{Cl}^-$Cl−), but sodium chloride is $\text{NaCl}$NaCl (one $+1$+1 to one $-1$−1). Aluminium chloride is $\text{AlCl}_3$AlCl3​. For sulfates, sodium sulfate is $\text{Na}_2\text{SO}_4$Na2​SO4​, while magnesium sulfate is simply $\text{MgSO}_4$MgSO4​ (the $+2$+2 and $-2$−2 already balance).

Exam Tip: The salt from sulfuric acid is a sulfate (ending "-ate", containing oxygen), not a sulfide. A sulfide (like iron sulfide, FeS) contains only sulfur with no oxygen and is not made from sulfuric acid. Get the salt's formula right before you balance — a wrong formula wrecks the whole equation.

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Deducing a Reactivity Order from Results

Prediction works both ways. Given a table of experimental results, you can work backwards to deduce the order of reactivity of the elements involved. The logic is simple: whichever metal reacts with (or displaces) the most others is the most reactive; whichever reacts with the fewest is the least reactive.

For example, suppose four metals A, B, C and D are added to each other's salt solutions, and you are told: A displaces B, C and D; B displaces only D; C displaces B and D; D displaces none. Then A (displaces three) is most reactive, C (displaces two) is next, B (displaces one) follows, and D (displaces none) is least reactive — giving A > C > B > D. The same reasoning ranks halogens from their displacement results, or metals from how vigorously they react with acid or water.

Often a question gives you more than one kind of evidence — say, how each metal behaves with water and with acid and in displacement — and expects you to combine them into a single order. The strongest answers weave the strands together rather than treating each in isolation: a metal that reacts with cold water, fizzes fast in acid, and displaces several others from their salts is clearly near the top, while one that ignores cold water, ignores acid, and displaces nothing is clearly near the bottom. Where two metals behave similarly in one test, look to a second test to separate them — this is exactly why experiments often use several different reactions to pin down a reactivity order precisely.

Exam Tip: To deduce an order from results, count: the element that displaces (or reacts with) the most others is the most reactive; the one that reacts with the fewest is the least reactive. When you have several tests, cross-check that every result is consistent with the single order you write down.

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Worked Examples

Worked Example 1 — Metal + acid

Predict the products and write the balanced equation for aluminium reacting with dilute hydrochloric acid.

Step 1 — pattern: metal + acid → salt + hydrogen. Aluminium is above hydrogen, so it reacts.