Predicting Reactions and Products

Chemistry would be impossibly hard to learn if every reaction had to be memorised separately. Fortunately, a small set of patterns lets you predict what will happen — and what will be made — for whole families of reactions. Armed with the reactivity series and the periodic table, you can work out the products of a metal reacting with an acid, a metal reacting with water, or one metal displacing another, and you can decide whether a reaction will happen at all. This lesson, the capstone of the "predicting" half of Topic C4 of OCR Gateway Science A, draws together those patterns and shows how to turn a prediction into a balanced equation.

By the end of this lesson you should be able to predict the products of metal-with-acid, metal-with-water and displacement reactions, name the salt formed from each acid, decide whether a given reaction will occur, and write balanced symbol equations for your predictions.

---

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 you can predict the products every time.

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​
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: Memorise the general word patterns in the left two columns. Once you can say "metal + acid → salt + hydrogen", predicting the actual products is just a matter of filling in the names and balancing.

---

Naming the Salt

When a metal (or its oxide, hydroxide or carbonate) 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; copper oxide with nitric acid gives copper nitrate.

Exam Tip: The salt from sulfuric acid is a sulfate (ending "-ate", containing oxygen), not a sulfide. A sulfide (e.g. iron sulfide, FeS) contains only sulfur with no oxygen and is not made from sulfuric acid. This is a classic trap.

---

Predicting Whether a Reaction Will Happen

Just as important as predicting the products is deciding whether a reaction occurs at all. Two rules from earlier in C4 do most 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. Displacement: a metal displaces another only if it is more reactive (higher in the series). A more reactive halogen displaces a less reactive halide.

For metal + water, 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.

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 metal more reactive?". If the answer is no, the correct prediction is "no reaction" — and that may be the mark.

---

Getting the Formula of the Salt Right

Predicting the name of a salt is only half the job; you also need its correct formula to write a balanced equation, and that depends on the charges of the ions. The rule is that the positive and negative charges must balance 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−​, $\text{CO}_3^{2-}$CO32−​, $\text{O}^{2-}$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− to balance), but sodium chloride is $\text{NaCl}$NaCl (one $+1$+1 to one $-1$−1). Aluminium chloride is $\text{AlCl}_3$AlCl3​ ($+3$+3 needs three $-1$−1). For sulfate salts: sodium sulfate is $\text{Na}_2\text{SO}_4$Na2​SO4​, but magnesium sulfate is $\text{MgSO}_4$MgSO4​ (the $+2$+2 and $-2$−2 already balance).

Exam Tip: Build a salt formula by balancing the charges: a $+2$+2 metal needs two $-1$−1 ions (e.g. $\text{MgCl}_2$MgCl2​) but only one $-2$−2 ion (e.g. $\text{MgSO}_4$MgSO4​). Get the formula right before you balance the equation — a wrong formula wrecks the whole answer.

---

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.

Step 2 — name the salt: hydrochloric acid → chloride, so the salt is aluminium chloride, $\text{AlCl}_3$AlCl3​ (aluminium forms $\text{Al}^{3+}$Al3+).

Step 3 — write and balance: $2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2$2Al+6HCl→2AlCl3​+3H2​. Check: Al 2=2; Cl 6=6; H 6=6. Balanced.

Answer: aluminium chloride and hydrogen; $2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2$2Al+6HCl→2AlCl3​+3H2​.

Worked Example 2 — Metal + water

Predict the products and write the balanced equation for potassium reacting with cold water.

Step 1 — potassium is very reactive, so it reacts with cold water → metal hydroxide + hydrogen.

Step 2 — products: potassium hydroxide ($\text{KOH}$KOH) and hydrogen ($\text{H}_2$H2​).

Step 3 — balance: $2\text{K} + 2\text{H}_2\text{O} \rightarrow 2\text{KOH} + \text{H}_2$2K+2H2​O→2KOH+H2​. Check: K 2=2; H 4=4; O 2=2. Balanced.

Answer: potassium hydroxide and hydrogen; $2\text{K} + 2\text{H}_2\text{O} \rightarrow 2\text{KOH} + \text{H}_2$2K+2H2​O→2KOH+H2​.

Worked Example 3 — Will it react?

Predict whether (a) copper reacts with dilute sulfuric acid and (b) magnesium reacts with copper(II) sulfate solution. Justify each.

Step 1 — (a) copper is below hydrogen, so it does not react with dilute acid → no reaction.

Step 2 — (b) magnesium is more reactive than copper, so it displaces copper → reaction occurs.

Step 3 — write (b): $\text{Mg} + \text{CuSO}_4 \rightarrow \text{MgSO}_4 + \text{Cu}$Mg+CuSO4​→MgSO4​+Cu (the blue solution fades and copper is deposited).

Answer: (a) no reaction (copper is below hydrogen); (b) reaction — magnesium displaces copper.

Worked Example 4 — Halogen displacement (Group 7, from C2)

Predict the products of adding chlorine to potassium iodide solution and write the balanced equation.

Step 1 — chlorine is more reactive than iodine (higher in Group 7), so it displaces iodine.