1. Reactivity is about the ease, conditions, and speed of reaction
Reactivity is about:
- Whether a substance reacts or not
- The conditions under which it can react
- The speed of reaction
You know that an O Level question is asking you about reactivity when it mentions the keywords above. Some examples include:
2019 P2 A3(a)
Explain why lead reacts with nitric acid but gold does not.
Lead is above hydrogen in the reactivity series, while gold is below hydrogen. This means that lead is more reactive and loses electrons more readily to form lead(II) ions.
Only metals above hydrogen in the reactivity series of metals do react with acids.
2017 P2 A2(b)
Silver chloride forms a grey solid rapidly on standing, silver bromide forms a grey solid slowly on standing, while silver iodide does not have any observable change.
What conclusion can you make from the observations about the relationship between reactivity of the halogen and the rate of breakdown of the silver halide.
Reactivity decreases down Group VII from chlorine, bromine, to iodine. As reactivity decreases, the silver halide breaks down faster to form grey silver metal on standing.
2016 P2 A1(b)
Describe the trend in reactivity down Group I and Group VII.
Reactivity increases down Group I, but decreases down Group VII.
Sometimes, the question can be very sneaky! Like the one in 2018, about the recovery of helium. However, if you interpret recovery as a chemical reaction, then the question is essentially asking you why it cannot react.
2018 P2 A5(b)
Suggest why helium cannot be recovered if it is released into the atmosphere.
Helium is a noble gas that is chemically unreactive and hence monoatomic. Thus, it cannot react with any substance to form a compound that can be easily recovered.
2. Explain reactivity in terms of electrons
| Metals | Non-metals |
|---|---|
| Oxidised during a reaction | Reduced during a reaction |
| Lose electrons to form cations | Gain electrons to form anions |
| More reactive down the group | Less reactive down the group |
Down the group, atoms get larger and their valence electrons are further away from the nucleus. Therefore, there is a weaker attraction between the nucleus and the electrons. Consequently:
- Metals are more reactive down the group as they can lose electrons more easily.
- However, non-metals are less reactive down the group, as they gain electrons less readily.
3. Inferring reactivity from experiments
The more reactive element displaces the less reactive one from its compound.
Metal displacement: Zn + CuSO4 → ZnSO4 + Cu
Halogen displacement: Cl2 + 2NaBr → 2NaCl + Br2
Displacement reactions can be observed. For example, when zinc displaces copper from blue copper(II) sulfate solution, the resulting zinc sulfate solution turns colourless. Likewise, when chlorine displaces bromine from colourless sodium bromide, the resulting bromine formed turns the solution brown.
2015 P2 B9(a)(i)
A jet of fluorine gas is aimed at a filter paper soaked in a solution of potassium bromide. The solution on the filter paper quickly turned brown. Using an ionic equation, explain why.
F2(g) + 2Br–(aq) ⟶ 2F–(aq) + Br2(aq)
Since fluorine is more reactive than bromine, fluorine can displace bromine from potassium bromide to form brown aqueous bromine.
2015 P2 B9(a)(ii)
The experiment is repeated. A jet of chlorine gas and a jet of iodine gas are each aimed at separate filter papers soaked in a solution of potassium bromide. State and explain what you would expect to see in each experiment.
For the experiment with chlorine gas, the solution turns brown as chlorine is more reactive than bromine, hence displacing bromine from potassium bromide.
For the experiment with iodine gas, the solution remains colourless as iodine is less reactive than bromine.
O Level questions usually present experimental data in the form of a table, like the one below from the 2017 paper. A strategy is to read the table row by row, whereby you look at one metal at a time. The metal that can cause a reaction in the most number of solutions is the most reactive.
| copper(II) sulfate | magnesium sulfate | cobalt(II) sulfate | chromium(III) sulfate | |
|---|---|---|---|---|
| copper | no change | no change | no change | |
| magnesium | brown solid forms in colourless solution | brown solid forms in colourless solution | brown solid forms in colourless solution | |
| cobalt | brown solid forms in pink solution | no change | no change | |
| chromium | brown solid forms in green solution | no change | grey solid forms in green solution |
2017 P2 A5
With reference to the table above, which is the most reactive metal? Explain.
Magnesium is the most reactive metal.
Magnesium can displace copper from copper(II) sulfate, cobalt from cobalt(II) sulfate, and chromium from chromium(III) sulfate. This suggests that it is more reactive than the three other metals, whereby it loses electrons more readily.
2017 P2 A6
With reference to the table above, what is the colour of chromium(III) solution? Explain.
Chromium(III) solution is green in colour. When chromium displaces copper in copper(II) sulfate to form chromium(III) sulfate, the resulting solution is green.
4. Correlation between reactivity of metal and stability of compounds
The more reactive the metal, the more stable its metal oxide and metal carbonates.
This means that potassium oxide is the most stable in the series above, evident in how it cannot be reduced to potassium by carbon or hydrogen. Instead, potassium oxide must undergo electrolysis to form potassium.
Similarly, potassium carbonate is the most stable carbonate in the series. It needs the highest temperature to thermally decompose to form potassium oxide and carbon dioxide.
2018 P2 A7(c)
In two separate experiments, hydrogen was passed over heated magnesium oxide and heated copper(II) oxide. Describe and explain the observations, if any.
Observations:
Black copper(II) oxide becomes pink copper, with a loss in mass. However, magnesium oxide remains white, with no observable change,
Explanation:
Copper, but not magnesium, is below hydrogen in the reactivity series. Therefore, copper(II) oxide is less stable and can be easily reduced to copper by hydrogen gas.
2015 P2 A2(a)(i)
Metals are extracted from their oxides by reduction.
Calcium oxide is reduced at 2100 °C while lead oxide is reduced at 400 °C. Explain.
Calcium is more reactive than lead, and hence calcium oxide is more stable than lead oxide. Therefore, a higher temperature is needed to reduce calcium oxide than to reduce lead oxide.
2015 P2 A2(a)(ii)
Metal oxides also react with some metals.
Metals: copper, zinc, magnesium, iron
Metal oxides: silver oxide, sodium oxide, calcium oxide, potassium oxide
Which metal and which metal oxide are most likely to react together to give the most vigorous reaction?
Metal oxides: silver oxide, sodium oxide, calcium oxide, potassium oxide
Magnesium and silver oxide. Magnesium is the most reactive metal in the list while silver oxide is the least stable (and hence most reactive) metal oxide.
5. Understanding reactivity is important in the industry
Differences in reactivity have enabled many important technologies, like the sacrificial protection of iron and the production of electricity in a simple cell.
2016 P2 A2(b)
A shop sells a spray-on rust treatment. The spray stops iron from rusting even if the paint on the bicycle is scratched. The spray contains particles of zinc.
Explain how zinc prevents rust from forming.
Zinc is more reactive than iron, because it loses electrons more readily to form a cation. Therefore, zinc will corrode preferentially in place of iron. This prevents iron from rusting, whereby it reacts with oxygen and water to form hydrated iron(III) oxide.
2019 P2 A7(a)
Explain why different combinations of metals in a simple cell produce different voltages.
Different combinations of metals have varying differences in reactivity. When the difference in reactivity is larger, the voltage is also larger.
