0620 · 9.4Reactivity series

Reactivity series and key ideas

0:00Magnesium bubbles in dilute hydrochloric acid, while copper shows no reaction under the same conditions. Both are metals, but their chemical behaviour is different. The reactivity series organises these differences into a useful order. By the end, you will understand how that order predicts reactions and how experiments reveal a metal’s position. We need a few starting ideas about particles. An atom is electrically neutral because it contains equal numbers of protons and electrons.

0:26Losing electrons produces a positive ion because the number of protons remains unchanged. An aqueous solution contains substances dissolved in water, so dissolved ions can move through the liquid. Reactivity describes how readily a substance undergoes a chemical reaction. For metals, we compare their behaviour with substances such as water, dilute acids and solutions containing other metals’ ions. A more reactive metal generally reacts more readily under comparable conditions. However, temperature, surface area and protective coatings can affect what we actually observe.

1:00The required order is potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold. Read from the top downwards to move from more reactive to less reactive substances. Carbon and hydrogen are not metals, but their positions provide useful reference points. In this video, hydrogen will help us predict reactions with dilute hydrochloric acid. Treat the series as a ranking, not a scale with equal steps. The gap between neighbouring names does not represent a fixed difference in reaction speed.

1:32Position tells us which substance is more reactive, rather than exactly how quickly a particular experiment will proceed. You still need to consider the conditions stated in the question.

Metals reacting with water and steam

1:42Potassium, sodium and calcium all react with cold water. Each produces hydrogen and a metal hydroxide. For example, sodium plus water gives sodium hydroxide plus hydrogen. The hydroxide makes the water alkaline, while escaping hydrogen produces bubbles around the reacting metal. Potassium reacts very vigorously, and the hydrogen it releases can ignite. Sodium reacts vigorously, moving across the surface and usually melting into a ball. Calcium reacts less vigorously, producing bubbles as it is gradually used up.

2:14These differences agree with their order in the series: potassium above sodium, and sodium above calcium. The important comparison is that all three can react with cold water, but their reactions have different vigour. Do not describe calcium as unreactive simply because sodium reacts faster. Calcium is still high enough in the series to react readily with water. These demonstrations require appropriate laboratory precautions, especially for potassium and sodium. The balanced equations on the board show that atoms are conserved during these reactions.

2:44Two sodium atoms react with two water molecules, producing two units of sodium hydroxide and one hydrogen molecule. Calcium forms calcium hydroxide instead. Notice that the metal determines the hydroxide, but hydrogen is produced in both examples. Magnesium is below calcium, so its behaviour with water is different. Its reaction with cold water is very slow, but heated magnesium reacts with steam. Magnesium plus steam gives magnesium oxide plus hydrogen.

3:13Notice that the product here is an oxide, not the hydroxide formed in the cold water reactions we just considered. Steam provides different reaction conditions from cold water. Heating the magnesium and using steam make its reaction fast enough to observe. The series therefore does not mean every metal above hydrogen reacts rapidly with cold water. Always connect your prediction to the actual reactant and conditions.

Reactions with dilute hydrochloric acid

3:36With dilute hydrochloric acid, magnesium, zinc and iron react to form a chloride salt and hydrogen. Their surfaces produce bubbles as the gas escapes, and the metal gradually dissolves. Under comparable conditions, magnesium usually reacts most vigorously, followed by zinc, then iron. Their positions above hydrogen explain why these reactions occur. Magnesium plus hydrochloric acid gives magnesium chloride plus hydrogen. Zinc forms zinc chloride, while iron produces iron(II) chloride. The word two tells us that the iron ions have a 2+ charge.

4:09You do not need to guess a different gas for each metal because all three reactions release hydrogen. Copper, silver and gold do not react with dilute hydrochloric acid under normal laboratory conditions. Each is below hydrogen in the series. They cannot displace hydrogen from this acid, whereas magnesium, zinc and iron can. This is a prediction about dilute hydrochloric acid, not a claim that those metals never undergo any chemical reactions.

4:35The balanced magnesium equation is on the board. One magnesium atom reacts with two hydrogen chloride units to produce magnesium chloride and one hydrogen molecule. For an explanation, connect the observation to the ranking. Saying that magnesium bubbles describes the result; saying it is above hydrogen explains why hydrogen can be released. These observations can also appear in the practical paper or Alternative to Practical paper. Hydrogen is identified using a lighted splint, which produces a squeaky pop.

5:04When comparing reaction rates, control acid concentration, volume and temperature, as well as the metal’s exposed surface area. Otherwise, different conditions could make your comparison misleading. Experimental results can reveal an order, but only as far as the evidence allows. If one metal reacts more vigorously than another under comparable conditions, that supports placing it higher. No reaction with dilute hydrochloric acid places copper, silver and gold below hydrogen. That result alone does not establish the order between those three metals.

Electron transfer in displacement reactions

5:34This next part is for the Extended paper. Relative reactivity can be explained by a metal’s tendency to form positive ions. More reactive metals have a greater tendency to lose electrons and become positively charged. This connects the visible reaction to the particle changes happening at the metal’s surface. A zinc atom forms a zinc ion with a 2+ charge by losing two electrons. Magnesium, iron and copper also form ions with a 2+ charge in the examples here.

6:02Silver ions have a 1+ charge. Losing electrons is oxidation, linking reactivity to the redox ideas you have already studied. A displacement reaction occurs when a more reactive metal replaces a less reactive metal from a compound. In aqueous solutions, the incoming metal reacts with the dissolved ions of another metal. The more reactive substance loses electrons and enters the solution as positive ions. Those electrons allow the other metal’s ions to become neutral atoms.

6:30Consider zinc placed in copper(II) sulfate solution. Zinc plus copper(II) sulfate gives zinc sulfate plus copper. Zinc is above copper, so it can displace copper from the solution. Copper is deposited as a solid while zinc enters the liquid as zinc ions. At particle level, zinc atoms lose electrons, forming zinc ions with a 2+ charge. Copper ions gain those electrons and become copper atoms. The sulfate ions remain in solution without undergoing this change, so they are spectator ions.

7:02The ionic equation on the board leaves them out, showing only zinc atoms and copper ions forming zinc ions and copper atoms. Displacement therefore involves electron transfer, rather than one solid simply pushing another substance out of the way.

Predicting and deducing metal order

7:16The reverse experiment gives no displacement reaction. Copper placed in zinc sulfate solution cannot displace zinc because copper is less reactive. A metal also cannot displace itself from a solution containing its own ions. The direction of displacement depends on relative reactivity, not on which substance was added first. For these five metals, the order is magnesium, zinc, iron, copper, silver. Magnesium can displace each metal below it from a solution of that metal’s ions.

7:45Zinc displaces iron, copper and silver, but not magnesium. Iron displaces copper and silver, while copper displaces silver only. Silver cannot displace any of the other four metals from their aqueous ions. You can make each prediction by comparing two positions rather than memorising the entire grid. First identify the solid metal, then identify the metal represented by the dissolved ions. Displacement occurs only when the solid metal is higher in the series.

8:12Let us deduce an order from these illustrative experimental results. Magnesium displaces zinc, so magnesium must be more reactive than zinc. The next result shows zinc displacing iron, placing iron below zinc. Combining those comparisons gives magnesium above zinc above iron, rather than three disconnected observations. Iron displaces copper, so copper belongs below iron in the developing sequence. Finally, copper displaces silver, placing silver at the bottom. The complete order is magnesium, zinc, iron, copper, silver, from most to least reactive.

8:47Each placement follows from a specific result, which is the reasoning an examiner expects. A non-reaction also provides evidence when the experiment is suitable. For example, zinc failing to displace magnesium agrees with zinc being less reactive. However, do not automatically interpret every inactive surface as proof of low reactivity. Protective layers can prevent the reactants from reaching the metal underneath.

Aluminium oxide and exam answers

9:11This next part is for the Extended paper. Aluminium is high in the reactivity series, yet it often appears unreactive. Its surface reacts quickly with oxygen in the air, forming a thin, protective layer of aluminium oxide. This coating prevents water and other reactants from reaching the aluminium underneath, greatly reducing further reaction. The oxide layer does not turn aluminium into a less reactive metal. Instead, it forms a barrier between the metal and its surroundings.

9:37If that protection is removed, the underlying aluminium can react much more readily. Explain its apparent unreactivity using the protective oxide layer, not a position below hydrogen. Common mistakes come from confusing visible speed with the underlying ranking. A coated aluminium surface can appear inactive despite the metal’s high position. Another error is predicting a hydroxide when magnesium reacts with steam; the required product is magnesium oxide. Also remember that copper ions in solution are not the same particles as neutral copper atoms.

10:10An exam might ask you to describe zinc’s reaction with dilute hydrochloric acid. A suitable answer is that bubbles of hydrogen are produced. Zinc dissolves as zinc chloride forms. If asked to explain why copper does not react, state that copper is below hydrogen in the reactivity series. For iron added to silver nitrate solution, predict a displacement reaction because iron is more reactive than silver. Iron has a greater tendency to form positive ions.

10:36Its atoms lose electrons, while silver ions gain electrons to form silver metal. Keep each answer tied to the command word: describe observations, explain reasons, and state an order clearly. The reactivity series connects experimental behaviour with a metal’s tendency to form positive ions. Position helps predict reactions with water, dilute hydrochloric acid and aqueous metal ions. More reactive metals displace less reactive ones, while experimental results let you reconstruct the ranking.

11:05Aluminium reminds us that a protective surface can hide the behaviour of the metal underneath.