Chemical reactions, Year 10: the reactivity series, oxidation and reduction

Reduction

HintIt is the reverse of oxidation, and in everyday speech the word means making something smaller.

The whyReduction and oxidation are opposites: a substance that gains oxygen is oxidised, and one that loses oxygen is reduced. Here the copper oxide is reduced to copper, while the hydrogen that takes the oxygen is oxidised to water.

Lead oxide

HintLook for the reactant that ends the reaction holding less oxygen than it began with.

The whyLead oxide starts joined to oxygen and ends as lead with none, so it has lost oxygen: it is reduced. The carbon gains that oxygen and is oxidised to carbon dioxide.

Magnesium

HintFind the reactant that has gained oxygen by the time it reaches the right-hand side.

The whyMagnesium starts as the element and ends joined to oxygen in MgO, so it has gained oxygen and is oxidised. The carbon dioxide loses its oxygen and is reduced to carbon.

Whether it gains or loses oxygen during the reaction

HintCompare how each substance starts with how it finishes.

The whyOxidation and reduction describe a change, not what a formula contains. A substance is oxidised only if it finishes with more oxygen than it started with; copper oxide that turns into copper has lost oxygen, so it is reduced.

oxide

HintThe name of this family of compounds is built from the name of the gas.

The whyMagnesium gives magnesium oxide, copper gives copper oxide, and so on. The metal has gained oxygen, which is why the reaction is called an oxidation.

positive

HintMetal atoms give away electrons when they react; think what charge that leaves behind.

The whyWhenever a metal reacts, its atoms lose electrons and become charged ions. Potassium does this very readily and copper does not, which is why potassium is near the top of the reactivity series and copper is near the bottom.

Calcium, magnesium, zinc, copper

HintOne of them fizzes in cold water; one of them will not even react with dilute acid.

The whyThe order to learn for the eight named metals runs potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper. Calcium reacts with cold water, magnesium and zinc both react with dilute acid (magnesium faster), and copper reacts with neither.

Copper

HintIt is the only one of the four that sits below hydrogen in the reactivity series.

The whyA metal reacts with a dilute acid only if it is more reactive than hydrogen, because it has to take the place of the hydrogen in the acid. Magnesium, zinc and iron are above hydrogen; copper is below it, so nothing happens.

Z, X, Y

HintA metal can only push another one out of its compound if it is the more reactive of the pair.

The whyZ displaces X, so Z is more reactive than X. Y cannot displace X, so Y is less reactive than X. That gives Z, then X, then Y.

potassium hydroxide; hydrogen

HintOne product is an alkali that stays dissolved; the other is a gas that catches fire and burns with a lilac flame.

The whyPotassium, sodium and lithium all react with cold water in the same way, giving the metal hydroxide and a flammable gas. The hydroxide dissolves, which is why the water turns alkaline.

Reduced

HintMany pupils use the memory aid OIL RIG; the answer is in the second half of it.

The whyOxidation is loss of electrons and reduction is gain of electrons, whether or not oxygen is involved. For example, a copper ion that gains two electrons to become a copper atom has been reduced.

Zn + Cu²⁺ → Zn²⁺ + Cu

HintLeave out the sulfate, which is unchanged, and show only the two metals swapping roles.

The whyEach zinc atom loses two electrons to become Zn²⁺, and each copper ion gains two to become a copper atom. The sulfate ions are in solution before and after, so they are left out. (On these cards the small numbers in a formula sit on the line and charges are raised, as in Cu²⁺.)

The sulfate ion, SO4²⁻

HintIt is the part of the formula that appears, untouched, on both sides of the arrow.

The whyCopper sulfate and magnesium sulfate are both dissolved, so their ions are separate in the water. The sulfate ions are there at the start and still there at the end, so the ionic equation is just Mg + Cu²⁺ → Mg²⁺ + Cu.

Magnesium, Mg

HintLook for the one that begins with no charge and ends up positive.

The whyThe magnesium atom loses two electrons to become Mg²⁺, and loss of electrons is oxidation. The zinc ion gains those two electrons, so it is reduced.

They gain electrons (they are reduced)

HintThe metal atoms give something away; ask where it goes.

The whyEach iron atom loses two electrons to become Fe²⁺, so the iron is oxidised. The hydrogen ions each take one electron and pair up as hydrogen gas, so they are reduced: Fe + 2H⁺ → Fe²⁺ + H2. That is why a metal reacting with an acid is a redox reaction.

A redox reaction

HintThe name is stitched together from the opening letters of the two processes.

The whyElectrons lost by one species must be gained by another, so oxidation and reduction always happen together. Displacement reactions, and the reactions of metals with acids, are both of this kind.