Many reactions involve the transfer of oxygen or hydrogen between substances. These two ideas — oxidation and reduction — always go together, which is why such reactions are called redox reactions (reduction–oxidation).
Oxidation and reduction
Oxidation is the gain of oxygen or the loss of hydrogen by a substance. Reduction is the opposite: the loss of oxygen or the gain of hydrogen. Look at the reaction between copper oxide and hydrogen on heating: CuO + H2 → Cu + H2O. Here copper oxide loses oxygen, so it is reduced to copper; at the same time hydrogen gains oxygen, so it is oxidised to water. Both happen in the same reaction, so it is a redox reaction. The substance that gives oxygen (or removes hydrogen) is the oxidising agent; the substance that removes oxygen (or supplies hydrogen) is the reducing agent. In the example above, CuO is the oxidising agent and H2 is the reducing agent.
Another clear example is ZnO + C → Zn + CO: zinc oxide is reduced to zinc (loses oxygen) and carbon is oxidised to carbon monoxide (gains oxygen). Recognising which substance gains and which loses oxygen is the key skill here.
Corrosion
Corrosion is the slow eating away of a metal by the action of air, moisture or chemicals on its surface. The most familiar example is the rusting of iron, in which iron reacts with oxygen and water (moisture) to form a flaky, reddish-brown layer of hydrated iron(III) oxide. Rusting needs both air (oxygen) and water; if either is removed, iron does not rust. Corrosion weakens structures such as bridges, railings, car bodies and ships, causing huge economic loss. We prevent it by painting, oiling or greasing, by galvanisation (coating iron with a layer of zinc), by electroplating, or by making alloys such as stainless steel (iron mixed with chromium and nickel).
Rancidity
When fats and oils in food are oxidised by the oxygen in air, they develop an unpleasant smell and taste — this spoilage is called rancidity. Fried snacks, butter and chips become stale this way. To slow it down we add antioxidants (substances that resist oxidation), keep food in air-tight containers, store it in a refrigerator, and flush packets of chips with an unreactive gas such as nitrogen so that oxygen cannot reach the food. So both corrosion and rancidity are everyday consequences of oxidation.
In CuO + H2 → Cu + H2O, identify what is oxidised, what is reduced, and name the agents.
Solution- Copper oxide loses oxygen and becomes copper, so CuO is reduced.
- Hydrogen gains oxygen and becomes water, so H2 is oxidised.
- The substance that supplies oxygen (CuO) is the oxidising agent.
- The substance that removes oxygen (H2) is the reducing agent.
Answer: CuO is reduced (oxidising agent); H2 is oxidised (reducing agent).
Identify the substance oxidised in ZnO + C → Zn + CO.
Solution- Zinc oxide loses oxygen and becomes zinc — ZnO is reduced.
- Carbon gains oxygen and becomes carbon monoxide.
- Gaining oxygen means carbon is oxidised.
Answer: Carbon (C) is oxidised to CO; it acts as the reducing agent.
State the two conditions necessary for the rusting of iron and explain how galvanisation prevents it.
Solution- Rusting needs oxygen (air) and water (moisture) acting on iron together.
- If either air or water is kept away, iron does not rust.
- Galvanisation coats iron with a layer of zinc.
- The zinc layer keeps air and moisture away, and being more reactive, zinc corrodes in preference to iron.
Answer: Air (oxygen) and water are both needed; galvanisation (zinc coating) blocks them and protects the iron beneath.
Why do chips manufacturers flush the packet with nitrogen gas?
Solution- Chips contain fats and oils that turn rancid when oxidised by air.
- Rancidity is caused by oxygen in the air reaching the food.
- Nitrogen is an unreactive gas, so it does not oxidise the food.
- Filling the packet with nitrogen removes oxygen, preventing rancidity.
Answer: Nitrogen displaces oxygen, so the fats are not oxidised and the chips stay fresh longer.
Define rancidity and list two methods to prevent it.
Solution- Rancidity is the spoilage of fatty or oily food by oxidation, giving a bad smell and taste.
- One method: add antioxidants to the food.
- Another method: store food in air-tight containers or in a refrigerator.
Answer: Rancidity is oxidation-caused spoilage of fats and oils; prevent it using antioxidants and air-tight/refrigerated storage (or nitrogen flushing).
Explain why corrosion is described as a redox process, using rusting as the example.
Solution- In rusting, iron combines with oxygen from the air in the presence of water.
- Iron gains oxygen, so iron is oxidised to hydrated iron(III) oxide (rust).
- The oxygen is the oxidising agent that is itself reduced.
- Because oxidation occurs at the metal surface, corrosion is fundamentally an oxidation (redox) process.
Answer: Iron is oxidised by atmospheric oxygen to form rust, so corrosion is an oxidation (redox) reaction.