Oxidation-Reduction in Everyday Life
The iron gate of a house in Puebla looks shiny when it is freshly painted, but after a few rainy seasons reddish-brown spots appear. In the kitchen, the stove burns LP gas, and a cut apple turns dark on the table. What do these three things have in common? They are all redox reactions, and by the end of this page you will see why.
Redox: A Quick Review
A redox reaction is one in which electrons are transferred. You can recognize it because the oxidation number of some element changes.
-
To be oxidized is to lose electrons: the oxidation number increases.
-
To be reduced is to gain electrons: the oxidation number decreases.
-
The reducing agent is the substance that is oxidized (it gives electrons to another).
-
The oxidizing agent is the substance that is reduced (it takes electrons from another).
They always happen together: the electrons that one species loses are gained by another.
An iron fence exposed to rain becomes covered with rust over time. Explain which species is oxidized and which one acts as the oxidizing agent, and mention two everyday…
An iron fence exposed to rain becomes covered with rust over time. Explain which species is oxidized and which one acts as the oxidizing agent, and mention two everyday ways to protect iron from oxidation.
The Rusting of Iron
Iron (Fe) exposed to moist air reacts with the oxygen in the air. The product is hydrated iron(III) oxide, or rust. A simplified form of the reaction:
$$4\,\text{Fe}(s) + 3\,\text{O}_2(g) \rightarrow 2\,\text{Fe}_2\text{O}_3(s)$$Let's look at the oxidation numbers:
Water and salt speed up the process: that is why in Veracruz or Cancún, near the sea, iron rusts faster.
How Is Iron Protected?
All forms of protection aim to keep iron from touching oxygen and water:
-
Paint or enamel: forms a barrier.
-
Oil or grease: used on tools and bicycles.
-
Galvanizing: a layer of zinc (Zn is oxidized first and protects the iron). Galvanized sheet metal and water tanks are made this way.
-
Alloy: stainless steel contains chromium, which forms a protective layer.
An iron fence exposed to rain becomes covered with rust over time. Explain which species is oxidized and which one acts as the oxidizing agent, and mention two everyday…
An iron fence exposed to rain becomes covered with rust over time. Explain which species is oxidized and which one acts as the oxidizing agent, and mention two everyday ways to protect iron from oxidation.
Combustion: LP Gas in the Stove
A combustion is a fast reaction with oxygen that gives off heat and light. LP gas contains mostly butane. Its complete combustion:
$$2\,\text{C}_4\text{H}_{10}(g) + 13\,\text{O}_2(g) \rightarrow 8\,\text{CO}_2(g) + 10\,\text{H}_2\text{O}(g)$$Here you don't see electrons moving and there are no ions, so is it redox? Yes: the oxidation numbers change.
If there is not enough oxygen (a yellow or orange flame on the stove, soot on the comal), the combustion is incomplete and $\text{CO}$ is produced, a toxic, colorless gas. That is why stoves and heaters need ventilation.
On the stove of a Mexican kitchen, LP gas (mainly butane, C₄H₁₀) burns with oxygen from the air, producing carbon dioxide and water. Identify which substance is oxidized…
On the stove of a Mexican kitchen, LP gas (mainly butane, C₄H₁₀) burns with oxygen from the air, producing carbon dioxide and water. Identify which substance is oxidized (the fuel) and which one is reduced (the oxidizer), and explain why combustion is a redox reaction even though we cannot see electrons moving.
Oxidants and Antioxidants
An oxidant takes electrons from other substances; it is itself reduced. An antioxidant is a substance that is easily oxidized: it gives its electrons to the oxidant and so protects another substance.
-
Chlorine (bleach): hypochlorite, $\text{ClO}^-$, oxidizes dye stains (leaving them colorless) and the bacteria in water. That is why chlorine whitens clothes and disinfects.
-
Vitamin C (ascorbic acid): found in guava, orange, and lime. It gives up electrons easily.
The chlorine used to bleach clothes and disinfect water acts as an oxidizing agent, while vitamin C from guava or orange is advertised as an "antioxidant". Explain, in…
The chlorine used to bleach clothes and disinfect water acts as an oxidizing agent, while vitamin C from guava or orange is advertised as an "antioxidant". Explain, in terms of electron transfer, what an oxidizer does and what an antioxidant does, and why adding lime juice (rich in vitamin C) to a cut fruit slows down its browning in the air.
Benefits and Environmental Cost
Redox reactions are useful, but they have consequences:
| Redox process | Benefit | Environmental cost |
|---|---|---|
| Combustion of LP gas, gasoline, or firewood | Cooking, transportation, heat | $\text{CO}_2$ (greenhouse effect), CO, and particulates; air pollution in cities like Mexico City |
| Rust | None | Loss of bridges, pipes, and structures; more steel has to be produced, which uses energy |
| Chlorine in water | Eliminates microorganisms and prevents disease | Can form toxic byproducts if used in excess |
| Bleaches | Cleaning and hygiene | If poured down the drain in large amounts, they harm aquatic life |
Responsible consumption: keep stoves properly adjusted (blue flame), paint and repair iron to extend its life, reuse and recycle steel, use chlorine in the recommended dose, and never mix it with other cleaning products (it can release toxic gases).
On the stove of a Mexican kitchen, LP gas (mainly butane, C₄H₁₀) burns with oxygen from the air, producing carbon dioxide and water. Identify which substance is oxidized…
On the stove of a Mexican kitchen, LP gas (mainly butane, C₄H₁₀) burns with oxygen from the air, producing carbon dioxide and water. Identify which substance is oxidized (the fuel) and which one is reduced (the oxidizer), and explain why combustion is a redox reaction even though we cannot see electrons moving.
The chlorine used to bleach clothes and disinfect water acts as an oxidizing agent, while vitamin C from guava or orange is advertised as an "antioxidant". Explain, in…
The chlorine used to bleach clothes and disinfect water acts as an oxidizing agent, while vitamin C from guava or orange is advertised as an "antioxidant". Explain, in terms of electron transfer, what an oxidizer does and what an antioxidant does, and why adding lime juice (rich in vitamin C) to a cut fruit slows down its browning in the air.