# Redox Reaction

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A redox reaction is a reaction where reduction and oxidation take place at the same time.

Oxidation reaction involves:

• Gain of oxygen
• Loss of hydrogen
• Loss of electrons
• Increase in oxidation number

Reduction reaction involves:

• Loss of oxygen
• Gain of hydrogen
• Gain of electrons
• Decrease in oxidation number

Reducing agent is the substance that:

• causes another substance to undergo reduction; and
• is (itself) oxidized in the redox reaction.

Oxidizing agent is the substance that:

• causes another substance to undergo oxidation; and
• is (itself) reduced in the redox reaction

Now, we will look at redox reaction in terms of the different factors (oxygen, hydrogen, …)

### Redox Reaction In Terms Of Oxygen Gain/Loss

When a substance gains oxygen, it is said to be oxidised.

When a substance loses oxygen, it is said to be reduced.

Example:

$$\text{H}_{2} (\text{g}) + \text{PbO} (\text{s}) \rightarrow \text{H}_{2}\text{O} (\text{l}) + \text{Pb} (\text{s})$$

• Hydrogen, $\text{H}_{2}$ undergoes oxidation because it gains oxygen to form water. $\text{H}_{2}$ is the reducing agent.
• $\text{PbO}$ undergoes reduction because it loses oxygen to form lead, $\text{Pb}$. $\text{PbO}$ is the oxidising agent.

### Redox Reaction In Terms Of Hydrogen Gain/Loss

When a substance loses hydrogen, it is said to be oxidised.

When a substance gains hydrogen, it is said to be reduced.

Example:

$$\text{H}_{2}\text{S} (\text{g}) + \text{Cl}_{2} (\text{g}) \rightarrow \text{S} (\text{s}) + 2 \text{HCl} (\text{g})$$

• $\text{H}_{2}\text{S}$ undergoes oxidation because it loses hydrogen to form $\text{S}$. $\text{H}_{2}\text{S}$ is the reducing agent.
• $\text{Cl}_{2}$ undergoes reduction because it gains hydrogen to form $\text{HCl}$. $\text{Cl}_{2}$ is the oxidising agent.

### Redox Reaction In Terms Of Electron Gain/Loss

When a substance loses electron, it is said to be oxidised.

When a substance gains electrons, it is said to be reduced.

Example:

$$\text{Zn} (\text{s}) + \text{Cu}^{2+} (\text{aq}) \rightarrow \text{Zn}^{2+} (\text{aq}) + \text{Cu} (\text{s})$$

• Zinc, $\text{Zn}$, undergoes oxidation because it loses 2 electrons to form zinc ions, $\text{Zn}^{2+}$. $\text{Zn}$ is the reducing agent.
• Copper(II) ions, $\text{Cu}^{2+}$, undergoes reduction because it gains 2 electrons to form copper, $\text{Cu}$. $\text{Cu}^{2+}$ is the oxidising agent.

### Redox Reaction In Terms Of Oxidation Number Increase/Decrease

Oxidation number is an indicator of the degree of oxidation of an atom in a chemical compound.

When a substance undergoes an increase in oxidation number, it is said to be oxidized.

When a substance undergoes a decrease in oxidation number, it is said to be reduced.

In order to understand an example involving oxidation number, you will need to understand how to calculate the oxidation number of a substance. The calculation of the oxidation number will be covered in the next post.