Redox Reactions – Oxidation, Reduction, Redox Reactions, Their Applications, Practice Problems and FAQ

Pollution is a threat to the existence of Earth and the life that lives on it. Conventional energy sources such as thermal power plants, burning of fossil fuels, etc have polluted our planet and have degraded the quality of life.

Is there any pollution-free or less polluting energy sources that can be used as an alternative to conventional energy sources?

There are many sustainable and non-polluting energy sources like solar energy, wind energy, geothermal energy, etc, but one of the most efficient and less polluting sources of energy is fuel cells.

What are fuel cells?

A fuel cell is an electrochemical cell that uses an electrochemical reaction to produce electrical energy from fuel. If the fuel used in a fuel cell is pure hydrogen, then only water and hot air are produced as by-products, which are non-polluting. emitted from the fuel cell. Redox reactions are the underlying principle of Fuel cells.

What are redox reactions? We’ll get to know about redox reactions on this concept page.

TABLE OF CONTENTS

• Oxidation and Reduction
• Oxidising Agent and Reducing Agent
• Redox Reaction
• Daily Life Examples of Redox Reactions
• Practice Problems
• Frequently Asked Questions – FAQ

Oxidation and Reduction

The term "reduction reaction" refers to any chemical process in which an element gains electrons. It implies that an element has undergone reduction if its oxidation state decreases.

The term “oxidation reaction” refers to any chemical process in which an element loses electrons. It implies that an element has undergone oxidation if its oxidation state increases.

The oxygen in the atmosphere is reduced, and iron metal is oxidised to produce rust, an iron oxide.

$Fe\left(s\right)+O_2\left(g\right)+x{H}_{2}O\left(l\right)\to F{e}_2{O}_3.x{H}_{2}O\left(s\right)$

In the reaction given above, the oxidation state of oxygen changes from 0 in O₂(g) to -2 in $F{e}_2{O}_3.x{H}_{2}O\left(s\right)$. Since the oxidation state of oxygen decreases from 0 to -2, O₂(g) has undergone reduction. The oxidation state of iron in the given reaction changes from 0 in Fe(s) to +3 in $F{e}_2{O}_3.x{H}_{2}O\left(s\right)$. Since the oxidation state of iron increases from 0 to +3, Fe(s) has undergone oxidation.

Oxidising Agent and Reducing Agent

Oxidising agent / Oxidant

An oxidising agent or an oxidant is a chemical species that can oxidise others while itself undergoing reduction in a chemical reaction. In other words, an oxidising agent is a chemical species that accepts electrons in a redox reaction.

Reducing Agent / Reductant

A reducing agent or a reductant is a chemical species that can reduce others while itself undergoing oxidation in a chemical reaction. In other words, a reducing agent is a chemical species that donates electrons in a redox reaction.

Redox Reaction

A redox reaction is a chemical reaction in which two different reactants react and electrons are transferred between them. By examining the change in the oxidation state of both the reacting species, it is possible to identify the transfer of electrons between the involved reactants.

When both reduction and oxidation occur simultaneously, the chemical reaction is referred to as a reduction-oxidation or redox reaction. The oxidised species loses electrons, while the reduced species gains electrons.

The diagram below shows how an electron is transferred between two reactants during a redox reaction in great detail.

Any redox reaction can be divided into two halves namely, the oxidation half-reaction and the reduction half-reaction. When written independently, each of these half-reactions must be balanced in order for the number of electrons in each to be equal.

Daily Life Examples of Redox Reactions

Redox Reaction in Combustion

Combustion is classified as an oxidation-reduction reaction, making it a redox reaction. Redox reactions serve as the internal mechanism of space rockets. The mixture of ammonium perchlorate and powdered aluminium inside the rocket boosters causes an oxidation-reduction reaction.

For example, the chemical equation below can be used to express methane combustion.

${CH}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+{2H}_{2}O\left(g\right)$

In this reaction, the oxidation state of carbon changes from -4 in CH₄(g) to +4 in CO₂(g). Since the oxidation state of carbon increases from -4 to +4, it has undergone oxidation i.e. CH₄(g) acts as a reducing agent.

Similarly, the oxidation state of oxygen changes from 0 in O₂(g) to -2 in CO₂(g) and H₂O(g). Since the oxidation state of oxygen decreases from 0 to -2, it has undergone reduction. i.e. O₂(g) acts as an oxidising agent.

Redox Reaction in Photosynthesis

In green plants, photosynthesis is the process by which water and carbon dioxide are converted into carbohydrates in the presence of sunlight.

The involved reaction is

In this reaction, the oxidation state of carbon changes from +4 in CO₂(g) to 0 in C₆H₁₂O₆ (aq). Since the oxidation state of carbon decreases from +4 to 0, it has undergone reduction i.e. CO₂(g) acts as an oxidising agent.

Similarly, the oxidation state of oxygen changes from -2 in H₂O(l) to 0 in O₂(g). Since the oxidation state of oxygen increases from -2 to 0, it has undergone oxidation i.e. H₂O(l) act as a reducing agent.

Practice Problems

Q1. Which of the following statements is correct about redox reactions?

1. In redox processes, reducing agents are electron-accepting species that experience a decrease in oxidation state.
2. The electroplating of various metals on jewellery is a redox reaction application.
3. In redox processes, oxidising agents are electron-donating substances that donate excess electrons and experience an increase in oxidation state.
4. All of these

Answer: B

Solution:

1. A reducing agent or a reductant is a chemical species that can reduce others while itself undergoing oxidation in a chemical reaction. In other words, a reducing agent is a chemical species that donates electrons in a redox reaction. Therefore, the statement given in option A is incorrect.

2. In the electroplating process, redox processes are employed to deposit a thin layer of a substance on an object. An electroplating procedure is used to create gold-plated jewellery. Therefore, the statement given in option B is correct.

3. An oxidising agent or an oxidant is a chemical species that can oxidise others while itself undergoing reduction in a chemical reaction. In other words, an oxidising agent is a chemical species that accepts electrons in a redox reaction.

So, option B is the correct answer.

Q2. Pick out the non-redox reaction among the given reactions?

$$\begin{gathered} A. ZnO\left(s\right)+H_2\left(g\right)\to Zn\left(s\right)+H_{2}O\left(aq\right) \\ B. {CH}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+{2H}_{2}O\left(g\right) \\ C. {HNO}_3\left(aq\right)+CuO\left(s\right)\to Cu\left(N{O}_3{)}_2\right(aq)+H_{2}O(aq) \\ D. All of these \end{gathered}$$

Answer: C

Solution:

1. $ZnO\left(s\right)+H_2\left(g\right)\to Zn\left(s\right)+H_{2}O\left(aq\right)$

In the reaction given in option A, the oxidation state of zinc changes from +2 in ZnO to 0 in Zn(s). Since the oxidation state of zinc decreases from +2 to 0, it has undergone reduction. Similarly, the oxidation state of hydrogen changes from 0 in H₂(g) to +1 in H₂O(aq). Since the oxidation state of hydrogen increases from 0 to +1, it has undergone oxidation.

Reduction: ZnO (+2)Zn(0);

Oxidation: H₂(0)H₂O (+1)

As oxidation and reduction are taking place in this reaction, it is a redox reaction.

2. ${CH}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+{2H}_{2}O\left(g\right)$

In the reaction given in option B, the oxidation state of carbon changes from -4 in CH₄(g) to +4 in CO₂(g). Since the oxidation state of carbon increases from -4 to +4, it has undergone oxidation. Similarly, the oxidation state of oxygen changes from 0 in O₂(g) to -2 in CO₂(g) and H₂O(g). Since the oxidation state of oxygen decreases from 0 to -2, it has undergone reduction.

$Oxidation: C{H}_4\left(-4\right)\to C{O}_2\left(+4\right);$

$Reduction: {\mathit{O}}_2\left(0\right)\to {\mathit{H}}_2\mathit{O}\mathit{ }\left(-2\right)$

As oxidation and reduction are taking place in this reaction, it is a redox reaction.

3. ${HNO}_3\left(aq\right)+CuO\left(s\right)\to Cu\left(N{O}_3{)}_2\right(aq)+H_{2}O(aq)$

In the reaction given in option C, the oxidation states of the elements [ $H\left(+1\right), Cu\left(+2\right), O\left(-2\right), N\left(+5\right)$] remains the same. Therefore, it is not a redox reaction.

So, option C is the correct answer.

Q3. Select the incorrect statement about redox reactions.

1. All decomposition reactions are redox reactions.
2. Oxidation is the addition of an electronegative element or the removal of an electropositive element.
3. Reduction is the addition of an electropositive element or the removal of an electronegative element.
4. None of the above

Answer: A

Solution:

1. All decomposition reactions are not redox reactions. For example, the decomposition of calcium carbonate into calcium oxide and carbon dioxide is not a redox reaction

Oxidation state of Ca in reactant and product is +2.

Oxidation state of C in reactant and product is +4.

Oxidation state of O in reactant and product is -2.

Since the oxidation states of the elements remain the same, it is not a redox reaction. Thus, the statement given in option A is incorrect.

2. When an electronegative element is added, the process is oxidation.

${2H}_{2}O\left(g\right)+2F_2\left(g\right)\to 4HF\left(g\right)+O_2\left(g\right)$ (oxidation of oxygen: $\left(-2\to 0\right)$)

When an electropositive element is removed, the process is oxidation.

${2KI+H}_2{O}_2\left(l\right)\to I_2\left(g\right)+2KOH\left(g\right)$ (oxidation of iodide: $\left(-1\to 0\right)$)

Thus, the statement given in option B is correct.

3. When an electropositive element is added, the process is reduction.

$2Ca\left(s\right)+O_2\left(g\right)\to 2CaO\left(s\right)$ (Reduction of oxygen :($0\to -2)$)

When an electronegative element is removed, the process is reduction.

$2{FeCl}_3\left(aq\right)+H_2\left(g\right)\to {2FeCl}_2\left(aq\right)+2HCl\left(g\right)$ (Reduction of iron:$(+3\to +2)$)

Thus, the statement given in option C is correct.

So, option A is the correct answer.

Q4. Which of the following reactions is a redox reaction?

$$\begin{gathered} A. {ZnCO}_3\left(s\right)\to {ZnO\left(s\right)+CO}_2\left(g\right) \\ B. {SO}_3\left(g\right)+H_{2}O\left(g\right)\to H_2{SO}_4\left(g\right) \\ C. FeC{l}_3\left(aq\right)+HCl\left(aq\right)\to HFeC{l}_4\left(aq\right) \\ D. None of these \end{gathered}$$

Answer: D

Solution:

1. ${ZnCO}_3\left(s\right)\to {ZnO\left(s\right)+CO}_2\left(g\right)$

In the reaction given in option A, the oxidation states of the elements [ $Zn\left(+2\right), O\left(-2\right), C\left(+4\right)$] remain the same. Therefore, the reaction given in option A is not a redox reaction.

2. ${SO}_3\left(g\right)+H_{2}O\left(g\right)\to H_2{SO}_4\left(g\right)$

In the reaction given in option B, the oxidation states of the elements [ $S\left(+6\right), O\left(-2\right), H\left(+1\right)$] remain the same. Therefore, the reaction given in option B is not a redox reaction.

3. $FeC{l}_3\left(aq\right)+HCl\left(aq\right)\to HFeC{l}_4\left(aq\right)$
4. In the reaction given in option C, the oxidation states of the elements [ $H\left(+1\right), Cl\left(-1\right), Fe\left(+3\right)$] remain the same. Therefore, the reaction given in option C is not a redox reaction.

In the reactions given in options A, B and C, no element's oxidation state changes, demonstrating that neither oxidation nor reduction is taking place. Therefore, none of the reactions in the given options is a redox reaction.

So, option D is the correct answer.

Frequently Asked Questions – FAQ

Q1. Do any redox reactions take place in our bodies?
Answer: As we all know, the primary source of energy in humans is cellular respiration, Water is oxidised to produce oxygen during the respiration process, while carbon dioxide is reduced. This includes a number of redox processes. Only redox processes can transform the food we eat into energy. Thus, our bodies undergo many redox reactions.

Q2. How can we figure out if a redox reaction is taking place in an acidic medium or a basic medium?
Answer: The reaction occurs in the acidic solution if H⁺or any acid is present on either side of the chemical equation. The reaction occurs in the basic solution if OH^- or any base is present on either side of the chemical equation. The reaction occurs in the neutral solution if the chemical equation contains neither H⁺ nor OH^- nor acid nor base.

Q3. Why does nitric acid act as only an oxidising agent whereas nitrous acid can act as both an oxidising and a reducing agent?
Answer: Since nitrogen in nitric acid has reached its maximum oxidation state of +5, it cannot undergo any more oxidation and is, therefore, reduced to lower oxidation states only. Therefore, HNO₃ only has oxidising properties. On the other hand, because N is in the +3 oxidation state and has the chemical formula HNO₂, it can either undergo oxidation to have +5 oxidation state or reduction to have -3 oxidation state.

Q4. Is it correct that all chemical reactions are redox reactions?
Answer: Though redox reactions are abundant in nature, all chemical reactions are not redox reactions. Acid-base neutralisation reactions, double decompositions, and precipitation reactions are examples of non-redox reactions.