Reduction – Classical and Modern Definition of Reduction, Reducing Agents, Practice Problems and FAQ

Railways are one of the most comfortable modes of transportation. We experience less disturbance, turbulence and jerks when travelling in a train as the rails are smooth. The intense pressure created when the train is in motion causes shaking and perhaps cracking. The lifespan of the steel rails and train safety are both at risk due to this problem.

So, how we can resolve the interface issue?

Well, a technique known as Thermite welding can be used to create smooth rails in which they ignite the welding component and insert it into the crucible. Slag and molten steel will be produced throughout the welding process.

$2Al\left(s\right)+{Fe}_2{O}_3\left(s\right)+heat\to 2Fe\left(s\right)+{Al}_2{O}_3\left(s\right)$

The joint between two steel rails will be fused by the hot, molten steel (Fe(s)). In this reaction, Fe in Fe₂O₃(s) is converted to Fe(s) by reduction.

We now understand that reduction is used in thermite welding, but what is reduction?

This concept page gives a brief account of reduction, so let’s get to know about reduction without any further ado!

TABLE OF CONTENTS

• Reduction
• Classical Idea of Reduction
• Modern Definition of Reduction
• Reducing Agents or Reductants
• Difference between Reduction and Oxidation
• Redox Reactions
• Practice Problems
• Frequently Asked Questions – FAQ

Reduction

Back in the day, the term ‘reduction; was used to describe the reaction of an element with hydrogen. The definition was later modified to include all processes in which electrons are gained, regardless of whether hydrogen was present after it was discovered that the element gains electrons when it is reduced.

Reduction is the process of the addition of hydrogen or an electropositive element or group or the removal of oxygen or an electronegative element or group. The reduction also refers to the gain of electrons by the element in consideration or the decrease in the oxidation state of the element in consideration.

Classical Idea of Reduction

According to classical concepts, reduction is a process in which hydrogen or any other electropositive element is added, or oxygen or another electronegative element is removed.

The process of an atom or ion gaining one or more electrons is known as reduction, according to the electronic notion.

Reduction can be explained using four different cases.

Case 1: Addition of hydrogen

$2H_2\left(g\right)+F_2\left(g\right)\to 2HF\left(g\right)$ (Reduction fluorine:(0-1))

Case 2: Removal of oxygen

$Zn\left(s\right)+CO\left(g\right)\to ZnO\left(s\right)+C\left(s\right)$ (Reduction of C:(-20))

Case 3: Addition of an electropositive element

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

Case 4: Addition of an electronegative element

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

Modern Definition of Reduction

Reduction is any chemical reaction in which electrons are gained by an element. Actually, it signifies that the element whose oxidation state decreases has undergone reduction.

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

$Reduction: O_2\left(0\right)\to F{e}_2{O}_3.x{H}_{2}O(-2)$

$Oxidation: Fe\left(0\right)\to F{e}_2{O}_3.x{H}_{2}O(+3)$

The oxygen present in the atmosphere is reduced and iron metal is oxidised to produce the iron oxide known as rust. It decreases the oxidation state of oxygen. O₂(g) is in 0 oxidation state, When it is reduced, the oxidation state changes to (-2) in Fe₂O₃.xH₂O(s). Hence, the oxidation state decreases from 0 to (-2).

Example: $H_{2}O\left(g\right)+F_2\left(g\right)\to 2HF\left(g\right)+O_2\left(g\right)$

$Reduction: F_2 \left(0\right)\to HF(-1)$

$Oxidation: H_{2}O(-2)\to O_2 \left(0\right)$

The fluorine molecule is reduced and water is oxidised in this process. The oxidation state of oxygen changes from (-2) in H₂O to 0 in O₂. The oxidation state of fluorine decreases (0) in F₂ to -1 in HF .

Reducing Agents or Reductants

A reducing agent or a reductant is a compound that can reduce others while oxidising itself in a chemical reaction. In other words, a reducing agent is a reagent that loses electrons in a redox reaction.

${2KI\left(s\right)+H}_2{O}_2\left(l\right)\to I_2\left(g\right)+2KOH\left(g\right)$

In the reaction given above, I in KI(s) is in -1 oxidation state and changes to 0 oxidation state in I₂(g). Thus, the oxidation state of I increases, which implies that KI(s) is oxidised to I2(g) istelf. Hence, KI(s) is a reducing agent.

Difference between Reduction and Oxidation

Redox Reactions

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

Example: $FeO\left(s\right)+H_2\left(g\right)\to Fe\left(s\right)+H_{2}O\left(aq\right)$

$Reduction: FeO (+2)\to Fe\left(0\right);$

$Oxidation: H_2\left(0\right)\to H_{2}O (+1)$

There is clearly a change in the oxidation states of H and Fe, which involves both oxidation and reduction. As a result, the ensuing reaction is a redox reaction.

Practice Problems

1. Which of the following statements about reduction is correct?

1. Reduction of an element occurs when the oxidation state decreases.
2. Reduction of an element occurs when the oxidation state increases.
3. Reduction of an element occurs when it loses electrons.
4. Reduction of an element occurs when there is the addition of hydrogen only.

Answer: A

Solution: The addition of hydrogen is not a necessary condition for a reduction process. When an element gains electrons, its oxidation state decreases. The reduction of an element occurs when there is the addition of hydrogen or any other electropositive element and only on the addition of hydrogen.

Thus, the statements given in options B, C and D are incorrect, but the statement given in option A is correct.

So, option A is the correct answer.

2. According to classical or previous concepts, reduction is a process

1. in which oxygen is added
2. in which hydrogen is removed
3. in which an electropositive element is added
4. in which an electronegative element is added

Answer: C

Solution: In accordance with traditional or earlier conceptions, reduction is a reaction in which hydrogen or another electropositive element is added and oxygen or another electronegative element is removed.

For example, Na has 0 oxidation state. When H₂, an electropositive element is added, the oxidation state of Na decreases from 0 to -1 in NaH, and the reduction of sodium takes place.

$H_2\left(g\right)+2Na \left(s\right)\to 2NaH\left(s\right)$

So, option C is the correct answer.

3. Which of the following elements is reduced in the given reaction?

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

1. S
2. H
3. O
4. None of these

Answer: D

Solution: In the given reaction, ${SO}_3\left(g\right)+H_{2}O\left(g\right)\to H_2{SO}_4\left(g\right)$

• S has +6 oxidation state in both SO₃(g) and H2SO₄(g). Hence, there is no gain or loss of electrons.

• O has -2 oxidation state in SO₃(g),H₂O(g) and H₂SO₄(g). Hence, there is no gain or loss of electrons.

• H has +1 oxidation state in both H₂O(g) and H₂SO₄(g). Hence, there is no gain or loss of electrons.

If there is no gain or loss of electrons in a reaction, neither reduction nor oxidation has taken place.

So, option D is the correct answer.

4. In the given reaction, which of the following elements gets reduced?

${2MnO}_4^{-}+10I^{-}+16H^{+}\to 2{Mn}^{2+}+5I_2+8H_{2}O$

1. Mn
2. I
3. O
4. None of these

Answer:

Solution: In the given reaction, ${2MnO}_4^{-}+10I^{-}+16H^{+}\to {2Mn}^{2+}+5I_2+8H_{2}O$

• Mn has +7 oxidation state in MnO₄- and +2 oxidation state in Mn²⁺. Hence, there is decrease in oxidation state from +7 to +2 . Therefore, it has gained electrons, or in other words, it undergoes reduction.

• O has -2 oxidation state in MnO₄-and H₂O. Hence, there is no gain or loss of electrons, or in other words, it undergoes neither oxidation nor reduction.

• I has -1 oxidation state in I- and 0 oxidation state in I₂. Therefore, it has lost electrons, or in other words, it undergoes oxidation. Thus, there is an increase in the oxidation state from -1 to 0.

Reduction takes place when there is a decrease in the oxidation state. Hence, in this reaction, Mn is reduced.

So, option A is the correct answer.

Frequently Asked Questions – FAQ

1. Do reduction and oxidation occur simultaneously?
Answer: Yes, oxidation and reduction occur simultaneously. This is because the electrons lost by the element undergoing oxidation must be gained or accepted by another element. Therefore oxidation and reduction occur simultaneously.

2. What is the relationship between reduction and reducing agents?
Answer: There is a simple relationship between reducing agents and reduction. The reducing agent is a substance that causes the reduction of others by losing electrons. Thus, reducing agents reduce the other species and themselves get oxidised.

3. Every time two elements interact, is it a redox reaction?
Answer: Though redox reactions are abundant in nature, not all chemical reactions are redox reactions. Any reaction in which the oxidation state of an atom changes is referred to as a redox reaction. In general, neutralisation reactions and double displacement reactions are non-redox chemical reactions.

4. Do you know the origin of the term "oxidation state" and why "reduction state" isn't used instead?
Answer: Early on, the term "oxidation" was used to describe the reaction of an element with oxygen. Antoine Lavoisier, a French chemist, coined the term "oxidation." The definition was later modified to include all processes in which electrons are lost, regardless of whether oxygen was present after it was discovered that the element loses electrons when it is oxidised. So, it is by convention that we began to streamline things by using the term ‘oxidation state’ for both reduction and oxidation after making a few additional enhancements.