Difference Between Reducing Agent and Oxidizing Agent

Main Difference – Reducing Agent vs Oxidizing Agent

Reducing agents and oxidizing agents are chemical compounds involved in redox reactions. These compounds are the reactants of a redox reaction. The main difference between reducing agent and oxidizing agent is that reducing agent can lose electrons and be oxidized whereas oxidizing agent can gain electrons and be reduced.

Key Areas Covered

1. What is a Reducing Agent
      – Definition, Properties, Reaction Mechanisms, Examples
2. What is an Oxidizing Agent
      – Definition, Properties, Reaction Mechanisms, Examples
3. What is the Difference Between Reducing Agent and Oxidizing Agent
      – Comparison of Key Differences

Key Terms: Half Reaction, Oxidation, Oxidation State, Oxidizing Agent, Redox Reaction, Reducing Agent, Reduction

What is a Reducing Agent

A reducing agent is a substance that can be oxidized by losing some of its electrons. The loss of electrons causes the reducing agent to get a positive charge since the charge of an atom is dependent on the balancing of the positive charge of the nucleus by the negative charge of the electrons. Therefore, after losing electrons, there aren’t enough negative charges to balance the corresponding positive charge of the nucleus. Thus a positive charge is left. This charge is called the oxidation state of the atom.

A reducing agent can be a substance containing either the same element or different elements. In order to be a reducing agent, a compound that is composed of several elements should have at least one element that is in a lower possible oxidation state so that this element can oxidize into a higher oxidation state, losing its electrons. For example, SO₃^2- can act as a reducing agent. The sulfur atom is in +4 oxidation state there. The highest oxidation number that sulfur can hold is +6. Therefore, the +4 state sulfur can be oxidized to the +6 oxidation state.

In redox reactions, the overall reaction is obtained from the half reactions that occur in that system. The two half reactions are the oxidizing reaction and the reduction reaction. The oxidizing reaction always represents the oxidation of the reducing agent.

In organic chemistry, Red-Al or reducing aluminum compound is a commonly used reducing agent. The following image shows the functional groups that are reduced by this compound.

Figure 1: The reactions of Red-Al.

Oxidation Reactions of Reducing Agents

Followings are the types of reactions reducing agents undergo.

Oxidation of Zero Oxidation State into Positive Oxidation State

Lithium (Li) is a strong reducing agent because it readily loses an electron obtaining a +1 oxidation state. The half reaction would be,

Li →     Li⁺¹   +       e^–

Oxidation of a Positive Oxidation State into a Higher Positive Oxidation State

H₂C₂O₄ is also a good reducing agent. The oxidation state of the C atom is +3. The highest oxidation state that the C atom can have is +4.  Therefore, can oxidize into CO₂. The half reaction is,

H₂C₂O₄ →     2CO₂           +       2H⁺   +       2e^–

Oxidation of a Negative Oxidation State into a Zero Oxidation State

O₂ can be produced from O^2- in oxides. For example, Ag₂O can be oxidized into Ag and O₂.

2Ag₂O       →     4Ag  +       O₂

Oxidation of a Negative Oxidation State into a Positive Oxidation State

Oxidation of H₂S into H₂SO₄ cause the oxidation number of sulfur to change from -2 to +6.

S^2-                  +       4H₂O         →     SO₄^2-            +       8H⁺   +       8e^–

What is an Oxidizing Agent

An oxidizing agent is a substance that can be reduced by obtaining electrons. Therefore it is called an electron receiver or acceptor in redox reactions. The half reaction of reduction is the reaction that the oxidizing agents undergo. When electrons are obtained from outside, there are more negative charges that cannot be completely neutralized by the nucleus. Therefore, the atom obtains a negative charge.  But if this reduction occurs in a positively charged atom, it can obtain a lower positive charge or a neutral charge.

Figure 2: The reducing agent C2H4O cause the reduction of Ag+ into Ag. There, the oxidation number of aldehyde carbon (I) is oxidized to (III) in the carboxylic carbon atom.

In oxidizing agents, the reduction causes the oxidation state of the atom to get decreased. For example, if there’s an atom having a positive charge (such as Na⁺), it can be reduced to zero oxidation state (Na⁺ into Na).  Similarly, an atom or molecule having a zero charge (such as O₂) can be reduced to a negative charge (O₂ into 2O^2-).

Reduction Reactions of Oxidizing Agents

Reduction of oxidizing agents can occur mainly in following ways.

Reduction of Zero Oxidation State to a Negative Oxidation State

Oxygen (O₂) and ozone (O₃) can act as oxidizing agents. They reduce into O^2-. This reduced form can be included in different forms such as O^2- in H₂O and CO₂.

O₂         +       4H⁺   +       4e^–    →     2H₂O

Reduction of Positive Oxidation into a Lower Positive Oxidation State

Manganese (Mn) of MnO₄^– can be reduced into Mn⁺² or MnO₂ (Mn⁺⁴).

         MnO₄^–       +       8H⁺   +       5e^–      →     Mn⁺² +       4H₂O

Reduction of Positive Oxidation State to a Zero Oxidation State

HF (-1 oxidation state of F) can be reduced into F₂ (zero oxidation state of F).

2 HF     →     F₂      +       H₂

2F^–        →     F₂      +       2e^–

Reduction of a Positive Oxidation State into Negative Oxidation State

Sulfur in SO₄^-2 (+6 oxidation state) can be reduced into H₂S (-2 oxidation state).

SO₄^2-      +       8H⁺   +       8e^–    →     S^2-     +       4H₂O        

Difference Between Reducing Agent and Oxidizing Agent

Definition

Reducing Agent: A reducing agent is a substance that can be oxidized by losing some of its electrons.

Oxidizing Agent: An oxidizing agent is a substance that can be reduced by obtaining electrons.

Oxidation State

Reducing Agent: The oxidation state of reducing agent increases.

Oxidizing Agent: The oxidation state of oxidizing agent decreases.

Electron Exchange

Reducing Agent: Reducing agent acts as the electron donor.

Oxidizing Agent: Oxidizing agent acts as the electron receiver.

Change of Oxidation State in the Agent

Reducing Agent: Reducing agent is oxidized during the reaction.

Oxidizing Agent: Oxidizing agent is reduced during the reaction.

Change of Oxidation State in the Other Reactants

Reducing Agent: Reducing agent causes the reduction of another reactant.

Oxidizing Agent: Oxidizing agent causes the oxidation of another reactant.

Conclusion

Reducing agents and oxidizing agents are chemical compounds involved in redox reactions. The main difference between reducing agent and oxidizing agent is that reducing agent can lose electrons and be oxidized whereas oxidizing agent can gain electrons and be reduced.

References:

1.”Strong Oxidizing Agents.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. Available here. 03 July 2017.
2.”Oxidation – Reduction Reactions.” Oxidizing and Reducing Agents. N.p., n.d. Web.  Available here. 03 July 2017.
3.”Reducing Agent – Definition and Example | Reductant.” Chemistry. Byjus Classes, 09 Nov. 2016. Web.  Available here. 03 July 2017.

Image Courtesy:

1. “Red-Al Reductions” By Jimesq – Own work (CC0) via Commons Wikimedia
2. “Redox Tollens Oxidationszahlen C” Von DMKE – Eigenes Werk (CC BY-SA 2.5) via Commons Wikimedia