Oxidizing and Reducing Agents
Oxidizing and reducing agents are key terms used in describing the reactants in redox reactions that involve transferring electrons between reactants to form products. Here, we will look at what defines an oxidizing and reducing agent, how to determine an oxidizing and reducing agent in a chemical reaction, and the importance of this concept in real world applications.
Oxidizing and Reducing Agents
An oxidizing agent, or oxidant, gains electrons and is reduced in a chemical reaction. Also known as the electron acceptor, the oxidizing agent is normally in one of its higher possible oxidation states because it will gain electrons and be reduced. Examples of oxidizing agents include halogens, potassium nitrate, and nitric acid.
A reducing agent, or reductant, loses electrons and is oxidized in a chemical reaction. A reducing agent typically is in one of its lower possible oxidation states and is known as the electron donor. A reducing agent is oxidized because it loses electrons in the redox reaction. Examples of reducing agents include the earth metals, formic acid, and sulfite compounds.
Remember that a reducing agent will oxidize the other substance and lose electrons; therefore, its oxidation state will increase. An oxidizing agent reduces the other substance and gains electrons. To help eliminate confusion, here are a few mnemonic devices to help you remember how to determine oxidizing and reducing agents.
"Oil Rig" and "Leo the lion said Ger/Roar"
How to Identify Reducing and Oxidizing Agents
Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)
Oxidation half reaction
2 Br- (aq) → Br2 (aq)
Oxidation States: -1 0
Reduction Half Reaction
Cl2 (aq) → 2 Cl- (aq)
Oxidation States: 0 -1
Oxidizing and reducing agents are important in industrial applications. They are used in processes such as purifying water, bleaching fabrics and storing energy (such as in batteries and gasoline). Oxidizing and reducing agents are especially crucial in biological processes such as metabolism and photosynthesis. For example, organisms utilize electron acceptors such as NAD+ to harvest energy from redox reactions as in the hydrolysis of glucose:
C6H12O6 + 2ADP + 2P + 2NAD+ → 2CH3COCO2H + 2ATP + 2NADH
All combustion reactions are also examples of redox reactions. A combustion reaction occurs when a substance reacts with oxygen to create heat. One example is the combustion of octane, the principle component of gasoline:
2 C8H18 (l) + 25 O2 (g) → 16 CO2 (g) + 18 H2O (g)
Combustion reactions are a major source of energy for modern industry.
By looking at each element's oxidation state on the reactant side of a chemical equation in comparison to the same element's oxidation state on the product side, one can determine if the element is being reduced or oxidized. Thus, one is able to conclude the oxidizing and reducing agents of a chemical reaction.
1. Identify the oxidizing agent and the reducing agent in the following redox reaction.
MnO2(s) + 4 H+(aq) + 2 Cl-(aq) → Mn2+(aq) + 2 H2O(l) + Cl2(g)
2. In the reaction, 2 NO2(g) + 7 H2(g) → 2 NH3(g) + 4 H2O(g), is hydrogen an oxidizing agent or a reducing agent? Explain.
3. a) An element that is oxidized is a(n) __________ agent.
b) An element that is reduced is a(n) __________ agent.
4. What is the reducing agent in the following redox reaction: 5 SO32- + 2 MnO4- + 6 H+ → 5 SO42- + 2 Mn2+ + 3 H2O?
5. What is the oxidizing agent in the following redox reaction: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)?
Now that you know about oxidizing and reducing agents, test yourself with some of these problems.
6. Determine the oxidizing and reducing agent of the following chemical equation for aerobic respiration.
C6H12O6 (s) + 6 O2 (g) --> 6 CO2 (g) + 6 H2O (l)
7.Is A the oxidizing or reducing agent? Is B the oxidizing or reducing agent? Which one is reduced and which one is oxidized?
8. In a redox reaction, there must be
A) an oxidizing agent and no reducing agent
B) a reducing agent and no oxidizing agent
C) a reducing agent and an oxidizing agent
D) no reducing or oxidizing agent
9. Which of the following will be a strong reducing agent, which of the following will be a strong oxidizing agent?
NO3-, NO, N2H4, NH3
More practice on identifying oxidizing and reducing agents can be found here.
1. Cl- is the oxidizing agent because it gains one electron (starting with an oxidation state of -1 in Cl- and increasing to 0 in Cl2). Remember that gaining electrons means it is "reduced". MnO2 is the reducing agent because it loses two electrons (starting with an oxidation state of +4 in MnO2 and decreasing to +2 in Mn2+). Keep in mind that losing electrons means it is "oxidized".
2. In this reaction, hydrogen loses one electron. Hydrogen is oxidized, thus making it the reducing agent.
3. a) An element that is oxidized is a reducing agent because the element loses electrons.
b) An element that is reduced is an oxidizing agent because the element gains electrons.
4. SO32- is the reducing agent because it lost two electrons, sulfur goes from an oxidation state of +4 in SO32- to an oxidation state of +6 in SO42-.
5. Cu2+ (aq) is the oxidizing agent because it gains two electrons, going from an oxidation state of +2 in Cu2+(aq) to an oxidation state of 0 in Cu(s).
6.The oxidizing agent is oxygen. The reducing agent is glucose.
The oxygen is being reduced, so it is an oxidizing agent. The glucose is being oxidized, so it is a reducing agent.
7. When A loses electrons, it is being oxidized, thus is a reducing agent.
8. C. In a redox reaction, there is always an oxidizing and reducing agent.
When B gains electron, it is being reduced, thus is an oxidizing agent.
A is oxidized. B is reduced.
9. NO3- is most likely to be a strong oxidizing agent. NH3 is most likely to be a strong reducing agent. You determine the likelihood for oxidation or reduction by comparing the oxidation numbers of nitrogen. Since NO3- has the highest oxidation number of +5, compared to the other molecules, it will most likely be the oxidizing agent. Since NH3 has an oxidation state of -3, it has the lowest oxidation state and will most likely be the reducing agent.
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