An oxidizing agent is a substance that causes another substance to oxidize by taking electrons from it. It is often called an electron acceptor because it accepts electrons during a reaction.
Oxidizing agents themselves are reduced in the process (they gain electrons). Examples of common oxidizing agents are oxygen (O2), hydrogen peroxide (H2O2), chlorine (Cl2), and potassium permanganate (KMnO4). A reducing agent is a substance that causes another substance to be reduced by donating electrons to it. It is often called an electron donor because it loses electrons during the reaction. The reducing agents themselves are oxidized in the process (they lose electrons). Examples of common reducing agents include hydrogen gas (H2), metal hydrides (such as NaBH4), carbon monoxide (CO), and metals such as zinc (Zn) and aluminum (Al).
For example, in the reaction between sodium and chlorine to form sodium chloride:
2Na + Cl2 → 2NaCl
Na is the reducing agent as it is oxidized, whereas Cl2 is the oxidizing agent as it is reduced.
Prepare solutions of ferrous sulphate (FeSO4) and potassium permanganate (KMnO4) in separate beakers. Transfer about 10 cm3 of ferrous sulphate solution into a test tube. Add about 10 cm3 of dilute H2SO4 to it. Now add a few drops of KMnO4 solution to the test tube. What happens?
FeSO4 reduces KMnO4, so its purple color is discharged. KMnO4 oxidizes FeSO4 in this reaction. FeSO4 is the reducing agent, whereas KMnO4 is the oxidizing agent.
A color change during a chemical reaction may indicate a redox reaction.
Potassium permanganate (VII) is an oxidizing agent.
Often used to test for the presence of reducing agents.
When acidified KMnO4 is added to the reducing agent, it changes from purple to colorless.
The above reaction discharges the purple color of KMnO4.
Therefore, the solution must contain a reducing agent that reduces MnO4- ions (purple) to Mn2+ ions (colorless).
During this reaction, the oxidation number of Mn changes from +7 to +2.
Prepare an aqueous solution of potassium iodide and transfer it to a 10 cm3 test tube. Add about 5 cm3 of hydrogen peroxide to it. What's going on? The solution turns reddish-brown, indicating the formation of iodine.
Potassium iodide is a reducing agent. Often used to test for the presence of oxidizing agents. When a potassium iodide solution is added to an acidified hydrogen peroxide solution, the solution turns reddish-brown. The appearance of this color is due to the formation of iodine (I2).
KI is oxidized, and H2O2 is reduced.
This redox reaction can be confirmed by a color change from colorless to reddish-brown.
2KI + H2O2 + H2SO4 → K2SO4 + 2H2O + I2
Consider the following reaction that takes place in the manufacture of steel:
Fe2O3 + 3CO → 2Fe + 3CO2
To identify the oxidizing and reducing agents, work out the oxidation states of all the elements involved in the reaction.
Fe2O3: Fe = +3, O = -2
CO: C = +2, O = -2
Fe: 0
CO2: C = +4, O = -2
(i) Carbon is being oxidized because there is an increase in its oxidation state.
(ii) Fe is being reduced because there is a decrease in its oxidation state.
(iii) The reactant CO contains the C that is being oxidized, so CO is the reducing agent.
(iv) The reactant Fe2O3 contains the Fe that is being reduced. So Fe2O3 is the oxidizing agent.
Oxidizing or reducing agent is the whole molecule or formula unit and not the atom that has undergone a change in oxidation number.
Tungsten is used to make filaments for electric bulbs because it has the highest melting point and high electrical resistance. This metal is obtained from tungsten (VI) oxide, WO3, by reducing it with hydrogen gas.
WO3 + 3H2 → W + 3H2O
Identify the oxidizing and reducing agents in this reaction.
Problem-solving strategy:
Step 1: Work out the oxidation states of all the elements involved in the reaction.
Step 2: Note the element that is undergoing an increase in its oxidation state. Since it is being oxidized, the reactant that contains this element is the reducing agent.
Step 3: Note the element that is undergoing a decrease in its oxidation state. Since it is being reduced, the reactant that contains this element is the oxidizing agent.
Solution:
First, assign oxidation numbers to each atom.
WO3: W = +6, O = -2
H2: H = 0
W: 0
H2O: H = +1, O = -2
Because the oxidation number of W decreases, WO3 is an oxidizing agent. Similarly, the oxidation number of H increases, therefore H2 is the reducing agent.
1. Identify oxidizing and reducing agents in the following reactions:
a) 2S + Cl2 → S2Cl2
b) 2Na + Br2 → 2NaBr
2. Differentiate between oxidizing and reducing agents.
Redox in photography
Silver is a very soft metal. Silver atoms have weak interactions and are loosely packed together. Silver tarnishes in air when it comes in contact with trace quantities of H2S or SO2 in the atmosphere or food such as eggs, which are rich in sulfur compounds. Silver tarnish is silver sulfide that gives silver a blackish appearance. Due to this reason, decorative and practical objects made of solid silver gradually turn black and lose their shining appearance. Decorative and practical objects are plated with a thin layer of silver. Atoms in thin layers firmly adhere to the metal atoms of the object and form a durable layer. An article thickly plated with silver contains many layers of silver atoms. Such layers form a soft covering. These layers gradually turn black.