Question

Which of the following statements is correct regarding redox reactions? (a) An increase in oxidation number of an element is called reduction. (b) A decrease in oxidation number of an element is called oxidation. (c) A reagent which lowers the oxidation number of an element in a given substance is reductant. (d) A reagent which increases the oxidation number of an element in a given substance is reductant.

Short answer

Statement (c) is correct: A reagent which lowers the oxidation number of an element in a given substance is reductant.

Step-by-step solution

Exploring Option (a)

Assess if an increase in the oxidation number of an element corresponds to reduction. In redox reactions, an increase in oxidation number signifies a loss of electrons, which is known as oxidation, not reduction. Therefore, statement (a) is incorrect.

Exploring Option (b)

Examine if a decrease in the oxidation number of an element corresponds to oxidation. The decrease in the oxidation number indicates a gain of electrons, which is referred to as reduction, not oxidation. Thus, statement (b) is also incorrect.

Exploring Option (c)

Determine if a reagent that lowers the oxidation number of an element is called a reductant. A reagent causing a decrease in oxidation number is indeed providing electrons to another substance, thus it is a reductant or reducing agent. Therefore, statement (c) is correct.

Exploring Option (d)

Assess if a reagent that increases the oxidation number of an element is called a reductant. A reagent that increases the oxidation number is removing electrons from another substance, which means it is an oxidant or oxidizing agent. Hence, statement (d) is incorrect.

Key concepts

Oxidation Number

The oxidation number, also known as oxidation state, is a concept essential for understanding redox reactions. It is a number assigned to an element in a compound that represents the number of electrons lost or gained by that atom, if the compound were to be broken up into its constituent elements. The rules for assigning oxidation numbers are based on a set of conventions:

• Elements in their elemental form have an oxidation number of zero.
• For monoatomic ions, the oxidation number is equal to the charge of the ion.
• Oxygen usually has an oxidation number of -2, except in peroxides where it is -1 or when bonded to fluorine.
• Hydrogen is normally +1, but it can be -1 when bonded as a hydride to metals.
• The sum of oxidation numbers in a neutral compound is zero, while in polyatomic ions, it is equal to the ion's charge.

Understanding how to determine oxidation numbers is pivotal when analyzing redox reactions because changes in oxidation numbers reveal which atoms are oxidized and which are reduced.

Reduction

Reduction in chemistry refers to the gain of electrons by an atom or ion. It is one-half of a redox reaction, which is a reaction that involves the transfer of electrons between substances. Reduction can be identified by a decrease in the oxidation number of an element. This is because gaining electrons reduces the overall positive charge of the atom - electrons being negatively charged.

For example, when iron(III) oxide reacts with carbon monoxide to form iron and carbon dioxide, the iron(III) is reduced to iron metal. The oxidation number of iron decreases from +3 in iron(III) oxide to 0 in iron metal. Reduction is pivotal to numerous biological and industrial processes, such as respiration and the extraction of metals from ores.

Reductant

A reductant, also known as a reducing agent, is a substance that donates electrons to another species in a redox reaction, thereby being oxidized itself. The reductant causes the reduction of another atom or molecule, traditionally by adding electrons to them.

Reductants are characterized by having elements with a low oxidation number that increases over the course of the reaction. For example, in the reaction between hydrogen gas (H₂) and fluorine gas (F₂) to form hydrogen fluoride (HF), hydrogen acts as the reductant because it gives up its electrons, with its oxidation number increasing from 0 to +1.

Oxidizing Agent

An oxidizing agent, or oxidant, is the component in a redox reaction that accepts electrons from another species. As a result, it causes the other species to be oxidized, and it is itself reduced during the process. The oxidizing agent has a high affinity for electrons and is characterized by a decrease in its oxidation number.

A classic example of an oxidizing agent is molecular oxygen (O₂) in combustion reactions. When substances burn in the presence of oxygen, they are oxidized (lose electrons), and oxygen is reduced, gaining those electrons, which decreases its oxidation state from 0 to -2 as it forms oxides.