Understanding Reduction Reactions

A chemical reaction is called a reduction reaction if a substance gains electrons or its oxidation state decreases. Reduction is always paired with oxidation in what is known as a redox reaction. In simple terms, reduction can be understood in several ways:

• Gain of electrons: An atom or ion accepts electrons.
• Decrease in oxidation state: The numerical value of the oxidation state of an element goes down.
• Gain of hydrogen: A substance gains hydrogen atoms (historically, in organic chemistry).
• Loss of oxygen: A substance loses oxygen atoms (historically, often seen in metal oxides).

We need to examine each given reaction to determine which one represents a reduction reaction, specifically looking for a substance that is being reduced.

Analyzing Each Reaction Option

Reaction 1: \(2Mg(s) + O_2(g) \rightarrow 2MgO(s)\)

In this reaction, Magnesium (Mg) reacts with Oxygen (O₂) to form Magnesium Oxide (MgO).

• Magnesium (Mg) changes from oxidation state 0 in \(Mg(s)\) to +2 in \(MgO(s)\). This is an increase in oxidation state, so Mg is oxidized.
• Oxygen (O) changes from oxidation state 0 in \(O_2(g)\) to -2 in \(MgO(s)\). This is a decrease in oxidation state, so O₂ is reduced.

This is an oxidation reaction of magnesium.

Reaction 2: \(S(s) + O_2(g) \rightarrow SO_2(g)\)

In this reaction, Sulfur (S) reacts with Oxygen (O₂) to form Sulfur Dioxide (SO₂).

• Sulfur (S) changes from oxidation state 0 in \(S(s)\) to +4 in \(SO_2(g)\). This is an increase in oxidation state, so S is oxidized.
• Oxygen (O) changes from oxidation state 0 in \(O_2(g)\) to -2 in \(SO_2(g)\). This is a decrease in oxidation state, so O₂ is reduced.

This is an oxidation reaction of sulfur.

Reaction 3: \(2HgO\left( s \right)\overset{heat}{\mathop{\to }}\,2Hg\left( l \right)+{{O}_{2}}\left( g \right)\)

In this reaction, Mercury(II) Oxide (HgO) decomposes upon heating to form Mercury (Hg) and Oxygen (O₂).

• Mercury (Hg) in \(HgO(s)\) is in the +2 oxidation state. It changes to oxidation state 0 in \(Hg(l)\). This is a decrease in oxidation state (+2 to 0), so Hg is reduced. Alternatively, \(HgO\) loses oxygen.
• Oxygen (O) in \(HgO(s)\) is in the -2 oxidation state. It changes to oxidation state 0 in \(O_2(g)\). This is an increase in oxidation state (-2 to 0), so O is oxidized.

In this reaction, Mercury(II) Oxide is reduced to Mercury. This reaction fits the definition of a reduction reaction, specifically the reduction of the metal compound.

Reaction 4: \(Mg(s) + S(s) \rightarrow MgS(s)\)

In this reaction, Magnesium (Mg) reacts with Sulfur (S) to form Magnesium Sulfide (MgS).

• Magnesium (Mg) changes from oxidation state 0 in \(Mg(s)\) to +2 in \(MgS(s)\). This is an increase in oxidation state, so Mg is oxidized.
• Sulfur (S) changes from oxidation state 0 in \(S(s)\) to -2 in \(MgS(s)\). This is a decrease in oxidation state, so S is reduced.

While Sulfur is reduced in this reaction, the decomposition of a metal oxide to its metal form (as in option 3) is a more classic example often highlighted as a reduction process.

Identifying the Reduction Reaction

Comparing the reactions, Reaction 3 clearly shows a substance (Mercury(II) Oxide) being reduced to its elemental form (Mercury) by losing oxygen and a decrease in the oxidation state of Mercury. Therefore, the decomposition of Mercury(II) Oxide is a reduction reaction.

Revision Table: Oxidation vs. Reduction

Additional Information on Redox Reactions

Redox reactions are fundamental chemical processes involving the transfer of electrons. They always occur in pairs; one substance is oxidized while another is reduced. The substance that causes oxidation (by accepting electrons) is called the oxidizing agent, and the substance that causes reduction (by donating electrons) is called the reducing agent. In reaction 3, \(HgO\) acts as the oxidizing agent (specifically, the \(Hg^{2+}\) ion is reduced) and it is reduced. While oxygen is produced, this reaction is primarily categorized by the reduction of the metal oxide.