Reduction and oxidation reactions

When an atom gains electrons, we say that the atom has been:

• Reduced

• Oxidised

• Deionised

• Decomposed

Answer explanation

Reduction = gains electrons

Oxidation = loses electrons

Two acronyms/mnemonics that people use to help them remember:

"OIL RIG" = "Oxidation Is Loss, Reduction Is Gain"

"LEO the lion says GER" = "Lose Electrons: Oxidation; Gain Electrons: Reduction"

2NaCl → 2Na + Cl₂

This reaction is an example of:

• neither an oxidation nor a reduction reaction.

• an oxidation reaction.

• a reduction reaction.

• an oxidation and reduction reaction.

Answer explanation

In this reaction, the starting reactant is an ionic compound; NaCl consists of Na⁺ and Cl^- ions.

The Na⁺ gains a negatively charged electron to become Na, and the Cl^- loses electrons to become Cl₂. Both reduction and oxidation are occurring.

This holds true for any reaction that involves a transfer of electrons; in order for one species to gain electrons, another species has to lose electrons. Reduction must always be accompanied by oxidation.

Mg + 2AgNO₃ → 2Ag + Mg(NO₃)₂

In this reaction, the species that is being reduced is:

• Mg

• Ag

• Mg²⁺

• Ag⁺

Answer explanation

When identifying the species being oxidised and reduced, compare each element on either side of the equation.

In this reaction, Mg is becoming Mg²⁺, and Ag⁺ is becoming Ag. Thus, Mg is being oxidised to Mg²⁺ (loses electrons, becomes more positive) and Ag⁺ is being reduced to Ag (gains electrons, becomes less positive).

Note that the NO₃^- ion is unchanged throughout the reaction, and is simply changing partners from Ag⁺ to Mg²⁺; it is acting as a "spectator ion".

Zn + CuSO₄ → Cu + ZnSO₄

In this reaction, the species acting as the oxidising agent is:

• Cu²⁺

• Zn²⁺

• Cu

• Zn

Answer explanation

An oxidising agent is a chemical that causes another species to become oxidised. It does this by accepting electrons from the oxidised species, and thus is being reduced.

Cu²⁺ is being reduced to Cu (gains electrons), and is thus the oxidising agent.

Remember:

Oxidising agent = species being reduced

Reducing agent = species being oxidised

SnCl₂ + 2FeCl₃ → SnCl₄ + 2FeCl₂

In this reaction, the species acting as the reducing agent is:

• Sn²⁺

• Sn⁴⁺

• Fe³⁺

• Fe²⁺

Answer explanation

Slightly more complicated as all species are positive cations, but the same principle holds.

Sn²⁺ → Sn⁴⁺ (more positive = loses electrons = oxidised; reducing agent)

Fe³⁺ → Fe²⁺ (less positive = gains electrons = reduced; oxidising agent)

A solution of Pb(NO₃)₂ will not react with:

• Zn

• Fe

• Sn

• Cu

Answer explanation

On p 141 of the textbook:

Metal-ion solutions will not react with less reactive metals (weaker reducing agents). In the reactivity series of metals, the only metals less reactive than Pb are Cu, Ag, and Au. Thus, Cu will not react with a solution of Pb(NO₃)₂.

When magnesium metal is burned in air, it becomes a fine white powder. The magnesium metal has been:

• Oxidised

• Reduced

• Neither oxidised nor reduced

• Both oxidised and reduced

Answer explanation

In combustion, a substance is reacted with oxygen, causing it to form an oxide.

Another way to think of oxidation is a gain of oxygen (thus the name "oxidation"), as it is losing electrons to oxygen in the reaction. Conversely, if a species loses oxygen it is being reduced.