Question

Specify which of the following are oxidation–reduction reactions, and identify the oxidizing agent, the reducing agent, the substance being oxidized, and the substance being reduced. a. \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(a q) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)\) b. \(\mathrm{HCl}(g)+\mathrm{NH}_{3}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\) c. \(\mathrm{SiCl}_{4}(i)+2 \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 4 \mathrm{HCl}(a q)+\mathrm{SiO}_{2}(s)\) d. \(\mathrm{SiCl}_{4}(l)+2 \mathrm{Mg}(s) \rightarrow 2 \mathrm{MgCl}_{2}(s)+\mathrm{Si}(s)\) e. \(\mathrm{Al}(\mathrm{OH})_{4}-(a q) \rightarrow \mathrm{AlO}_{2}^{-}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\)

Short answer

For the given reactions: a) Oxidation-reduction reaction. Oxidizing agent: Ag+, Reducing agent: Cu, Substance being oxidized: Cu, Substance being reduced: Ag+. b) Not an oxidation-reduction reaction. c) Not an oxidation-reduction reaction. d) Oxidation-reduction reaction. Oxidizing agent: SiCl4, Reducing agent: Mg, Substance being oxidized: Mg, Substance being reduced: SiCl4. e) Not an oxidation-reduction reaction.

Step-by-step solution

Determine the oxidation states of elements in each reaction

In order to determine if an oxidation-reduction reaction has occurred, we must first find the oxidation states of each element in the reactants and products. Remember that the oxidation state is the positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

Compare oxidation states before and after the reaction

If the oxidation state of an element changes during the reaction, this implies a transfer of electrons and thus an oxidation-reduction reaction. a. \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(a q) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)\) Oxidation states: Cu: 0 -> +2, Ag: +1 -> 0 Cu is oxidized, and Ag is reduced. The reaction is an oxidation-reduction reaction. The oxidizing agent is Ag+, the reducing agent is Cu, the substance being oxidized is Cu, and the substance being reduced is Ag+. b. \(\mathrm{HCl}(g)+\mathrm{NH}_{3}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\) Oxidation states: H: +1 -> +1, Cl: -1 -> -1, N: -3 -> -3, H in NH3: +1 -> +1 No change in oxidation states. This reaction is not an oxidation-reduction reaction. c. \(\mathrm{SiCl}_{4}(i)+2 \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 4 \mathrm{HCl}(a q)+\mathrm{SiO}_{2}(s)\) Oxidation states: Si: +4 -> +4, Cl: -1 -> -1, H: +1 -> +1, O: -2 -> -2 No change in oxidation states. This reaction is not an oxidation-reduction reaction. d. \(\mathrm{SiCl}_{4}(l)+2 \mathrm{Mg}(s) \rightarrow 2 \mathrm{MgCl}_{2}(s)+\mathrm{Si}(s)\) Oxidation states: Si: +4 -> 0, Cl: -1 -> -1, Mg: 0 -> +2 Si is reduced, and Mg is oxidized. The reaction is an oxidation-reduction reaction. The oxidizing agent is SiCl4, the reducing agent is Mg, the substance being oxidized is Mg, and the substance being reduced is SiCl4. e. \(\mathrm{Al}(\mathrm{OH})_{4}-(a q) \rightarrow \mathrm{AlO}_{2}^{-}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\) Oxidation states: Al: +3 -> +3, O in OH: -2 -> -2, H: +1 -> +1, O in AlO2: -2 -> -2 No change in oxidation states. This reaction is not an oxidation-reduction reaction.

Key concepts

Oxidizing Agent

In an oxidation-reduction reaction, the oxidizing agent plays a crucial role by gaining electrons from another substance. This process facilitates the oxidation of the other reactant. Consider the reaction:

• \[ \mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(aq) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(aq) \]

In this example, the silver ions (\(\mathrm{Ag}^+\)) act as the oxidizing agent. They are reduced by accepting electrons from copper (Cu), which in this case is oxidized. This acceptance of electrons is what defines them as the oxidizing agent. Always remember, the oxidizing agent experiences a decrease in its oxidation state.

Reducing Agent

The reducing agent in an oxidation-reduction reaction donates electrons and thus helps to reduce another substance. By donating electrons, the reducing agent itself becomes oxidized. Taking the same reaction:

• \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(aq) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(aq) \)

Copper (Cu) acts as the reducing agent. It donates electrons to silver ions, causing the silver to be reduced. As a result, copper loses electrons, which is why it is oxidized. This dual process highlights the interconnected nature of oxidation and reduction in these reactions.

Oxidation State

The oxidation state, also known as oxidation number, indicates the degree of oxidation or reduction of an atom in a substance. This is represented as a positive or negative number. For example, in:

• \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(aq) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(aq) \)

Copper changes its oxidation state from 0 to +2, indicating that it is oxidized. On the other hand, silver changes from +1 to 0, illustrating reduction. Keeping track of these changes in oxidation states is essential to identifying oxidation-reduction reactions and determining the substances involved.

Reduction

In chemical reactions, reduction refers to the gain of electrons by a molecule, atom, or ion. As a result, the oxidation state of the substance decreases. Using:

• \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(aq) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(aq) \)

The silver ions (\(\mathrm{Ag}^+\)) undergo reduction when they gain electrons from copper, forming metallic silver (Ag). This electron gain decreases the oxidation state of silver from +1 to 0. Reduction always accompanies oxidation, as seen in such reactions.

Electron Transfer

Electron transfer is the fundamental process driving oxidation-reduction reactions. It involves the movement of electrons from one reactant (the reducing agent) to another (the oxidizing agent). In the reaction of copper and silver ions:

• \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(aq) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(aq) \)

Copper atoms donate electrons to silver ions, exemplifying electron transfer. The electrons move from copper, which becomes oxidized, to silver ions, which become reduced. Recognizing this transfer helps us understand the dynamics of chemical reactions and the transformations that elements undergo.