Question

Classify each reaction as oxidation-reduction, acid-base, or precipitation. (a) \(\mathrm{CdCl}_{2}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{~S}(\mathrm{aq}) \longrightarrow \mathrm{CdS}(\mathrm{s})+2 \mathrm{NaCl}(\mathrm{aq})\) (b) \(2 \mathrm{Ca}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{CaO}(\mathrm{s})\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \longrightarrow \mathrm{CaCl}_{2}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)\)

Short answer

(a) Precipitation (b) Oxidation-reduction (c) Acid-base

Step-by-step solution

Analyze Equation (a)

The equation \( \mathrm{CdCl}_{2}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{~S}(\mathrm{aq}) \longrightarrow \mathrm{CdS}(\mathrm{s})+2 \mathrm{NaCl}(\mathrm{aq}) \) involves the formation of a solid \( \mathrm{CdS}(\mathrm{s}) \) from aqueous solutions. This is a characteristic of a precipitation reaction where a solid forms from two aqueous solutions. Therefore, this is a precipitation reaction.

Analyze Equation (b)

The equation \( 2 \mathrm{Ca}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{CaO}(\mathrm{s}) \) involves the reaction of a metal \( \mathrm{Ca} \) with \( \mathrm{O}_{2} \). \( \mathrm{Ca} \) loses electrons (is oxidized) and \( \mathrm{O}_{2} \) gains electrons (is reduced). This exchange of electrons identifies it as an oxidation-reduction (redox) reaction.

Analyze Equation (c)

The equation \( \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \longrightarrow \mathrm{CaCl}_{2}(\mathrm{aq})+2 \mathrm{H}_{2}\mathrm{O}(\ell) \) involves the reaction of \( \mathrm{Ca}(\mathrm{OH})_{2} \), a base, with \( \mathrm{HCl} \), an acid. They neutralize each other to form \( \mathrm{CaCl}_{2} \) and water. Thus, this is an acid-base reaction.

Key concepts

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, are processes where the oxidation state of atoms is changed due to the exchange of electrons. In simple terms, these reactions involve the transfer of electrons from one substance to another. This exchange is what distinguishes these reactions from other types.

In any redox reaction, one reactant will lose electrons, becoming oxidized, while another will gain electrons, becoming reduced. You can remember this process with the mnemonic **"OIL RIG"**: **Oxidation Is Loss**, and **Reduction Is Gain**.

• **Oxidizing Agent:** The substance that gains electrons and is reduced.
• **Reducing Agent:** The substance that loses electrons and is oxidized.

For example, in Reaction (b), calcium (\(\mathrm{Ca}\)) gets oxidized as it loses electrons, and oxygen (\(\mathrm{O}_{2}\) ) is reduced as it gains electrons. This type of reaction is crucial in a variety of applications, from the way batteries work to biological processes like respiration.

Acid-Base Reactions

Acid-base reactions occur when an acidic substance donates hydrogen ions (H⁺) to a base, which accepts them. These reactions are essential in chemistry because they are everywhere, from industrial processes to our digestive system.

The core concept here is neutrality. Acids and bases neutralize each other and produce a salt and water as byproducts in most reactions.

• **Acid:** A substance that increases the concentration of hydrogen ions in solution.
• **Base:** A substance that increases the concentration of hydroxide ions (\( \mathrm{OH}^{-} \)) in solution.

In Reaction (c), calcium hydroxide (\(\mathrm{Ca(OH)}_{2}\)), a base, reacts with hydrochloric acid (\(\mathrm{HCl}\)), an acid. They undergo a neutralization reaction, resulting in calcium chloride (\(\mathrm{CaCl}_{2}\)) and water. This classical form of acid-base reaction illustrates the exchange and balance between an acid and a base.

Precipitation Reactions

Precipitation reactions involve the formation of a solid, known as a precipitate, from the reaction of two aqueous solutions. When these solutions are mixed, the ions of the dissolved substances combine to form an insoluble compound that separates from the solution.

This type of reaction is particularly important in fields like analytical chemistry where it's used to isolate and identify compounds.

• **Soluble:** Substances that dissolve well in a solvent like water.
• **Insoluble:** Substances that do not dissolve well and form precipitates.

In the case of Reaction (a), when cadmium chloride (\(\mathrm{CdCl}_{2}\)) reacts with sodium sulfide (\(\mathrm{Na}_2 \mathrm{S}\)), the product, cadmium sulfide (\(\mathrm{CdS}\)), forms as a solid. This solid is the precipitate that emerges from the solution. Precipitation reactions help in separating substances based on their solubility traits.