Question

Classify each chemical reaction as a synthesis, decomposition, single- displacement, or double-displacement reaction. (a) \(\mathrm{K}_{2} \mathrm{~S}(a q)+\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}(a q) \longrightarrow 2 \mathrm{KNO}_{3}(a q)+\mathrm{CoS}(s)\) (b) \(3 \mathrm{H}_{2}(g)+\mathrm{N}_{2}(g) \longrightarrow 2 \mathrm{NH}_{3}(g)\) (c) \(\mathrm{Zn}(s)+\mathrm{CoCl}_{2}(a q) \longrightarrow \mathrm{ZnCl}_{2}(a q)+\mathrm{Co}(s)\) (d) \(\mathrm{CH}_{3} \mathrm{Br}(g) \underset{\text { UV light }}{\longrightarrow} \mathrm{CH}_{3}(g)+\mathrm{Br}(g)\)

Short answer

(a) Double-displacement reaction, (b) Synthesis reaction, (c) Single-displacement reaction, (d) Decomposition reaction.

Step-by-step solution

Identify the Type of Reaction in (a)

In reaction (a), two compounds react to form two different compounds. This is indicative of a double-displacement reaction, where parts of two ionic compounds are exchanged.

Identify the Type of Reaction in (b)

In reaction (b), multiple reactants (elements) combine to form a single compound. This is characteristic of a synthesis reaction, where simpler substances combine to form a more complex compound.

Identify the Type of Reaction in (c)

Reaction (c) shows one element (Zn) and one compound reacting to form a new compound and a new element. This is a single-displacement reaction, where one element replaces another in a compound.

Identify the Type of Reaction in (d)

Reaction (d) demonstrates a single compound breaking down into its constituent elements under UV light. This is an example of a decomposition reaction, where a compound breaks down into simpler substances.

Key concepts

Synthesis Reaction

A synthesis reaction, also known as a combination reaction, is a fundamental type of chemical change where two or more reactants combine to form a single product. This process usually involves elements or simple compounds that join together to form a more complex compound.

For example, when hydrogen gas (\text{H}_2) and nitrogen gas (\text{N}_2) react together, they form ammonia (\text{NH}_3), which can be represented by the chemical equation: \[\begin{equation}3 \text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\text{H}_2(g) + \text{N}_2(g) \longrightarrow 2 \text{NH}_3(g)\end{equation}\]This reaction is an essential process in the Haber process for producing ammonia, a valuable chemical in agricultural fertilizers. Synthesis reactions are vital in both industrial applications and the natural formation of compounds.

Decomposition Reaction

A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. These can be elements or simpler compounds and usually require an input of energy in the form of heat, light, or electricity.

An example is the decomposition of methyl bromide (\text{CH}_3\text{Br}) under UV light, which breaks down into methane (\text{CH}_3) and bromine gas (\text{Br}_2). The equation for this reaction is: \[\begin{equation}\text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g) \text{CH}_3\text{Br}(g)\underset{\text{ UV light }}{\longrightarrow} \text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\text{CH}_3(g)+\text{Br}_2(g)\end{equation}\]Decompositions are often involved in chemical tests when heating a substance to see its components, as well as in the body when complex molecules are broken down into simpler forms for digestion.

Single-Displacement Reaction

In single-displacement reactions, one element replaces another element in a compound, resulting in a new element and a new compound. These reactions occur when a more reactive element displaces a less reactive element from its compound.

The reaction between zinc metal (\text{Zn}) and cobalt chloride (\text{CoCl}_2) is a classic example of this type, and can be represented by the following equation: \[\begin{equation}\text{Zn}(s) + \text{CoCl}_2(aq) \longrightarrow \text{ZnCl}_2(aq) + \text{Co}(s)\end{equation}\]This type of reaction is often used to demonstrate reactivity series in chemistry, showcasing which metals can displace others from solutions of their salts. Single-displacement reactions are also utilized in the extraction of metals from their ores and in electrochemistry.

Double-Displacement Reaction

Double-displacement reactions involve the exchange of parts between two compounds, resulting in the formation of two new compounds. Also known as metathesis, these reactions typically occur in aqueous solutions where the cations and anions of two different compounds swap places.

A reaction between potassium sulfide (\text{K}_2\text{S}) and cobalt(II) nitrate (\text{Co(NO}_3)_2) is an example of a double-displacement reaction. This can be seen in the following balanced chemical equation: \[\begin{equation}\text{K}_2\text{S}(aq) + \text{Co(NO}_3)_2(aq) \longrightarrow 2 \text{KNO}_3(aq) + \text{CoS}(s)\end{equation}\]These reactions are important for understanding the reactions that occur in precipitation, where the formation of a solid, or precipitate, is often observed, and in neutralization reactions between acids and bases that result in the formation of water and a salt.