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) \(\mathrm{CH}_{3} \mathrm{Br}(g) \underset{\text { UV light }}{\longrightarrow} \mathrm{CH}_{3}(g)+\mathrm{Br}(g)\)

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.

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.

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 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.