Question

Classify each reaction in as many ways as possible. (a) \(\mathrm{NaCl}(a q)+\mathrm{AgNO}_{3}(a q) \longrightarrow\) \(\mathrm{AgCl}(s)+\mathrm{NaNO}_{3}(a q)\) (b) \(2 \mathrm{Rb}(s)+\mathrm{Br}_{2}(g) \longrightarrow 2 \mathrm{RbBr}(s)\) (c) \(\mathrm{Zn}(s)+\mathrm{NiBr}_{2}(a q) \longrightarrow \mathrm{Ni}(s)+\mathrm{ZnBr}_{2}(a q)\) (d) \(\mathrm{Ca}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(a q)+\mathrm{H}_{2}(g)\)

Short answer

Reaction (a) is a double displacement and precipitation reaction. Reaction (b) is a synthesis and redox reaction. Reaction (c) is a single displacement and redox reaction. Reaction (d) is a single displacement, redox, metal-water, and gas evolution reaction.

Step-by-step solution

Classify Reaction (a)

The reaction between NaCl(aq) and AgNO3(aq) results in the formation of a solid precipitate, AgCl(s), and a remaining aqueous solution of NaNO3(aq). This reaction is classified as a double displacement reaction, also known as metathesis, and as a precipitation reaction since it produces a solid from two aqueous solutions.

Classify Reaction (b)

The reaction between solid rubidium (Rb) and gaseous bromine (Br2) forms solid rubidium bromide (RbBr). This reaction is classified as a synthesis or combination reaction because two reactants combine to form a single product. It is also a redox reaction as rubidium gets oxidized and bromine gets reduced.

Classify Reaction (c)

The reaction involving solid zinc (Zn) and aqueous nickel bromide (NiBr2) yields solid nickel (Ni) and aqueous zinc bromide (ZnBr2). This reaction is classified as a single displacement reaction, where one element (Zn) displaces another in a compound (Ni in NiBr2). It is also a redox reaction because there is an exchange of electrons between Zn and NiBr2.

Classify Reaction (d)

The reaction where calcium (Ca) reacts with water (H2O) to form calcium hydroxide (Ca(OH)2) and hydrogen gas (H2) is a single displacement reaction and a redox reaction. Moreover, it is a type of chemical reaction named after the specific metal involved, called a metal-water reaction, and since a new compound and a gas are produced it can also be classified as a gas evolution reaction.

Key concepts

Double Displacement Reaction

When two compounds exchange parts to form two new compounds, we observe what is known as a double displacement reaction. These reactions often occur in aqueous solutions where ions are free to move and swap partners. A classic example is the reaction between sodium chloride (NaCl) and silver nitrate (AgNO3), where the ions switch places, resulting in the formation of silver chloride (AgCl) and sodium nitrate (NaNO3). Silver chloride precipitates out of the solution as a solid, giving this reaction a characteristic change easy to spot in the lab.

Double displacement reactions are vital in chemistry because they can be used for many applications, like water purification, ingredient swapping in pharmaceutical synthesis, and simply as a tool to study reaction dynamics.

Precipitation Reaction

A precipitation reaction involves the formation of a solid, termed a precipitate, when two aqueous (water-based) solutions are mixed. For instance, when a solution of NaCl is mixed with AgNO3, insoluble AgCl forms and falls out of the solution. This reaction is pivotal in the field of analytical chemistry where it helps in qualitative analysis to identify the presence of specific ions in a solution.

You might encounter precipitation reactions not only in the lab but also in nature or in various industries, such as when hard water leads to the build-up of limescale, which is the result of a precipitation reaction occurring within water pipes or boilers.

Redox Reaction

A redox reaction is a chemical process in which one substance is oxidized (loses electrons) while another is reduced (gains electrons). Redox is a contraction of 'reduction-oxidation.' In the synthesis of rubidium bromide (RbBr), rubidium atoms lose electrons to become oxidized, while bromine molecules gain electrons to become reduced. These reactions are fundamental to a vast array of processes, including those that happen in batteries, natural biological mechanisms, and even the rusting of iron. Understanding redox reactions is important not just for your chemistry class, but for grasping how energy is transferred and transformed in the world around us.

Single Displacement Reaction

This type of reaction involves an element displacing another element within a compound, resulting in a new element and a new compound. A primary school textbook example would be zinc (Zn) reacting with nickel bromide (NiBr2). Zinc displaces nickel from its compound to form zinc bromide (ZnBr2) and elemental nickel (Ni). Single displacement reactions are not only theoretically interesting, but they are also practical. They are utilized in metal extraction and purification, making them crucial for the metallurgy industry.

Synthesis Reaction

When two or more reactants come together to form a single product, we call it a synthesis reaction. In our case study, rubidium (Rb) reacts with bromine (Br2) to yield rubidium bromide (RbBr). These reactions are integral to the manufacturing of a multitude of substances, ranging from simple salts to complex polymers. They are also the types of reactions that often happen spontaneously in nature, helping to form the very materials that make up the Earth.

Metal-Water Reaction

Reactions between metals and water can be quite energetic and are a specific subset of single displacement reactions. The reaction of calcium (Ca) with water (H2O) to produce calcium hydroxide (Ca(OH)2) and hydrogen gas (H2) is a perfect example. These reactions are not only important for academic study but also in understanding and controlling corrosion, as many metals react with moisture in the environment. Moreover, the hydrogen gas produced in these reactions is of interest for energy storage and fuel applications.

Gas Evolution Reaction

Some chemical reactions result in the formation of a gas, which is observable as bubbling or fizzing in the reaction mixture. This is true for the reaction between calcium and water in which hydrogen gas is released. Gas evolution reactions are common in both laboratory and industrial settings and provide visual cues that a reaction is taking place. These types of reactions are utilized in the design of airbags, carbonated beverages, and in various laboratory tests to identify substances based on the gas evolved.