Question

Classify the following redox reactions as combination. decomposition, or displacement: (a) \(2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (b) \(\mathrm{Mg}+2 \mathrm{AgNO}_{3} \longrightarrow \mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}+2 \mathrm{Ag}\) (c) \(\mathrm{NH}_{4} \mathrm{NO}_{2} \longrightarrow \mathrm{N}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{H}_{2}+\mathrm{Br}_{2} \longrightarrow 2 \mathrm{HBr}\)

Short answer

(a) Decomposition, (b) Displacement, (c) Decomposition, (d) Combination.

Step-by-step solution

Understand Reaction Types

There are three types of reactions we'll classify here: combination (two or more substances combine to form a single product), decomposition (a single substance breaks down into two or more products), and displacement (an element displaces another in a compound).

Classify Reaction (a)

For reaction (a) \(2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\), identify that a single compound, \(\mathrm{H}_{2} \mathrm{O}_{2}\), decomposes into two different substances, \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{O}_{2}\). This is a decomposition reaction.

Classify Reaction (b)

For reaction (b), \(\mathrm{Mg}+2 \mathrm{AgNO}_{3} \longrightarrow \mathrm{Mg}(\mathrm{NO}_{3})_{2}+2 \mathrm{Ag}\), notice that \(\mathrm{Mg}\) displaces \(\mathrm{Ag}\) in the compound \(\mathrm{AgNO}_{3}\), thus forming \(\mathrm{Mg}(\mathrm{NO}_{3})_{2}\) and \(\mathrm{Ag}\). This is a displacement reaction.

Classify Reaction (c)

For reaction (c), \(\mathrm{NH}_{4} \mathrm{NO}_{2} \longrightarrow \mathrm{N}_{2}+2 \mathrm{H}_{2} \mathrm{O}\), the single compound \(\mathrm{NH}_{4} \mathrm{NO}_{2}\) breaks apart into two substances: \(\mathrm{N}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\). Thus, this is a decomposition reaction.

Classify Reaction (d)

For reaction (d), \(\mathrm{H}_{2}+\mathrm{Br}_{2} \longrightarrow 2 \mathrm{HBr}\), two different substances, \(\mathrm{H}_{2}\) and \(\mathrm{Br}_{2}\), combine to form a single product, \(\mathrm{HBr}\). Hence, this is a combination reaction.

Key concepts

Combination Reactions

Combination reactions are fascinating processes in chemistry where two or more substances unite to form a single, more complex product. These reactions are also known as synthesis reactions because new compounds are synthesized from simpler ones. A classic example is when hydrogen gas \(\mathrm{H}_{2}\) reacts with bromine gas \(\mathrm{Br}_{2}\) to produce hydrobromic acid, which is represented by the equation \(\mathrm{H}_{2}+\mathrm{Br}_{2} \longrightarrow 2 \mathrm{HBr}\). Here, the \(\mathrm{H_{2}}\) molecules combine with \(\mathrm{Br_{2}}\) molecules, forming a single compound, hydrogen bromide, \(\mathrm{HBr}\).
Key points about combination reactions include:

• Two or more reactants form one product.
• These reactions are typically exothermic, meaning they release energy in the form of heat or light.
• Combination reactions are frequently observed in nature and everyday processes, such as rust formation, where iron combines with oxygen to form iron oxide.

Decomposition Reactions

Decomposition reactions involve the breakdown of a single compound into two or more simpler substances. These reactions are essential in both industrial and biological processes, often driven by energy input in the form of heat, light, or electricity.
An example is the decomposition of hydrogen peroxide as seen in the equation \(2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\). In this reaction, hydrogen peroxide \(\mathrm{H}_{2} \mathrm{O}_{2}\) decomposes into water \(\mathrm{H}_{2} \mathrm{O}\) and oxygen gas \(\mathrm{O}_{2}\). This type of reaction is important in various applications, including the use of hydrogen peroxide as a disinfectant.
Important characteristics of decomposition reactions include:

• The reactant is a single compound, which dissociates into multiple products.
• These reactions can be endothermic, requiring energy input.
• Decomposition reactions are pivotal in natural processes like the decomposition of organic matter.

Displacement Reactions

Displacement reactions are intriguing chemical processes where one element replaces another in a compound. This exchange often results in the formation of new products and is a signal of activity energy or reactivity between substances.
For instance, in the reaction \(\mathrm{Mg}+2 \mathrm{AgNO}_{3} \longrightarrow \mathrm{Mg}(\mathrm{NO}_{3})_{2}+2 \mathrm{Ag}\), magnesium \(\mathrm{Mg}\) displaces silver \(\mathrm{Ag}\) from silver nitrate \(\mathrm{AgNO}_{3}\) forming magnesium nitrate and silver as the products. This type of reaction highlights the reactivity series of elements, where magnesium, a more reactive metal, easily replaces silver.
Key points about displacement reactions include:

• An element in a compound is replaced by another element.
• These reactions generally occur in solutions or mixtures.
• Displacement reactions are used in metallurgical processes and the extraction of metals.