Question

Classify each of the following reactions as a combination reaction, decomposition reaction, displacement reaction, or combustion reaction. (a) When solid calcium oxide, \(\mathrm{CaO}\), is exposed to gaseous sulfur trioxide, \(\mathrm{SO}_{3},\) solid calcium sulfate, \(\mathrm{CaSO}_{4}\), is formed. (b) Calcium metal (solid) reacts with water to produce a solution of calcium hydroxide, \(\mathrm{Ca}(\mathrm{OH})_{2},\) and hydro- gen gas. (c) When solid sodium hydrogen sulfite, \(\mathrm{NaHSO}_{3}\), is heated, solid sodium sulfite, \(\mathrm{Na}_{2} \mathrm{SO}_{3},\) sulfur dioxide gas, \(\mathrm{SO}_{2},\) and water vapor are formed. (d) Magnesium reacts with bromine to give magnesium bromide, \(\mathrm{MgBr}_{2}\)

Short answer

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

Step-by-step solution

Identify a Combination Reaction

A combination reaction occurs when two or more substances combine to form a single compound. Look for cases where there are two reactants forming one product.

Analyze Reaction (a)

Reaction: \( \mathrm{CaO (s) + SO_3 (g) \rightarrow CaSO_4 (s)} \). Two reactants, \(\mathrm{CaO}\) and \(\mathrm{SO}_3\), combine to form one product, \(\mathrm{CaSO}_4\). This is a combination reaction.

Identify a Displacement Reaction

Displacement reactions are those where an element displaces another in a compound, changing the set of compounds.

Analyze Reaction (b)

Reaction: \( \mathrm{Ca (s) + 2H_2O (l) \rightarrow Ca(OH)_2 (aq) + H_2 (g)} \). \(\mathrm{Ca}\) displaces \(\mathrm{H_2}\) in water to form \(\mathrm{Ca(OH)_2}\) and \(\mathrm{H_2}\) gas. This is a displacement reaction.

Identify a Decomposition Reaction

A decomposition reaction involves a single compound breaking down into two or more simpler substances.

Analyze Reaction (c)

Reaction: \( \mathrm{2NaHSO_3 (s) \rightarrow Na_2SO_3 (s) + SO_2 (g) + H_2O (g)} \). A single compound \(\mathrm{NaHSO_3}\) decomposes into \(\mathrm{Na_2SO_3}\), \(\mathrm{SO_2}\), and \(\mathrm{H_2O}\). This is a decomposition reaction.

Analyze Combination of Elements Reaction

When two elements react to form a compound, it is generally a combination reaction.

Analyze Reaction (d)

Reaction: \( \mathrm{Mg (s) + Br_2 (l) \rightarrow MgBr_2 (s)} \). Two elements \(\mathrm{Mg}\) and \(\mathrm{Br_2}\) combine to form one compound, \(\mathrm{MgBr_2}\). This is a combination reaction.

Key concepts

Combination Reaction

In chemistry, a combination reaction is when two or more substances fuse to create a single compound. This type of reaction is quite common and is often seen when elements combine to form a new compound. Let's take a closer look with the example from the exercise:

• When solid calcium oxide, \(\mathrm{CaO}\), is combined with sulfur trioxide gas, \(\mathrm{SO_3}\), they react to form a single product: calcium sulfate, \(\mathrm{CaSO_4}\). Here, two reactants merge into one product—a hallmark of combination reactions.

Combination reactions are important in creating compounds that might be used for industrial purposes or as new materials in various applications. Remember, the key is identifying multiple reactants forming just one product.

Decomposition Reaction

A decomposition reaction is essentially the opposite of a combination reaction. It occurs when a single compound breaks apart into two or more simpler substances. These reactions usually require an input of energy (such as heat) to proceed. Consider the example from the exercise:

• Heating solid sodium hydrogen sulfite, \(\mathrm{NaHSO_3}\), causes it to decompose into solid sodium sulfite, \(\mathrm{Na_2SO_3}\), sulfur dioxide gas \(\mathrm{SO_2}\), and water vapor. Here, one compound breaks into three products, illustrating a decomposition reaction.

Decomposition reactions are often used in processes where breaking down compounds is necessary, such as in waste treatment or recycling operations.

Displacement Reaction

Displacement reactions are interesting because they involve one element replacing another in a compound. This results in the formation of new products with distinct elemental compositions.

• For instance, when calcium metal reacts with water, it displaces hydrogen to form calcium hydroxide \(\mathrm{Ca(OH)_2}\) and hydrogen gas \(\mathrm{H_2}\). This is a displacement reaction, as calcium takes the place of hydrogen within the water molecule.

Displacement reactions are significant in many industrial applications such as the extraction of metals and manufacturing of chemicals. If you're trying to identify a displacement reaction, look for an element and a compound that yield a different element and a new compound.

Chemical Reaction Classification

Chemical reactions can be categorized into different types based on their characteristics and their resulting products. Understanding these classifications can give you a better grasp of how reactions proceed and are utilized in various chemical processes.

• Combination Reactions: Involve two or more reactants forming one product, such as the formation of calcium sulfate \(\mathrm{CaSO_4}\) from calcium oxide and sulfur trioxide.
• Decomposition Reactions: A single compound breaks into two or more products, as observed with sodium hydrogen sulfite breaking down under heat.
• Displacement Reactions: An element displaces another in a compound, such as calcium displacing hydrogen from water.

Understanding these basic classifications not only aids in predicting the products of certain reactions but is also critical for applying chemical principles in real-world scenarios like industrial synthesis and environmental chemistry.