Question

Classify the reactions represented by the following unbalanced equations by as many methods as possible. Balance the equations. a. \(\mathrm{I}_{4} \mathrm{O}_{9}(s) \rightarrow \mathrm{I}_{2} \mathrm{O}_{6}(s)+\mathrm{I}_{2}(s)+\mathrm{O}_{2}(g)\) b. \(\mathrm{Mg}(s)+\mathrm{AgNO}_{3}(a q) \rightarrow \mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\mathrm{Ag}(s)\) c. \(\mathrm{SiCl}_{4}(l)+\mathrm{Mg}(s) \rightarrow \mathrm{MgCl}_{2}(s)+\mathrm{Si}(s)\) d. \(\mathrm{CuCl}_{2}(a q)+\mathrm{AgNO}_{3}(a q) \rightarrow \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\mathrm{AgCl}(s)\) e. \(\mathrm{Al}(s)+\mathrm{Br}_{2}(l) \rightarrow \operatorname{AlBr}_{3}(s)\)

Short answer

(a) Decomposition reaction: \(\mathrm{I}_{4}\mathrm{O}_{9}(s) \rightarrow 2\mathrm{I}_{2}\mathrm{O}_{6}(s) + \mathrm{I}_{2}(s) + \frac{3}{2}\mathrm{O}_{2}(g)\) (b) Single displacement reaction: \(\mathrm{Mg}(s) + 2\mathrm{AgNO}_{3}(a q) \rightarrow \mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}(a q) + 2\mathrm{Ag}(s)\) (c) Single displacement reaction: \(\mathrm{SiCl}_{4}(l) + 2\mathrm{Mg}(s) \rightarrow 2\mathrm{MgCl}_{2}(s) + \mathrm{Si}(s)\) (d) Double displacement reaction: \(\mathrm{CuCl}_{2}(a q) + 2\mathrm{AgNO}_{3}(a q) \rightarrow \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(a q) + 2\mathrm{AgCl}(s)\) (e) Combination reaction: \(2\mathrm{Al}(s) + 3\mathrm{Br}_{2}(l) \rightarrow 2\operatorname{AlBr}_{3}(s)\)

Step-by-step solution

Solution for (a)

First, we will identify the type of reaction. This reaction involves the decomposition of one reactant into several products. Hence, it is a decomposition reaction. Now we will balance the equation: \(\mathrm{I}_{4}\mathrm{O}_{9}(s) \rightarrow 2\mathrm{I}_{2}\mathrm{O}_{6}(s) + \mathrm{I}_{2}(s) + \frac{3}{2}\mathrm{O}_{2}(g)\)

Solution for (b)

This reaction involves a single reactant being replaced by another element (Mg) in a compound. Hence, it is a single displacement reaction. Now we will balance the equation: \(\mathrm{Mg}(s) + 2\mathrm{AgNO}_{3}(a q) \rightarrow \mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}(a q) + 2\mathrm{Ag}(s)\)

Solution for (c)

This reaction involves an element (Mg) replacing another element (Si) in a compound. Hence, it is a single displacement reaction. Now we will balance the equation: \(\mathrm{SiCl}_{4}(l) + 2\mathrm{Mg}(s) \rightarrow 2\mathrm{MgCl}_{2}(s) + \mathrm{Si}(s)\)

Solution for (d)

This reaction involves two compounds exchanging ions to form two new compounds. Hence, it is a double displacement reaction. Now we will balance the equation: \(\mathrm{CuCl}_{2}(a q) + 2\mathrm{AgNO}_{3}(a q) \rightarrow \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(a q) + 2\mathrm{AgCl}(s)\)

Solution for (e)

This reaction involves two elements combining to form a single compound. Hence, it is a combination reaction. Now we will balance the equation: \(2\mathrm{Al}(s) + 3\mathrm{Br}_{2}(l) \rightarrow 2\operatorname{AlBr}_{3}(s)\)

Key concepts

Reaction Classification

Chemical reactions are processes that involve the transformation of one set of chemical substances to another. These transformations can be grouped into different classes or types based on the nature of the reaction occurring. By classifying reactions, it becomes easier to understand and predict the outcomes of chemical processes.

• Decomposition Reactions: A single compound breaks down into two or more simpler substances. For example, in the n ext{reaction } ext{(a)} n ext{ the compound } \( \mathrm{I}_{4} \mathrm{O}_{9} \text{ decomposes into } \mathrm{I}_{2} \mathrm{O}_{6}, \mathrm{I}_{2}, \text{and } \mathrm{O}_{2}. \)


• Single Displacement Reactions: One element displaces another in a compound. Reactions (b) and (c) in our exercise involve single elements (Mg) replacing other elements in compounds.


• Double Displacement Reactions: This involves the exchange of ions between two compounds to form two new compounds, as seen in reaction (d).


• Combination Reactions: Two or more substances combine to form a single new compound. We can see this type in reaction (e) when \( \mathrm{Al} \text{ and } \mathrm{Br}_{2} \text{ form } \mathrm{AlBr}_{3}. \)

Balancing Chemical Equations

Balancing chemical equations is crucial for representing the conservation of mass in a reaction. An equation must have the same number of each type of atom on both sides. This indicates that matter is neither created nor destroyed during a reaction.

• Steps to Balance: Identify the number of atoms of each element in both reactants and products. Adjust the coefficients to get equal numbers on each side. Never change the subscripts, only adjust the coefficients.


• Example: Look at the decomposition in (a). The unbalanced equation \( \mathrm{I}_{4} \mathrm{O}_{9} \rightarrow \mathrm{I}_{2} \mathrm{O}_{6} + \mathrm{I}_{2} + \mathrm{O}_{2} \) is balanced by adjusting coefficients to \( \mathrm{I}_{4} \mathrm{O}_{9} \rightarrow 2\mathrm{I}_{2} \mathrm{O}_{6} + \mathrm{I}_{2} + \frac{3}{2}\mathrm{O}_{2}. \)

This ensures that all iodine and oxygen atoms are accounted for.

Single Displacement Reaction

Single displacement reactions involve one element taking the place of another in a compound, making it a simple yet interesting reaction type.

• Characteristics: Typically involve metals and are often exothermic. A metal in its elemental form displaces another less reactive metal from a compound.


• Example: Consider reaction (b) \( \text{where } \mathrm{Mg} \text{ displaces silver in } \mathrm{AgNO}_{3}, \text{ forming } \mathrm{Mg}(\mathrm{NO}_{3})_{2} \text{ and } \mathrm{Ag}. \)

Observing this allows the understanding of reactivity series and predicting possible reactions.

Double Displacement Reaction

Double displacement reactions occur when parts of two ionic compounds are exchanged and form two new compounds.

• Characteristics: Often take place in aqueous solutions. Usually result in the formation of a precipitate, water, or a gas.


• Example: In the reaction (d), \( \mathrm{CuCl}_{2} \text{ and } \mathrm{AgNO}_{3} \) swap ions, forming \( \mathrm{Cu(NO}_{3})_{2} \text{ and } \mathrm{AgCl}. \)

This type of reaction is vital in processes like precipitation or neutralization in chemistry labs and industrial applications.

Decomposition Reaction

Decomposition reactions involve a single compound breaking down into two or more smaller compounds or elements. When energy, such as heat or electricity, is applied, the compound breaks apart.

• Characteristics: These reactions require energy input to break bonds. The reaction often starts with a single reactant that yields multiple products.


• Example: \( \mathrm{I}_{4}\mathrm{O}_{9} \rightarrow 2\mathrm{I}_{2}\mathrm{O}_{6} + \mathrm{I}_{2} + \frac{3}{2}\mathrm{O}_{2} \) from our exercise is a classic example where the reactant decomposes under appropriate conditions.

Understanding these reactions is pivotal for fields like chemistry and environmental science, where breaking down chemical substances is crucial.