Question

a. Why is the following reaction called a combination reaction? $$ \mathrm{H}_{2}(g)+\mathrm{Br}_{2}(g) \longrightarrow 2 \mathrm{HBr}(g) $$ b. Why is the following reaction called a double replacement reaction? $$ \mathrm{AgNO}_{3}(a q)+\mathrm{NaCl}(a q) \longrightarrow \mathrm{AgCl}(s)+\mathrm{NaNO}_{3}(a q) $$

Short answer

The reaction \(\text{H}_{2} + \text{Br}_{2} \rightarrow 2 \text{HBr}\) is a combination reaction as it forms a single product from multiple reactants. \(\text{AgNO}_{3} + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_{3}\) is a double replacement reaction as ions are exchanged to form new compounds.

Step-by-step solution

- Analyze the Combination Reaction

A combination reaction involves two or more reactants forming a single product. In the reaction \(\text{H}_{2}(g) + \text{Br}_{2}(g) \rightarrow 2 \text{HBr}(g)\), two gases, \(\text{H}_{2}\) and \(\text{Br}_{2}\), combine to form one gaseous product, \(\text{HBr}\). Therefore, it is called a combination reaction because multiple reactants create one product.

- Analyze the Double Replacement Reaction

A double replacement reaction involves two compounds exchanging ions to form two new compounds. In the reaction \(\text{AgNO}_{3}(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_{3}(aq)\), \(\text{Ag}^{+}\) from \(\text{AgNO}_{3}\) and \(\text{Cl}^{-}\) from \(\text{NaCl}\) form \(\text{AgCl}\), and \(\text{Na}^{+}\) from \(\text{NaCl}\) and \(\text{NO}_{3}^{-}\) from \(\text{AgNO}_{3}\) form \(\text{NaNO}_{3}\). Therefore, this is called a double replacement reaction because ions are exchanged between compounds.

Key concepts

Combination Reaction

A combination reaction is a type of chemical reaction where two or more substances, called reactants, come together to form a single substance, known as the product. This reaction often involves simple molecules combining to form more complex ones. For example, in the reaction, \(\text{H}_{2}(g) + \text{Br}_{2}(g) \rightarrow 2 \text{HBr}(g)\), hydrogen gas (\(\text{H}_{2}\)) and bromine gas (\(\text{Br}_{2}\)) combine to form hydrogen bromide (\(\text{HBr}\)). This is why it is termed a combination reaction.
Combination reactions are significant because they help in the formation of new compounds, which might have properties different from their reactants. These reactions are quite common in the formation of compounds in laboratory settings and in various industrial processes. By understanding how combination reactions work, students can better predict the outcomes of mixing certain chemicals.

Double Replacement Reaction

A double replacement reaction, also known as a double displacement reaction, is a type of chemical reaction where two compounds exchange components to form two new compounds. This exchange often involves ions from the reactants switching places. In the example reaction, \(\text{AgNO}_{3}(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_{3}(aq)\), silver nitrate (\(\text{AgNO}_{3}\)) and sodium chloride (\(\text{NaCl}\)) react to form silver chloride (\(\text{AgCl}\)) and sodium nitrate (\(\text{NaNO}_{3}\)).
During such reactions:

• The positive ion (cation) from one compound switches with the positive ion of the other.
• The new products are often a precipitate, a gas, or a molecular compound such as water.

These reactions are vital, particularly in creating various compounds used in day-to-day applications and in understanding the reactions occurring in natural processes. Knowing how to predict and balance these reactions helps students grasp more complex chemical interactions.

Reactants and Products

In any chemical reaction, the starting substances are known as reactants, and the substances formed as a result of the reaction are called products. Understanding the role of reactants and products is crucial for studying chemical reactions.
For instance, in the combination reaction \(\text{H}_{2}(g) + \text{Br}_{2}(g) \rightarrow 2 \text{HBr}(g)\), hydrogen (\(\text{H}_{2}\)) and bromine (\(\text{Br}_{2}\)) are the reactants that create the product hydrogen bromide (\(\text{HBr}\)). Similarly, in the double replacement reaction \(\text{AgNO}_{3}(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_{3}(aq)\), silver nitrate (\(\text{AgNO}_{3}\)) and sodium chloride (\(\text{NaCl}\)) are the reactants, while silver chloride (\(\text{AgCl}\)) and sodium nitrate (\(\text{NaNO}_{3}\)) are the products.
By recognizing the reactants and products in a reaction, students can determine how substances interact and transform. This foundational knowledge allows them to comprehend more complex reactions and predict the types of products that will result from various chemical interactions.