Types of Chemical Reactions

Double Displacement (Metathesis) Reactions

A major category of nonredox reactions is the double displacement reaction, also known as a metathesis reaction. In these reactions, the number of reactants is typically equal to the number of products, and two compounds exchange ions to form new combinations.

The general formula is: $\text{A}^+\text{X}^- + \text{B}^+\text{Y}^- \rightarrow \text{A}^+\text{Y}^- + \text{B}^+\text{X}^-$

It is crucial to remember that for the products to be chemically valid, the cation must always be paired with the opposite anion. However, simply mixing two aqueous solutions does not always result in a chemical reaction. For a double displacement reaction to be considered successful, there must be a net change driven by the formation of a precipitate, a gas, or a non-electrolyte/weak electrolyte.

Driving Forces in Aqueous Solutions

When reactions occur in water, we look for specific "driving forces" that pull the reaction forward by removing ions from the aqueous phase.

1. Precipitation Reactions

Precipitation occurs when two aqueous solutions are mixed and an insoluble ionic compound forms. This solid is known as a precipitate. To predict whether a precipitate will form, chemists use solubility rules. For instance, salts containing alkali metal ions ($\text{Li}^+$, $\text{Na}^+$, etc.) or nitrates ($\text{NO}_3^-$) are generally soluble, while many carbonates ($\text{CO}_3^{2-}$) are insoluble.

Example: $\text{NaCl(aq)} + \text{AgNO}_3\text{(aq)} \rightarrow \text{NaNO}_3\text{(aq)} + \text{AgCl(s)}$ In this case, the formation of solid silver chloride ($\text{AgCl}$) is the driving force.

2. Acid–Base (Neutralization) Reactions

An acid-base reaction occurs when an acid reacts with a base to produce a salt and water. These are often called neutralization reactions. The driving force here is the formation of water, which acts as a non-electrolyte (a substance that does not conduct electricity in solution).

General Formula: $\text{Acid} + \text{Base} \rightarrow \text{Water} + \text{Salt}$

Example: $\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{H}_2\text{O(l)} + \text{NaCl(aq)}$

3. Gas-Forming Reactions

In gas-forming reactions, one of the products is a gas that escapes the solution in the form of bubbles. These reactions often occur in two steps involving unstable intermediate species.

• Carbonates/Bicarbonates + Acid: These produce a metal salt, $\text{CO}_2$, and $\text{H}_2\text{O}$. An intermediate, carbonic acid ($\text{H}_2\text{CO}_3$), forms and immediately decomposes: $\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow [\text{CaCl}_2\text{(aq)} + \text{H}_2\text{CO}_3\text{(aq)}] \rightarrow \text{CaCl}_2\text{(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$
• Sulfites + Strong Acid: These produce a metal salt and sulfur dioxide ($\text{SO}_2$) gas.
• Ammonium Salts + Base: These produce a metal salt, ammonia ($\text{NH}_3$), and water. For example: $\text{NH}_4\text{Cl(aq)} + \text{NaOH(aq)} \rightarrow [\text{NaCl(aq)} + \text{NH}_4\text{OH(aq)}] \rightarrow \text{NaCl(aq)} + \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}$

Summary Table of Double Displacement Reactions

To help you study, use the following summary of the nonredox double displacement reactions:

Final Check: When identifying a reaction in a lab, look for these physical cues: temperature changes, color changes, gas bubbles (fizzing), or the appearance of a solid (precipitate).