Problem 81 - Chemistry and Chemical Reactivity

Question

Identify the spectator ion or ions in the reaction of nitric acid and magnesium hydroxide, and write the net ionic equation. What type of exchange reaction is this? $$\begin{aligned} 2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{NO}_{3}^{-}(\mathrm{aq})+\mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s}) \longrightarrow & \\ 2 \mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{Mg}^{2+}(\mathrm{aq})+2 \mathrm{NO}_{3}^{-}(\mathrm{aq})\end{aligned}$$

Step-by-Step Solution

Answer

The spectator ion is \( \mathrm{NO}_3^- \). The net ionic equation is \( 2 \mathrm{H}^+ (\mathrm{aq}) + \mathrm{Mg(OH)}_2 (\mathrm{s}) \rightarrow 2 \mathrm{H_2O} (\ell) + \mathrm{Mg}^{2+} (\mathrm{aq}) \). This is a neutralization reaction.

Step 1: Identify the Ions

Begin by identifying all ions involved in the reaction equation. From nitric acid, we have \( \mathrm{H}^+ \) and \( \mathrm{NO}_3^- \). Magnesium hydroxide (\( \mathrm{Mg(OH)}_2 \)) is a solid and doesn't dissociate in solution, thus it remains unchanged in its ionic form. In the products, \( \mathrm{H_2O} \) is a liquid and doesn't appear as ionic, while \( \mathrm{Mg}^{2+} \) and \( \mathrm{NO}_3^- \) remain in the aqueous state.

Step 2: Identify Spectator Ions

Spectator ions are those ions that appear unchanged on both sides of the equation. In this case, \( \mathrm{NO}_3^- \) appears on both the reactant and product sides in aqueous form and does not participate in the chemical reaction. Thus, \( \mathrm{NO}_3^- \) is the spectator ion.

Step 3: Write the Net Ionic Equation

The net ionic equation includes only those components that undergo a chemical change. Removing the spectator ion \( \mathrm{NO}_3^- \), we have: \[ 2 \mathrm{H}^+ (\mathrm{aq}) + \mathrm{Mg(OH)}_2 (\mathrm{s}) \rightarrow 2 \mathrm{H_2O} (\ell) + \mathrm{Mg}^{2+} (\mathrm{aq}) \] This equation shows the formation of water from hydrogen ions and magnesium hydroxide.

Step 4: Determine Reaction Type

The reaction between an acid and a base, like nitric acid and magnesium hydroxide, is a neutralization reaction, a type of double displacement reaction where acid and base form water and a salt (though the salt here isn't shown dissociating into spectator ions).

Key Concepts

Spectator Ions in Reactions

In chemistry, not all ions actively participate in a chemical reaction. Some ions remain unchanged throughout the process and are known as spectator ions. These ions are present in the reactants and products in aqueous solutions but do not contribute to the formation of new substances. For instance, in the reaction between nitric acid and magnesium hydroxide, the nitrate ions \( \mathrm{NO}_3^- \) are spectator ions.
They appear on both sides of the reaction equation without undergoing any change.
Identifying spectator ions is essential as it simplifies the reaction to only include the species that chemically interact, leading to the net ionic equation.
To find spectator ions:

• Identify all ions in the reactants and products.
• Find ions that do not change during the reaction.
• Omit these ions when writing the net ionic equation.

By understanding spectator ions, students can focus on the core aspects of a reaction, making chemical equations simpler and easier to interpret.

Neutralization Reaction Overview

Neutralization reactions occur when an acid reacts with a base, typically producing water and a salt. These reactions are pivotal in various chemical applications and everyday processes. For instance, when nitric acid \( \mathrm{H^+} \) reacts with magnesium hydroxide \( \mathrm{Mg(OH)_2} \), water is formed, showcasing the typical neutralization reaction process.
In this reaction:

• The hydrogen ions \( \mathrm{H^+} \) from the acid combine with the hydroxide ions \( \mathrm{OH^-} \) from the base to form water molecules \( \mathrm{H_2O} \).
• Neutralization helps in balancing pH levels and is fundamental in biochemical and industrial processes.

This kind of reaction is a subclass of double displacement reactions, which are characterized not only by the exchange of ions but specifically by the formation of water and the subsequent neutralization effect.

Understanding Double Displacement Reactions

Double displacement reactions involve the exchange of ions between two compounds, resulting in the formation of two new compounds. These reactions are common in aqueous solutions where ionic compounds are involved.
The general form can be summarized as:

• Compounds AB and CD react to form AD and CB.

In the example of nitric acid and magnesium hydroxide, the hydrogen ions from the acid interact with the hydroxide ions from the base, leading to the creation of water, while the remaining ions (magnesium and nitrate) form magnesium nitrate. Even though magnesium nitrate isn't shown as dissociated into ions in some equations, the essence of the reaction type remains: the swapping of ion pairs.
Key characteristics of double displacement reactions include:

• Formation of a precipitate or gas, showing a visible change.
• Neutralization effect in specific reactions, especially those involving acids and bases.

These features highlight the practical significance of these reactions in chemical analysis and synthesis.