Problem 65 - Chemistry and Chemical Reactivity

Question

Write the net ionic equation and identify the spectator ion or ions in the reaction of nitric acid and magnesium hydroxide. What type of reaction is this? $$\begin{array}{rl} 2 \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+2 \mathrm{NO}_{3}^{-}(\mathrm{aq})+\mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s}) & \rightarrow \\ 4 \mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{Mg}^{2+}(\mathrm{aq})+2 & \mathrm{NO}_{3}^{-}(\mathrm{aq}) \end{array}$$

Step-by-Step Solution

Answer

Net ionic equation: \( 2 \mathrm{H}_3\mathrm{O}^+ (\mathrm{aq}) + \mathrm{Mg(OH)}_2 (\mathrm{s}) \rightarrow 4 \mathrm{H}_2\mathrm{O}(\ell) + \mathrm{Mg}^{2+}(\mathrm{aq}) \). Spectator ions: \(\mathrm{NO}_3^-\). Type: Acid-base reaction.

Step 1: Write the Complete Ionic Equation

The given balanced molecular equation is already presented with dissociated ions. Rewrite it, showing each compound in its ionic form if applicable: \[ 2\mathrm{H}_3\mathrm{O}^+ (\mathrm{aq}) + 2 \mathrm{NO}_3^- (\mathrm{aq}) + \mathrm{Mg(OH)}_2 (\mathrm{s}) \rightarrow 4 \mathrm{H}_2\mathrm{O}(\ell) + \mathrm{Mg}^{2+}(\mathrm{aq}) + 2 \mathrm{NO}_3^-(\mathrm{aq}) \]\ We notice that nitric acid is presented as hydronium ions (\(\mathrm{H}_3\mathrm{O}^+\)) and nitrate ions (\(\mathrm{NO}_3^-\)) due to dissociation in aqueous solution.

Step 2: Identify and Remove Spectator Ions

Spectator ions do not participate in the reaction. In our equation, nitrate ions \((\mathrm{NO}_3^-)\) are present in the same form on both sides of the equation. Therefore, they are the spectator ions and can be removed. This simplifies the equation as they do not change during the reaction.

Step 3: Write the Net Ionic Equation

After removing the spectator ions \(\mathrm{NO}_3^-\), the net ionic equation consists only of the species that undergo a chemical change. The net ionic equation is then: \[ 2 \mathrm{H}_3\mathrm{O}^+ (\mathrm{aq}) + \mathrm{Mg(OH)}_2 (\mathrm{s}) \rightarrow 4 \mathrm{H}_2\mathrm{O}(\ell) + \mathrm{Mg}^{2+}(\mathrm{aq}) \] This equation shows that hydronium ions react with magnesium hydroxide to form water and magnesium ions.

Step 4: Determine the Type of Reaction

The reaction is an acid-base reaction, also known as a neutralization reaction. Here, hydronium (acid) reacts with magnesium hydroxide (base) to produce water and magnesium ions, effectively neutralizing each other.

Key Concepts

Spectator Ions

In any chemical reaction taking place in an aqueous solution, some ions do not participate in the actual chemical change. These are known as **spectator ions**. They start and end the reaction in the same state, essentially "watching" the action unfold without taking part. In the reaction between nitric acid and magnesium hydroxide, the spectator ions are the nitrate ions \(\mathrm{NO}_3^-\).

Here’s a simple way to spot them: if an ion appears on both sides of the chemical equation in the same form, it’s a spectator ion. Once identified, these ions can be removed when simplifying to the **net ionic equation**, allowing for a clearer view of the molecules and ions that are directly involved in the chemical change.

Removing spectator ions doesn’t alter the product of your experiment – these ions remain the same from start to finish. By focusing solely on the major players, namely the ions and compounds that undergo chemical transformations, you gain insight into the true nature of the reaction.

Acid-Base Reaction

An **acid-base reaction** is a fundamental type of chemical reaction that involves an acid and a base reacting with each other. In our exercise involving nitric acid and magnesium hydroxide, the reaction features hydronium ions \(\mathrm{H}_3\mathrm{O}^+\), which are a form of the acid, and magnesium hydroxide \(\mathrm{Mg(OH)}_2\) as the base.

When these react, the hydronium ions donate protons (hydrogen ions, \(\mathrm{H}^+\)) to the hydroxide ions \((\mathrm{OH}^-\)) within magnesium hydroxide. This transfers of protons is the hallmark of an acid-base reaction. It’s important to note that the hydronium ions \(\mathrm{H}_3\mathrm{O}^+\) often represent acids in a water-based solution because acids dissociate to give these ions in water.

The resulting reaction often forms water and an ionic compound, which in this case is magnesium ions \(\mathrm{Mg}^{2+}\) joining with the departing hydroxide. Acid-base reactions are a cornerstone of chemistry because they’re fundamental to understanding the behavior of substances in various chemical processes.

Neutralization Reaction

A special kind of acid-base reaction is the **neutralization reaction**. It’s aptly named as it results in the neutralization of the acid and base to form a neutral product, usually water, along with a salt. In the experiment with nitric acid and magnesium hydroxide, the net reaction simplifies to the creation of water \((\mathrm{H}_2\mathrm{O})\) and magnesium ions \((\mathrm{Mg}^{2+})\).

In this context of neutralization, the reaction can be summarized as the hydronium ions encountering hydroxide ions, releasing water as the hydrogen ions and hydroxide ions bond together. This reaction helps buffer the solutions, shifting their pH to a more neutral state in many cases, which is why such reactions form a principle foundation towards understanding pH balance in chemistry.

Neutralization reactions are not just limited to lab experiments; they are vital in everyday life, from how cleaning solutions work to how digestion occurs in the human body. Understanding the underlying concept of neutralization empowers better comprehension of how chemicals interact in our world.