Question

Using liquid ammonia as a solvent, sodium amide reacts with ammonium chloride in an acid-base neutralization reaction. Assuming that these compounds are completely dissociated in liquid ammonia, write molecular, ionic, and net ionic equations for the reaction. Which substance is the acid and which is the base?

Short answer

Molecular equation: NaNH2 + NH4Cl -> NaCl + NH3 + H2N2. Net ionic equation: NH2- + NH4+ -> NH3 + H2N2. Sodium amide (NaNH2) is the base and ammonium chloride (NH4Cl) is the acid.

Step-by-step solution

Write the molecular equation

First, write the molecular equation for the reaction between sodium amide (NaNH2) and ammonium chloride (NH4Cl) in liquid ammonia: \[\text{NaNH}_2 + \text{NH}_4\text{Cl} \rightarrow \text{NaCl} + \text{NH}_3 + \text{H}_2\text{N}_2\]

Write the complete ionic equation

Next, represent all strong electrolytes as ions to obtain the complete ionic equation: \[\text{Na}^+ + \text{NH}_2^- + \text{NH}_4^+ + \text{Cl}^- \rightarrow \text{Na}^+ + \text{Cl}^- + \text{NH}_3 + \text{H}_2\text{N}_2\]

Write the net ionic equation

Identify and cancel out the spectator ions (ions that do not participate in the reaction). After doing this, write the net ionic equation for the reaction: \[\text{NH}_2^- + \text{NH}_4^+ \rightarrow \text{NH}_3 + \text{H}_2\text{N}_2\]

Identify the acid and the base

In the context of liquid ammonia, the base is sodium amide, NaNH2, which donates a pair of electrons to form NH3 and H2N2. The acid is ammonium chloride, NH4Cl, which accepts a pair of electrons when NH4+ dissociates into NH3 and H+. Hence, NaNH2 is the base and NH4Cl is the acid in this reaction.

Key concepts

Chemical Equations

Understanding the basics of chemical equations involves recognizing the symbols and formulas used to represent substances in a reaction. In a chemical equation, reactants are listed on the left-hand side, while products are on the right, separated by an arrow indicating the direction of the reaction. For the reaction in liquid ammonia between sodium amide (NaNH₂) and ammonium chloride (NH₄Cl), the molecular equation is written showing the exact forms of reactants and products in their uncharged, molecular form.

In our example, the molecular equation is expressed as:
\[\text{NaNH}_2 + \text{NH}_4\text{Cl} \rightarrow \text{NaCl} + \text{NH}_3 + \text{H}_2\text{N}_2\]
Chemical equations follow the law of conservation of mass, meaning the number of atoms for each element should be balanced on both sides of the equation. It's crucial to ensure that the equation is balanced to reflect the matter's conservation during the reaction.

Ionic Equations

Ionic equations take the concept of chemical equations further by representing strong electrolytes as ions. This type of equation is more informative, showing how compounds dissociate in a solvent and interact at the ionic level.

The complete ionic equation for the sodium amide and ammonium chloride reaction in liquid ammonia is:
\[\text{Na}^+ + \text{NH}_2^- + \text{NH}_4^+ + \text{Cl}^- \rightarrow \text{Na}^+ + \text{Cl}^- + \text{NH}_3 + \text{H}_2\text{N}_2\]
In liquid ammonia, sodium amide and ammonium chloride are presumed to dissociate completely to their respective ions. Writing ionic equations is a step toward understanding the fundamental changes at the microscopic level, highlighting the significant species that participate in the reaction.

Net Ionic Equations

Net ionic equations are simplified versions of ionic equations that only show the particles that change during the chemical reaction. They are beneficial for emphasizing the actual chemical changes occurring in the reaction. Spectator ions, which do not participate in the reaction, are omitted because they do not undergo any chemical change.

The net ionic equation for our reaction is:
\[\text{NH}_2^- + \text{NH}_4^+ \rightarrow \text{NH}_3 + \text{H}_2\text{N}_2\]
This equation underscores the direct interaction between the amide ion (NH₂^-) and the ammonium ion (NH₄⁺), leading to the formation of ammonia (NH₃) and hydrazine (H₂N₂). It conveys a clearer picture of the acid-base neutralization reaction taking place in liquid ammonia.

Acid-Base Reactions

Acid-base reactions, such as the neutralization reaction in our example, involve the transfer of a proton (H⁺) from the acid to the base. Liquid ammonia acts as a solvent and can influence the behavior of acids and bases.

In the given scenario, sodium amide (NaNH₂) serves as the base because it provides a pair of electrons to form ammonia (NH₃) and hydrazine (H₂N₂), while the ammonium ion (NH₄⁺) is the acid as it donates a proton to form NH₃.

Recognizing an acid-base reaction in a non-aqueous solvent like liquid ammonia requires understanding the Brønsted-Lowry acid-base theory, which expands the definitions of acids and bases beyond their behavior in water. This theory defines an acid as a substance that can donate a proton, and a base as one that can accept a proton.