Question

Write the chemical equation for the autoionization of \(\mathrm{NH}_{3} .\)

Short answer

The equation is: \[ \text{2 NH}_3 (aq) \rightleftharpoons \text{NH}_4^+ (aq) + \text{NH}_2^- (aq) \]

Step-by-step solution

Identify Autoionization

Autoionization is a process where a molecule or compound reacts with itself to form ions. Here, we need to consider ammonia ( NH_3 g) in this process.

Write the Equation

In the autoionization of ammonia, one NH_3 molecule acts as a Brønsted-Lowry base, accepting a proton from another NH_3 molecule, which acts as a Brønsted-Lowry acid. This results in the formation of NH_4^+ (ammonium ion) and NH_2^- (amide ion). The balanced chemical equation is:\[ \text{2 NH}_3 (aq) \rightleftharpoons \text{NH}_4^+ (aq) + \text{NH}_2^- (aq) \]

Key concepts

Brønsted-Lowry Acid-Base Theory

The Brønsted-Lowry acid-base theory is a fundamental concept in chemistry which describes how acids and bases interact. According to this theory:

• An acid is a substance that can donate a proton (H⁺).
• A base is a substance that can accept a proton.

In the context of autoionization of ammonia, one ammonia molecule ( NH_3 ) donates a proton to another ammonia molecule. The first molecule acts as the Brønsted-Lowry acid because it loses an H⁺. The second molecule acts as the Brønsted-Lowry base as it gains the H⁺.

This interplay results in the formation of two ions: the ammonium ion ( NH_4^+ ) and the amide ion ( NH_2^- ). This simple exchange of protons characterizes the Brønsted-Lowry theory. This theory helps us understand the mechanisms behind many chemical reactions, where proton transfer is a key component.

Chemical Equilibrium

Chemical equilibrium is achieved in a reversible reaction when the rates of the forward and reverse reactions are equal. This means the concentrations of the reactants and products remain constant over time. In the case of the autoionization of ammonia, the equilibrium involves the balance between ammonia molecules transforming into ions and the ions recombining to reform ammonia.

The equilibrium equation can be represented as:\[ 2 \text{NH}_3 (aq) \rightleftharpoons \text{NH}_4^+ (aq) + \text{NH}_2^- (aq) \]

At equilibrium, the concentration of \( NH_3 \), \( NH_4^+ \), and \( NH_2^- \) do not change, even though the molecules continue to interact. The state of equilibrium also indicates that the extent of autoionization is not complete; there will always be both reactants and products present. Understanding chemical equilibrium enables chemists to predict the behavior of reactions under different conditions.

Ion Formation

Ion formation is a critical process in chemistry where neutral atoms or molecules gain or lose electrons, resulting in charged entities called ions. In the autoionization of ammonia, ion formation occurs when two \( NH_3 \) molecules interact:

• One molecule becomes \( NH_4^+ \) (ammonium ion) by adding a proton.
• The other becomes \( NH_2^- \) (amide ion) by losing a proton.

This process results in a net charge balancing effect: the positive charge from the ammonium ion and the negative charge from the amide ion. Ion formation during such autoionization changes the chemical properties of the species involved significantly, as ions usually have different solubilities, reactivities, and conductivities compared to their neutral counterparts. Understanding how ions form helps in predicting and explaining the behavior of substances in various chemical environments.