Problem 93 - Chemistry: The Central Science in SI Units, Global Edition

Question

Ammonia, \(\mathrm{NH}_{3}\), acts as an Arrhenius base, a Brønsted-Lowry base, and a Lewis base, in aqueous solution. Write out the reaction \(\mathrm{NH}_{3}\) undergoes with water and explain what properties of ammonia correspond to each of the three definitions of "base."

Step-by-Step Solution

Answer

Ammonia reacts with water as follows: \( NH_3 (aq) + H_2O (l) \rightleftharpoons NH_4^+ (aq) + OH^- (aq) \). Ammonia acts as an Arrhenius base because it increases the concentration of hydroxide ions in the solution. It acts as a Brønsted-Lowry base by accepting a proton from water, forming an ammonium ion (NH₄⁺). Finally, ammonia acts as a Lewis base by donating its lone pair of electrons on the nitrogen atom to form a covalent bond with a hydrogen atom from water.

Step 1: Write the reaction of ammonia with water

Ammonia reacts with water as follows: \[ NH_3 (aq) + H_2O (l) \rightleftharpoons NH_4^+ (aq) + OH^- (aq) \] Now, we will analyze this reaction according to the different definitions of bases.

Step 2: Arrhenius Base

An Arrhenius base is a substance that dissociates in water to produce hydroxide ions, OH⁻. In the ammonia-water reaction, we can observe that ammonia accepts a hydrogen ion (H⁺) from water, forming a hydroxide ion (OH⁻) in the process: \[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \] Here, ammonia acts as an Arrhenius base because it increases the concentration of hydroxide ions in the solution.

Step 3: Brønsted-Lowry Base

A Brønsted-Lowry base is a substance that can accept a proton (H⁺) from an acid. In the reaction of ammonia with water, ammonia (NH₃) accepts a proton from water (H₂O), which behaves like an acid, forming an ammonium ion (NH₄⁺): \[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \] In this case, ammonia is a Brønsted-Lowry base because it is a proton acceptor.

Step 4: Lewis Base

A Lewis base is a substance that can donate an electron pair to form a covalent bond with a Lewis acid. Ammonia has a lone pair of electrons on the nitrogen atom that can participate in the formation of a covalent bond: \[ NH_3 \xrightarrow{\text{donating electron pair}} NH_3^- \] When ammonia reacts with water, it uses its lone pair of electrons to form a covalent bond with a hydrogen atom from water: \[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \] Thus, ammonia is a Lewis base because it donates an electron pair to form a covalent bond with a hydrogen atom from water, behaving as a Lewis acid.

Key Concepts

Arrhenius Base

Arrhenius bases are substances that increase the concentration of hydroxide ions (OH⁻) when dissolved in water. These bases typically have components that release or form OH⁻ ions directly.

In the case of ammonia (\(\text{NH}_3\)), when it is dissolved in water, it reacts to form ammonium ions (\(\text{NH}_4^+\)) and hydroxide ions (\(\text{OH}^-\)). The reaction is:

• \(\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\)

The key point here is the production of \(\text{OH}^-\) ions, which is the hallmark of Arrhenius bases.

This production increases the basicity of the solution, and in doing so, \(\text{NH}_3\) conforms to the Arrhenius definition of a base.

Brønsted-Lowry Base

A Brønsted-Lowry base is described as any substance capable of accepting a proton (H⁺) from another substance, referred to as a Brønsted-Lowry acid. In the ammonia-water reaction, the water molecule (\(\text{H}_2 \text{O}\)) acts as a Brønsted-Lowry acid.

Ammonia acts as a base by accepting a proton from the water molecule. This proton transfer is why ammonia can be classified under the Brønsted-Lowry base definition. The reaction is represented by the equation:

• \(\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\)

When \(\text{NH}_3\) accepts the proton, it forms an ammonium ion \(\text{NH}_4^+\).

Therefore, in this context, ammonia is recognized as a proton acceptor, fulfilling the Brønsted-Lowry concept of a base.

Lewis Base

Lewis bases are defined as substances that can donate a pair of electrons to form a covalent bond with a Lewis acid. This definition shifts focus from protons to electrons, adding a different perspective to base behavior.

In the context of the ammonia-water reaction, ammonia functions as a Lewis base. It possesses a lone pair of electrons on its nitrogen atom, enabling it to donate these electrons to interact with a Lewis acid.
This lone pair donation allows ammonia to form a covalent bond with a proton from water, converting \(\text{NH}_3\) into \(\text{NH}_4^+\):

• \(\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\)

Thus, the electron-pair donation by ammonia confirms it as a Lewis base, broadening its classification as a base beyond just proton transfer.