Problem 93 - Chemistry The Central Science

Question

Ammonia, \(\mathrm{NH}_{3},\) acts as an Arrhenius base, a Bronsted-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

The reaction between ammonia (NH₃) and water (H₂O) is given by: \[ NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq) \] Ammonia fits all three base definitions: 1. Arrhenius base: It increases the concentration of hydroxide ions (OH⁻) in the solution. 2. Bronsted-Lowry base: It accepts a proton (H⁺) from water, forming ammonium ion (NH₄⁺). 3. Lewis base: It donates a pair of electrons from the nitrogen atom to form a bond with a proton (H⁺) from water, creating NH₄⁺.

Step 1: Write out the reaction between ammonia and water

Write the chemical equation representing the reaction between ammonia (NH₃) and water (H₂O) as follows: \[ NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq) \]

Step 2: Explain how ammonia acts as an Arrhenius base

According to the Arrhenius definition, a base is a substance that increases the concentration of hydroxide ions (OH⁻) in an aqueous solution. From the reaction equation in Step 1, we can see that when ammonia reacts with water, one of the products is a hydroxide ion (OH⁻). Therefore, ammonia meets the criteria of an Arrhenius base as it increases the concentration of OH⁻ ions in the solution.

Step 3: Explain how ammonia acts as a Bronsted-Lowry base

A Bronsted-Lowry base is a substance that can accept a proton (H⁺) from another substance. Analyzing the reaction equation in Step 1, we see that ammonia (NH₃) accepts a proton (H⁺) from water, forming the ammonium ion (NH₄⁺). Therefore, ammonia fulfills the Bronsted-Lowry definition of a base, as it accepts a proton from another substance (water, in this case).

Step 4: Explain how ammonia acts as a Lewis base

A Lewis base is a substance that can donate a pair of electrons to form a bond with another substance. In the reaction between ammonia and water, the nitrogen atom in ammonia has a lone pair of electrons. When the nitrogen atom donates this lone pair of electrons to form a bond with a proton (H⁺) from water, it creates the ammonium ion (NH₄⁺). This ability to donate a pair of electrons to form a bond with another substance (proton in this case) makes ammonia a Lewis base.

Key Concepts

Arrhenius Base

In chemistry, the Arrhenius base definition focuses on the presence of hydroxide ions. An Arrhenius base increases the hydroxide ion concentration

• Arrhenius bases in aqueous solutions boost hydroxide ions (OH⁻).
• Ammonia, when dissolved in water, results in the production of hydroxide ions.

For ammonia, the equation \[ NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq) \] illustrates the increase in the concentration of hydroxide ions.
The produced OH⁻ ions elevate the solution's basicity.
Hence, ammonia is an Arrhenius base since it increases the OH⁻ ions.

Bronsted-Lowry Base

The Bronsted-Lowry theory takes a different angle by focusing on proton transfer. Here, a base is a proton acceptor:

• Protons are simply hydrogen atoms missing an electron, represented by H⁺.
• A Bronsted-Lowry base accepts these protons, forming a new substance.

For ammonia,

• In the equation \( NH_3(aq) \), ammonia accepts the \( H^+ \) from water \( H_2O \).
• This forms the ammonium ion \( NH_4^+ \).

These actions classify ammonia as a Bronsted-Lowry base.

Lewis Base

Under the Lewis theory, bases donate electron pairs. This widens the definition of a base even more:

• A Lewis base is any compound that can donate an electron pair to form a bond.
• This doesn’t involve protons directly, as in Bronsted-Lowry.

For ammonia:

• The nitrogen atom has a lone pair of electrons.
• When ammonia acts with water, it donates this electron pair to an \( H^+ \) ion from water.
• It forms a bond, resulting in an \( NH_4^+ \) ion.

Thus, ammonia's ability to donate its electron pair makes it a Lewis base.