Dissolving Ammonium Chloride in Tap Water and the Resulting Chemical Reaction

Introduction

Ammonium chloride (NH4Cl) is an ionic salt composed of ammonium (NH4 ) and chloride (Cl-) ions. When mixed with tap water, this common household compound has some interesting chemical behavior. Let's explore what happens when 1.00 grams of ammonium chloride is mixed with 7.5 ml of tap water.

The Solution Formation Process

On mixing ammonium chloride with tap water, the process of dissolution leads to the formation of a colorless solution. The solution feels slightly cold, as the dissolution process is endothermic. The ammonium chloride dissociates into its constituent ions, NH4 and Cl-, which can be observed through this ionic dissociation process.

Chemical Reaction and Hydrolysis

It's important to note that a chemical reaction does occur when ammonium chloride is added to water. This reaction is known as hydrolysis, where water (H2O) reacts with the ammonium ion (NH4 ) to form ammonia (NH3) and hydronium ions (H3O ) in a water-based solution.

#x2192; NH4 H2O ? NH3 H3O

This hydrolysis reaction makes the water slightly acidic due to the presence of hydronium ions. The pH of the solution can be calculated to provide a more precise understanding of its acidic nature.

Calculating the pH of the Solution

Ammonium chloride undergoes partial ionization in water, with a Ka value of 5.6 x 10-10. This partial ionization can be modeled using the following equilibrium:

NH4 H2O ? NH3 H3O

Initial Condition:

NH4 : 1.0 M NH3: 0 M H3O : 1 x 10-7 M

Change Condition:

-x M (NH4 ) x M (NH3) x M (H3O )

Equilibrium Condition:

1.0 - x M (NH4 ) x M (NH3) x M (H3O )

Using the Ka expression for hydrolysis:

Ka [NH3][H3O ] / [NH4 ] (x)(x) / (1.0 - x)

Solving for x:

x 2.36 x 10-5 M

Thus, [H3O ] 2.36 x 10-5 1 x 10-7 2.37 x 10-5 M

And the pH of the solution is:

pH -log[2.37 x 10-5] 4.63

Conclusion and Summary

In summary, when 1.00 grams of ammonium chloride is mixed with 7.5 ml of tap water, a colorless and slightly cool solution is formed due to the endothermic dissolution process. This solution hydrolyzes to form ammonia and hydronium ions, making the water slightly acidic. The pH of the resulting solution is 4.63, indicating its acidic nature. This process of hydrolysis sets ammonium chloride apart from other salts, such as aluminum chloride, which reacts with water to evolve HCl and leave hydroxides behind.