Ammonium Carbonate Decomposes As Follows Calculate The Equilibrium Constant

The decomposition of ammonium carbonate (NH4)2CO3 is an important chemical reaction in equilibrium chemistry. This guide explains how to calculate the equilibrium constant for this reaction using the Law of Mass Action.

Introduction

The decomposition of ammonium carbonate follows this balanced chemical equation:

(NH4)2CO3(s) ⇌ 2NH3(g) + H2O(g) + CO2(g)

This reaction is endothermic and proceeds to the right as temperature increases. The equilibrium constant (K) for this reaction is a measure of the position of equilibrium at a given temperature.

Equilibrium Constant Formula

The equilibrium constant for a gas-phase reaction is calculated using the Law of Mass Action:

K = [NH3]² × [H2O] × [CO2]

Where:

K = equilibrium constant
[NH3] = concentration of ammonia gas
[H2O] = concentration of water vapor
[CO2] = concentration of carbon dioxide

Note that the solid ammonium carbonate does not appear in the equilibrium expression because its concentration does not change significantly during the reaction.

Calculation Example

Suppose we have a system at equilibrium where:

Concentration of NH3 = 0.20 M
Concentration of H2O = 0.15 M
Concentration of CO2 = 0.10 M

Using the equilibrium constant formula:

K = (0.20)² × 0.15 × 0.10 = 0.006

Therefore, the equilibrium constant for this system is 0.006.

Interpreting the Result

The equilibrium constant value of 0.006 indicates that the reaction favors the formation of solid ammonium carbonate. A small K value means the reactants are favored at equilibrium.

Factors that affect the equilibrium constant include:

Temperature (the reaction is endothermic)
Pressure (since gases are involved)
Initial concentrations of reactants and products

Frequently Asked Questions

What units are used for the equilibrium constant?

The equilibrium constant has no units because it's a ratio of concentrations. The units cancel out in the calculation.

How does temperature affect the equilibrium constant?

For endothermic reactions like this one, increasing temperature shifts the equilibrium to the right (products favored). The equilibrium constant increases with temperature.

Why doesn't solid ammonium carbonate appear in the equilibrium expression?

Solids and liquids don't appear in equilibrium expressions because their concentrations are constant and don't change during the reaction.