1.1 What is Chemistry?

19.2. Applications of Equilibrium Constants http://www.ck12.org

equilibrium, at which point the concentration of NO 2 is found to be 0.0204 M. Calculate the equilibrium constant
for the following reaction:

2NO 2 (g)N 2 O 4 (g)

Answer:

First, write the equilibrium constant expression for the given equation:

Kc=[[NON^2 O^4 ]
2 ]^2
We are given the equilibrium concentration of NO 2 , so now we need to find the concentration of N 2 O 4. This can be
accomplished by setting up a table that compares the initial and equilibrium amounts of each reaction component.
This type of table is often called anICE table, where ICE stands for initial, change, equilibrium. Start by drawing
out a blank table for the given reaction:

TABLE19.1:Example 19.4 ICE Table Initial Setup

2NO 2 (g)  N 2 O 4 (g)

Initial amount

Change

Equilibrium amount

Then, fill in the known information:

TABLE19.2:Example 19.4 ICE Table I

2NO 2 (g)  N 2 O 4 (g)

Initial amount 0.2000 mol 0 mol

Change

Equilibrium amount 0.0204 mol

ICE tables are often written in units of concentration. The values that we are using in this table have units of moles.
We are able to use moles rather than concentration because the reaction vessel is a rigid 1-L container, so the number
of moles is actually equal to the concentration (0.200mol =0.200mol/ 1-L =0.200M). As long as the total volume
of the container does not change over the course of the reaction, using either moles or concentration is an acceptable
way to set up this type of problem.

Because we are starting with no product, we know that the amount of product will increase and the amount of
reactant will decrease before an equilibrium is reached. Therefore, we can add the following information:

TABLE19.3:Example 19.4 ICE Table II

2NO 2 (g)  N 2 O 4 (g)

Initial amount 0.2000 mol 0 mol

Change -2x mol +x mol

Equilibrium amount 0.0204 mol x mol

These numbers are related to the coefficients from the balanced equation. If the amount of N 2 O 4 increases byx
moles, the amount of NO 2 must decrease by 2xmoles. Because we know the initial and equilibrium amounts of
NO 2 , we can solve forx: