(Much of this material comes courtesy of Chemistry: Principles and Reactions, 7th edition by Masterton, Hurley, and Neth.)
Goal:
To determine pressures of various gases at equilibrium
Procedure:
1. Write and balance the appropriate chemical equation (if it is not already provided).
2. Create an ICE table (as with the acid-base equilibria problems).
3. Determine the information requested.
Several examples are given below.
Example 1: Consider the following reaction:
The initial pressure of phosphorus pentachloride is 0.800 atm, and the equilibrium pressure of chlorine gas is 0.300 atm. Determine the equilibrium pressure constant .
Solution: The reaction is already provided, so Step 1 is bypassed. Moving forward to Step 2, since the equilibrium pressure of chlorine gas is given, and the reaction occurs in a 1:1:1 ratio, not only are there 0.300 atm of chlorine gas present at equilibrium, but there are 0.300 atm of phosphorus trichloride present as well, and 0.300 atm of phosphorus pentachloride must have been depleted. Using this information to create the ICE table yields:
Now, the equilibrium pressure constant expression can be derived; this is done very much like creating an equilibrium concentration constant expression for acids and bases. So doing:
Substituting the known equilibrium values into the above expression gives the following result:
Example 2: Nitrous oxide decomposes to form nitrogen and oxygen gas. Find the equilibrium pressure of nitrous oxide if 1.20 atm of nitrous oxide are initially present and .
Solution: The balanced chemical reaction is not given this time, so this must be dealt first. The following result is obtained:
Now, we proceed to Step 2. The amount of that disappears is unknown, so it can be denoted as . Since the reaction occurs in a 2:1:1 ratio, 2x atm of nitrous oxide are depleted, and x atm each of nitrogen and oxygen gas are produced. Creating an ICE table as before gives:
