Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
175
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 17
of hydrogen chloride. Thus, the value of K eq decreases. Lowering
the temperature of the system means that heat is removed, so the
equilibrium relieves the stress by shifting to the right, increasing
both the concentration of hydrogen chloride and K eq .
Practice Problems
8. Phosphorus pentachloride decomposes exothermically to form
phosphorus trichloride and chlorine.
PCl 5 (g) 3 PCl 3 (g) ϩ Cl 2 (g) ϩ heat
How would you regulate the temperature of this equilibrium in
order to do the following?
a. increase the concentration of PCl 5
b. decrease the concentration of PCl 3
c. increase the amount of Cl 2 in the system
d. decrease K eq
9. Predict how this equilibrium would respond to a simultaneous
decrease in both temperature and pressure.
N 2 O 4 (g) 3 2NO 2 (g) ⌬H° ϭ ϩ58 kJ
17.3 Using Equilibrium Constants
When K eq is known, the equilibrium concentration of a substance can
be calculated if you know the concentrations of all other reactants
and products. The following example problem shows you how to
determine an equilibrium concentration.
Example Problem 17-4
Calculating Equilibrium Concentrations
At 350°C, K eq ϭ 66.9 for the formation of hydrogen iodide from its
elements.
H 2 (g) ϩ I 2 (g) 3 2HI(g)
What is the concentration of HI if [H 2 ] ϭ 0.0295 mol/L and
[I 2 ] ϭ 0.0174 mol/L?
Write the equilibrium constant expression.
ϭ K eq
[HI] 2
ᎏ
[H 2 ][I 2 ]