Preview Version - To order please call 1.866.422.7310 or email info@edvantageinteractive.com
Consider the following equilibrium: H2(g) + F2(g) 2 HF(g) This equilibrium doesn’t respond to a volume change. It cannot partially restore the pressure by shifting in either direction since there are the same number of gas particles on each side of the equation.
Quick Check 1. According to Le Châtelier’s principle, how will an equilibrium respond to being compressed? _______________________________________________________________________________________ 2. What is partial pressure? _______________________________________________________________________________________ 3. According to Le Châtelier’s principle, how will an aqueous equilibrium respond to being diluted? _______________________________________________________________________________________
Sample Problem — Predicting How an Equilibrium Will Respond to a Decrease in Volume The system described by the equation below is compressed. In what direction will the system shift to restore equilibrium? When equilibrium is restored, how will the number of each type of molecule and the concentration of each substance compare to those before the system was compressed? PCl3(g) + Cl2(g) PCl5(g)
What to Think About 1. Recall that Le Châtelier’s principle says the system will shift to relieve some of the added pressure. 2. Consider the effect the stress has on the system to determine the number of each type of molecule. The stress changed the amount of space that the particles move around in, not the number of particles. Only the system’s response to the stress changes the number of particles. The forward reaction converts two molecules into one molecule. Thus a shift right reduces the total number of particles and the pressure of the system.
Concentration
3. Determine the effect of compression on the concentrations of the substances in the system. The situation regarding concentration is illustrated below: 12 10 8 6 4 2 PCl3 + Cl2 PCl5
How to Do It
The system must shift right to reduce the pressure. The number of PCl5 molecules will increase, while the number of PCl3 and Cl2 molecules will decrease.
Because the system was compressed, every substance has a higher concentration or partial pressure at the new equilibrium than it did at the initial equilibrium. (Although the shift can’t reduce all the chemicals’ concentrations or partial pressures, it is reducing most of them (2/3) by shifting to the right. The result is that some of the added pressure is relieved.)
In this diagram, all the original concentrations are doubled by the compression so the volume must have been halved.
© Edvantage Interactive 2019
Chapter 2 Chemical Equilibrium 87