Le Chatelier's Principle

Practice Problem #1: The reaction (HC2H3O2 <----> H+ + C2H3O2-) is in dynamic equilibrium. If the concentration of H+ is increased, the system will shift to re-establish equilibrium. What will be a result of this shift?

 

Correct Answer: The correct answer is C. More HC2H3O2 will be produced to keep the system "balanced" due to the increased concentration of H+. This is because Q>K after the addition of H+. Remember, the system always wants to re-establish equilibrium. 

 

Practice Problem #2: The reaction (N2 (g) + 3H2 (g) <---> 2NH3 (g)) is in dynamic equilibrium in a sealed piston. If the piston is compressed, in which direction will the reaction shift and why?

 

Correct Answer: The correct answer is B. The reaction will shift to the right because there are less moles of gas in the products than reactants. It does this because it wants to maintain the pressure it had at dynamic equilibrium, and with less volume, less moles of gas are required to produce the same pressure. 

 

Practice Problem #3: The reaction (H2 (g) + I2 (g) <---> 2HI (g)    ΔH = -51.0 kJ) is in dynamic equilibrium and occurs at 100°C. If the reaction is heated an additional 100°C, how will the system respond?

 

Correct Answer: The correct answer is A. The reaction will shift to the left. Since we can look at heat as a product or reactant based on enthalpy, it is as simple as adding to one side and shifting to the other. In this case, the reaction is exothermic and the heat is in the products. This means that when temperature is increased, the reaction will shift away from the heat in order to "cool down." 

 

Practice Problem #4: The reaction (PCl3 (g) + Cl2 (g) <---> PCl5 (g)) is in dynamic equilibrium. It is then released into a larger, sealed container with Ne (g) and PCL5 (g) so that the reaction becomes

(Ne (g) + PCl3 (g) + Cl2 (g) <--->  PCl5 (g) +Ne (g)) and the concentration of PCL5 (g) is greater than it was previously. How will the system respond in order to re-establish equilibrium? 

 

Correct Answer: The correct answer is B. Since Ne (g) is a noble and inert gas, we can totally exclude it from our analysis using Le Chatelier's Principle. PCL5 (g) was added and the system was put into a larger container. Both of these conditions will cause the reaction to shift to the left in order to re-establish equilibrium. It needs  to produce more moles of gas to maintain the same pressure in the larger container and shift away from PCL5 (g) because Q>K.