Le Chatelier’s Principle Or How to Get Away from Stress

Learning Objectives LO 6.8 The student is able to use LeChatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium. LO 6.10 The student is able to connect LeChatelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K.

Le Chatelier’s Principle When a system at equilibrium is subjected to a stress, the equilibrium will tend to shift in the direction to relieve the stress. What kind of “stress” could a chemical reaction be subjected to?? Change in concentration Change in pressure/volume Change in pressure affects which state of matter? Change in temperature Adding a catalyst? NO! Not a stress.

What are the effects of each stress? Change in concentration – system shifts to get back to the same ratio of reactants and products. Remember, the value of Keq will only change if the temperature is changed. Adding or removing a substance does not change the value of Keq. The instant the stress is applied, the value of Q is changed. The system will adjust (shift) to get Q equal to the correct ratio, Keq.

What are the effects of each stress? Add more N2? Equilibrium shifts to the right (). How do the concentrations of each gas compare to before the stress was applied? Add NH3? Eq. shifts to the left (). Take out NH3? Eq. shifts to the right ().

Change in Pressure/Volume – System shifts to equalize the pressure. Note – Changes in pressure and volume are related. A decrease in volume coincides with an increase in pressure. An increase in volume coincides with a decrease in pressure. Changing the pressure/volume only affects gases. Other states do not fill the entire container. Raise the pressure Shifts to side with fewer moles of gas. Since less space is available, the system shifts to the side that has a smaller volume. Lower the pressure Shifts to side with more moles of gas. Side that has a larger volume is favored.

Raise the pressure (lower volume) Eq. shifts  Shift is towards smallest total number of moles of gas, so the eq. shifts away from 4 moles (3 +1) towards 2 moles. Increase the volume (lower pressure) Eq. shifts  Shift equalizes pressure.

Indicate how the system below would respond to the following stresses. SO2 (g) is added. O2 (g) is removed. Pressure is decreased. Volume is decreased.