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 6.9 The student is able to use LeChatelier’s principle to design a set of conditions that will optimize a desired outcome, such as product yield. 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.

LeChatelier’s and Temperature Unlike changes to concentrations or changes to pressure/volume, the stress of a temperature change does not instantly change the value of Q. Instead, the value of Keq is changed when the system is subjected to a different temperature.

LeChatelier’s and Temperature In any equilibrium system, there will be both an exothermic and endothermic reaction. If the forward reaction is endothermic, the reverse reaction must be exothermic. A + B  C ΔH = + 32.5 kJ/mol C  A + B ΔH = - 32.5 kJ/mol

LeChatelier’s and Temperature An increase in temperature provides energy for the endothermic reaction to happen more. If the forward reaction is endothermic, it will be favored by an increase in temperature. Keq will increase. If the forward reaction is exothermic, it will be hindered by an increase in temperature (the reverse reaction will be favored). Keq will decrease.

N2(g) + 3 H2(g)  2 NH3(g)+ 92 kJ In what direction will the system have to shift after the following stresses are applied? H2 is added to the system. NH3 is added to the system. The pressure is increased. The volume is increased. The temperature is increased.