LeChâtelier’s Principle Regaining Equilibrium 6-2

LeChâtelier’s Principle This states that when a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change. Three types of stress are applied: Concentration Pressure and volume Temperature

Concentration When a reactant or product is added to a system at equilibrium, the system shifts away from the added component. (moves in a direction that uses up the excess component) But if a reactant or product is removed, the system shifts toward the removed component.

Changing Concentration If you add reactants (or increase their concentration). The forward reaction will speed up. More product will form. Equilibrium “Shifts to the right” Reactants products

Changing Concentration If you add products (or increase their concentration). The reverse reaction will speed up. More reactant will form. Equilibrium “Shifts to the left” Reactants products

Changing Concentration If you remove products (or decrease their concentration). The forward reaction will speed up. More product will form. Equilibrium “Shifts to the right” Reactants products

Changing Concentration If you remove reactants (or decrease their concentration). The reverse reaction will speed up. More reactant will form. Equilibrium “Shifts to the left”. Reactants  products Used to control how much yield you get from a chemical reaction.

Changing Temperature As you raise the temperature the reaction proceeds in the endothermic direction. (in order to use up the heat) As you lower the temperature the reaction proceeds in the exothermic direction. ( to create more heat) Reactants + heat  Products at high T Reactants + heat  Products at low T

Addition of heat Will cause this reaction to proceed to the left, creating more reactants. Will cause this reaction to proceed to the right, creating more products. N 2 (g) +3H 2 (g) 2NH 3(g) + 92 kJ CaCO 3 (s) kJ CaO (s) + CO 2(g)

Changes in Pressure As the pressure increases the reaction will shift in the direction of the least gases, (the number of moles). At high pressure 2H 2 (g) + O 2 (g) 2 H 2 O(g) At low pressure 2H 2 (g) + O 2 (g)  2 H 2 O(g) 3 moles verses 2: will go to the side with fewer moles

volume When the volume of a gaseous system at equilibrium is decreased, the system shifts in the direction that gives the smaller number of gas molecules opposite to pressure.

CaCO 3 (s) CaO (s) + CO 2(g) decreasing the volume of the following reaction will: shifts the equilibrium to the left ( 1 vs 2)

N 2 (g) +3H 2 (g) 2NH 3(g) PCl 3 (g) + 3NH 3(g) P(NH 2 ) 3 (g) + 3HCl (g) shifts the equilibrium to the right has no effect on equilibrium position Take the equilibrium between 2NO 2 (g) N 2 O 4 (g) brown gasclear gas Which direction is favored by increasing the volume? The right! Products favored

For each of the following predict whether the given change will cause the equilibrium to shift in favour of the products or the reactants, or neither. a) 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) Increase SO 2 Decrease total pressure Inject more O 2 at constant pressure Add a catalyst Decrease volume of the container right left right No change right

b) Ag 2 CrO 4 (s) + heat 2 Ag + (aq) + CrO 4 2- (aq) Add more Ag 2 CrO 4 (s) Increase temperature Add AgNO 3 (aq) Increase pressure Add NaCl(aq) (which precipitates AgCl) right left no change right No change

c) BaCO 3 (s) + heat BaO(s) + CO 2 (g) Add BaCO 3 (s) Increase temperature Add BaO(s) Add CO 2 (g) at constant volume. Increase volume of the container. No change right No change left right

N 2 O 4 (g) + energy 2 NO 2 (g) Addition of N 2 O 4 Addition of NO 2 Removal of N 2 O 4 Removal of NO 2 Decrease volume Increase temperature Decrease temperature Addition of a catalyst right left right left right left No change

Text book questions P: 568 # 5 P: 586 #4, 6,7,11