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Chem Notes - LeChatelier's Principle A change in any of the...

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LeChatelier's Principle: A change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner as to reduce or counteract the change. Factors that determine the equilibrium conditions of a system are: 1. Concentration 2. Temperature (K changes) 3. Volume (Pressure) for gaseous systems. Volume When a system is at equilibrium, Q = K. Changing the volume occupied by a gaseous system at equilibrium will change the concentrations of reactant and product gases and will change the pressure. This MAY cause the reactant quotient, Q, to change. Volume changes for systems that have no gases have no effect on the equilibrium. The equilibrium will shift to the side of the reaction with: ... fewer moles of gas if the volume is reduced , thus offsetting the increased pressure. ... more moles of gas if the volume is increased , thus offsetting the reduced pressure. The equilibrium will remain unchanged if the number of moles of gas is the same on both sides of the reaction. The difference between the number of moles of gaseous products and gaseous reactants will tell you how a system behaves when the volume is changed. n gas = (moles product gas - moles reactant gas) If n gas < 0, fewer moles of product gases. Decrease V, the reaction runs forward to reduce the pressure. If n gas > 0, fewer moles of reactant gases. Decrease V, the reaction runs backward to reduce the pressure. If n gas = 0, same number of moles of reactant and product gases. Changing V has no effect on the equilibrium. Example 1: 2 NO 2 (g) N 2 O 4 (g) ...... K = [N 2 O 4 ] / [NO 2 ] 2 Decreasing the volume at constant temperature will cause the concentrations of NO 2 (g) and N 2 O 4 (g) to both increase. The pressure in the system increases because of the higher concentrations. Since there are 2 moles of NO 2 (g) consumed for every mole of N 2 O 4 produced, the concentration increase for NO 2 will have a greater effect (larger denominator). Q is now less than K and the reaction will move to offset this by seeking to reduce the amount of NO 2 and thus the pressure. Example 2: 2 NO(g) N 2 (g) + O 2 (g) ....... K = [N 2 ] [O 2 ] / [NO] 2 Decreasing the volume at constant temperature will cause the concentrations of all three gases to increase. Since there are equal numbers of moles of gas on each side of the reaction, the concentration increases exactly offset one another and there is no effect on the equilibrium system, even though the overall pressure does go up. Example 3: CaCO 3 (s) Ca 2+ (aq) + CO 3 2- (aq) ....... K = [Ca 2+ ] [CO 3 2- ] Changing the volume on this system has no effect on the concentrations of the aqueous ions and thus has no effect on the equilibrium. Concentration When a system is at equilibrium, Q = K. Changing the concentration of reactants or products that are part of the equilibrium constant expression cause the reactant quotient, Q, to change. Q will no longer be equal to K and
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the system will seek to restore itself to a new equilibrium that offsets the change.
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