Le Châtelier’s Principle Le Chatelier’s Principle: if you disturb an equilibrium, it will shift to undo the disturbance. Remember, in a system at equilibrium, Keq is a constant. 3 Common ways to disturb an equilibrium:
Le Châtelier’s Principle • Effect of Temperature Changes • The equilibrium constant is temperature dependent. • For an endothermic reaction, heat can be considered as a reactant. • For an exothermic reaction, heat can be considered as a product. • Adding heat causes the reaction to shift to get rid of this extra heat
Effect of Temp. Change N2 + O2 2NO ΔHrxn= +180.5kJ What is effect of increasing T?
Effect of Change in Concentration For reaction Butane Isobutane Kc at 25oC = 2.50 What would be the effect of adding 1.5 M butane?
Common Ion Effect • At equilibrium the conc. of butane is 0.2M and Isobutane is 0.5M. If 2.00 mol / L isobutane is added, what will the new equilibrium concentrations be?
Effect of Volume Change in Gas Phase Equilibrium In reactions involving gases, if you change the size of the container, you will also change the gas concentration. The effect of a volume decrease would be to shift the equilibrium to the side with the fewer numbers of gas molecules
In the reaction: 2NO2 N2O4 Kc = 170 at 298K What would be the effect of halving the volume of the container?
Solubility Equilibria The solubilities of slightly soluble compounds can be written as equilibrium expressions. Write the equilibrium expression for the dissolving of silver bromide: Since the reactant is solid, only the products (ions) are involved in the Ksp expression Ksp (solubility product constant):
Ksp Values • Ksp values can be found on charts of Ksp constants at a given temperature • Relative solubilities of compounds can be compared IF the ion ratios in the two compounds are the same, but not if they’re different. • Compare solubilities of: AgCl and AgI • Compare solubilities of: Ca(OH)2 and CaSO4
Solubility Product Calculations Write the equilibrium expression for the dissolving of calcium fluoride: If the solubility of CaF2 is 2.4 x 10-4 mol/L at 25°C, find the Ksp
Solubility Product Calculations The Ksp for barium sulfate is 1.1 x 10-10. Find the solubility in mol/L and g/L.
The Ksp for magnesium fluoride is 5.2 x 10-11. Find the solubility in mol/L.
Common Ion Effect: when an ion common to the equilibrium is added, it affects the equilibrium balance. If you add AgCl to 1.0L of 0.55M NaCl, what mass of AgCl will dissolve? The Ksp of AgCl = 1.8 x 10-10
The Ksp for AgCl is 1.8 x 10-10. If the concentration of silver ions is 1.2 x 10-5, is the solution saturated, or will more dissolve?
The concentration of magnesium ions in solution is 1.5 x 10-6. If NaOH is added to make the hydroxide concentration 1.0 x 10-4, will Mg(OH)2 precipitate form? The Ksp for Mg(OH)2 is 5.6 x 10-12.
A solution of 100.0 mL of 0.0010M AgNO3 is prepared. Will AgCl precipitate if 5.0 mL of 0.025M HCl are added?