Chapter 17: Chemical Equilibrium Some reactions can proceed in either direction… these are called reversible reactions. Chemical equilibrium is that point where a reaction is balanced between reactants going to products and products going to reactants… it is seldom at 50% of each. The law of equilibrium states that at a particular temperature, the equilibrium point of a reaction is equal to the product of product concentrations over reactant concentrations and has a constant value called Keq.

Equilibrium cont’d With the generalized equation below, notice that the product concentrations are placed over each of the reactant concentrations (in brackets and multiplied…the brackets mean M=[]=mol/L…). If Keq > 1 then products are favored. If Keq < 1 then reactants are favored. Know this for the quiz and test. Write the Keq expression for the balanced equation at the bottom right… aA + bB < - > cC + dD

Equilibrium cont’d When the products and reactants are present in more than one state this represents heterogeneous equilibrium. When the products and reactants are present in the same physical state, this represents homogeneous equilibrium. You should be able to write Keq expressions and solve for Keq , given values.

17-2 Factors Affecting Equilibrium LeChatelier’s Principle states that if a stress is applied to a system, in equilibrium, the system shifts to relieve the stress, to make more products or make more reactants. Adding product or reactant causes the reaction to shift away from the side added to. Adding heat shifts away from the side with heat. Cooling (taking heat) shifts to the side with heat. Adding pressure adds to the side with the most moles of gas and shifts to the other side… reducing pressure takes away from the side with the most moles of gas and shifts to fill on that side.

The Solubility Product Constant The Ksp is the product of the products, where the products are dissociated ions. PbCl2(s) --> Pb2+(aq) + 2 Cl-(aq) Ksp = [Pb2+][Cl-]2 For all solubility calculations, write a Ksp first then look at the type of problem to calculate a solution. See Ksp chart pg 615 or on possibly useful page.

Ksp Cont’d Calculating basic Ksp… Write an equation showing dissociation. Write a Ksp equation. Find the Ksp value and solve for ions using If the value has a coefficient remember that is the power of the ion concentration.

Ksp Cont’d Predicting precipitates… If you have a problem that asks if a precipitate is formed, …. Write a dissociation for the precipitating ions. Plug values into the ions from the problem. Calculate the Qsp which is the temporary Ksp. If the Qsp is higher than the Ksp, there is a precipitate.

Ksp Cont’d If the problem has 2 solutions mentioned and one of the solutions has a ‘common ion’… Calculate the concentration of the common ion by using your basic Ksp calculation. Calculate the ‘other ion’ (not the common one) by adding the 2 sources of the common ion and calculating using the Ksp value from the chart.