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A.P. Chemistry Chapter Fourteen Outline Chemical Equilibrium Reactants↔Products Chemical Equilibrium: when the concentrations of reactants and products in a chemical reaction remain constant Product favored reactions: when products predominate Reactant favored reactions: when reactants predominate Dynamic Equilibrium: when the rate of the forward reaction is equal to the rate of the reverse reaction Although catalysts speed up reaction, the same equilibrium will be reached I. The equilibrium constant Kc = the equilibrium constant, which is a quotient of equilibrium concentrations of reactants and products for a given reactions at a given temperature The equilibrium constant expression can be obtained by multiplying together the rate constants for the forward reactions in all steps and then dividing by the rate constants for the reverse reaction in all steps II. Writing Equilibrium Constant Expressions The only concentrations that appear in an equilibrium constant expression are those of gases and of solutes in dilute solutions Example: AX(g) + BY(g) → 2CZ(g) Kc = [CZ]2/[AX][BY] Note that the coefficients become powers and the products are on the numerator and reactants on denominator Whenever the stoichiometric coefficients of a balanced equation are multiplied by some factor, the equilibrium constant for the new equation is the original equilibrium constant raised to the power of the multiplication factor Example: N2(g) + 3H2(g) → 2NH3(g) Kc = [NH3]2/[N2][H2]3 if the reaction is doubled than the new K would be the square of the original The equilibrium constant for a reaction and that for its reverse are reciprocals of one another If two chemical equations can be summed to give a third, than the equilibrium constant for the overall equation equals the product of the two equilibrium constants for the equations that were summed III. Determining Equilibrium Constants Write the balanced equation for the equilibrium reaction Write the equilibrium constant expression IV. V. VI. Set up a table containing initial concentrations, change in concentrations, and equilibrium concentrations for each substance included in the equilibrium constant expression. Enter all known info into table Use x to symbolize the change in concentration of one substance. Use the stoichiometric coefficients in the balanced equation to calculate the others in terms of x From initial concentrations and the changes in concentrations calculate the equilibrium concentrations in terms of x and enter it into table Use the simplest equation to solve for x, then use x to calculate what you are looking for Check your answer for reasonableness The Meaning of the Equilibrium Constant If Kc>>1 than the reaction is strongly product favored If Kc<<1, than the reaction is strongly reactant favored Remember that the reverse reaction K is the inverse of the forward reaction K When Kc approximates 1 than calculations are needed to determine equilibrium concentrations If a reaction has a large tendency to occur in one direction than the reverse reaction has little tendency to occur Using Equilibrium Constants To predict the direction of a reaction, first determine the reaction quotient, which has the same mathematical form as the equilibrium expression but is a ratio of actual concentrations in the mixture If Qc= Kc than the reaction is at equilibrium If Qc is less than Kc than the reaction will proceed towards product until equilibrium is reached If Qc > Kc than the reverse reaction will proceed until equilibrium is reached Shifting a Chemical Equilibrium: Le Chatelier’s Principle Le Chatelier’s Principle: Of a system is at equilibrium and the conditions are changed so that it no longer is at equilibrium; the system will react in a way that partially counteracts the change Changing pressure by changing the volume shifts an equilibrium only if the sum of the coefficients for gas phase reactants is different from the sum of the coefficients of the gas phase products For an endothermic reaction, an increase in temperature always means an increase in Kc, the reaction will become more product favored at higher temperatures For an exothermic reaction, an increase in temperature always means a decrease in Kc, the reaction will become less product favored at higher temperatures Entropy is a measure of randomness or disorder. Reaction favor increases in Entropy