Chapter 14 Chemical Equilibrium 14 1 Fetal Hemoglobin and Equilibrium o Equilibrium Constant K The concentrations of the reactants and products in a reaction at equilibrium A large value of K means the reaction lies far to the right at equilibrium A small value of K means that the reaction lies far to the left at equilibrium The larger K the more the reaction favors the products K is a measure of how far a reaction proceeds o Any system at equilibrium acts to maintain that equilibrium If any of the concentrations of the reactants products change the reaction shifts to counteract the change Ex Hemoglobin and Oxygen 14 2 The Concept of Dynamic Equilibrium o Reaction rates generally increase with increasing concentration of the reactants unless the reaction is zero order and decrease with decreasing concentration of the reactants o A reaction that can proceed in both the forward and reverse directions is said to be reversible o A reactant concentration decrease in turn decreases the rate of the forward reaction o Dynamic Equilibrium The condition in which the rate of the forward reaction equals the rate of the reverse reaction o As concentration of product increases and concentrations of reactants decrease the rate of forward reaction slows down and the rate of reverse reaction speeds up o Equilibrium is reached in a chemical reaction when the concentrations of the reactants o Called dynamic because the forward and reverse reactions are still occurring at the and products no longer change same rate 14 3 The Equilibrium Constant K o The equilibrium constant is a way to quantify the concentrations of the reactants and products at equilibrium o Write the general chemical equilibrium equation o Equilibrium Constant K The ratio at equilibrium of the concentrations of the products raised to their stoichiometric coefficients divided by the concentrations of the reactants raised to their stoichiometric coefficients PRODUCT REACTANT o Law of Mass Action The relationship between the balanced chemical equation and the expression of the equilibrium constant o Coefficients in the chemical equation become exponents in the expression of the equilibrium constant o A large Equilibrium Constant K 1 Forward reaction is favored Numerator of equilibrium constant is larger than the denominator Equilibrium point lies far to the right High concentration of products low concentration of reactants Forward reaction proceeds essentially to completion o A Small Equilibrium Constant K 1 The reverse reaction is favored Equilibrium point lies far to the left High concentration of reactants low concentrations of products Forward reaction does not proceed very far o An in between Equilibrium Constant K 1 Neither direction is favored Forward reaction proceeds about halfway o Relationships between the Equilibrium Constant and the Chemical Equation If a chemical equation is modified in some way then the equilibrium constant for the equation MUST be changed to reflect the modification 1 If you reverse the equation invert the equilibrium constant K 2 If you multiply the coefficients in the equation by a factor raise the equilibrium constant to the same factor Raise the whole thing 3 If you add two or more individual chemical equations to obtain an overall equation multiply the corresponding equilibrium constants by each other to obtain the overall equilibrium constant 14 4 Expressing the Equilibrium Constant in Terms of Pressure o For gaseous reactions the partial pressure of a particular gas is proportional to its concentration Therefore we can express the equilibrium constant in terms of the partial pressures of the reactants and products o Kc Equilibrium constant with respect to concentration in molarity o Kp Equilibrium constant with respect to partial pressure in atmospheres o Kp can equal Kc but doesn t always This is due to the fact that partial pressure of a gas in atmospheres is not the same as its concentration in molarity o Write equation 14 2 below o If the sum of the stoichiometric coefficients for the reactants equals the sum for the products then Kp Kc 14 5 Heterogeneous Equilibria Reactions Involving Solids and Liquids o Many chemical reactions involve pure solids and or pure liquids as reactants as products o Pure solids and pure liquids have a constant concentration as long as there is some solid or liquid present For this reason pure solids and pure liquids are excluded in the equilibrium constant expressions Remember The exclusions only count if solids liquids have s l subscripts in the chemical equation 14 6 Calculating the Equilibrium Constant from Measured Equilibrium Concentrations o The most direct way to obtain an experimental value for the equilibrium constant of a reaction is to measure the concentrations of the reactants and products in a reaction mixture at equilibrium o Remember We do not include units when expressing the value of the equilibrium constant so that Kc is unitless o The equilibrium concentrations of the reactants and products depend on the initial concentrations and in general will vary from one set of initial concentrations to another o The equilibrium constant is always the same at a given temperature regardless of the initial concentrations o Whether you start with only reactants or only products the reaction reaches equilibrium concentrations in which the equilibrium constant is the same In other words no matter what the initial concentrations are the reaction always goes in a direction so that the equilibrium concentrations give the same constant o In most cases we only need to know The initial concentrations of the reactants Equilibrium concentration of any one reactant or product The other equilibrium concentrations can be found from the stoichiometry of the reaction using the ICE table o ICE Table I Initial Concentration C Change in Concentration Since reactants are consumed during the reaction their changes are negative Since products are produced in reactions their changes are positive E Equilibrium concentration Once all the equilibrium constants are found substitute those numbers into the equilibrium expression of the reaction to find the equilibrium constant 14 7 The Reaction Quotient Predicting the Direction of Change o Reaction Quotient A quantity used to gauge the progress of a reaction with respect to equilibrium o The definition of a reaction quotient takes the same form as the definition of the equilibrium constant except for the fact that the reaction