I 15 1 The Concept of Equilibrium Chapter 15 Chemical Equilibrium Static equilibrium an equilibrium in which an object is at rest Chemical equilibrium when opposing reactions occur at equal rates A Equilibrium state a mixture of reactants and products whose concentrations no longer change with time Equilibrium mixture a mixture in which the concentrations of the components no longer change with time In order for reaction to reach equilibrium the reaction must be reversible For equilibrium to occur neither the reactants nor the products can escape from the system At equilibrium a particular ratio of concentration terms equals a constant B Equilibrium can be reached from either direction Law of mass action expresses the relationship between concentrations of the reactants and products present at equilibrium For the general equilibrium equation aA bB dD eEAccording to the law of mass action the equilibrium condition is described by the equilibrium constant expression K c D d E e equilibrium constant Ex for the Haber process N 2 g 3 H 2 g 2 N H 3 g the equilibrium constant expression is A a B b Where D d E e are the products A a B b are the reactants and K c is the 15 2 The Equilibrium Constant K c N H 3 2 N 2 H 2 3 The equilibrium constant expression depends only on the stoichiometry of the reaction not on its mechanism 1 2 Evaluating K c Equilibrium Constants in Terms of Pressure K p When reactants and products in a chemical reaction are gases it is possible to formulate the equilibrium constant expression in terms of partial pressure K p P D d P E e P A a PB b Where P A is the partial pressure of A in atmospheres and so forth It is possible to calculate K p or K c from the other From PV nRT so P n V where the quantity n is the change in number of moles of gas in the balanced chemical equation It equals the sum of the coefficients of the gaseous products minus the sum of the RT A RT K p K c RT n RT P A nA V coefficients of the gaseous reactants n moles of gaseous product molesof gaseousreactants 3 Equilibrium Constants and Units Equilibrium constants derived from thermodynamic measurements are defined in terms of activities rather than concentration or partial pressure The activity of an ideal mixture is the ratio of the concentration or pressure of the substance to a reference concentration 1M or reference pressure 1atm The units of these ratios always cancel out so activities have no units Furthermore the numerical value of the activity is equal to the concentration For pure solids and pure liquids the activities merely equal 1 so again no units The thermodynamic equilibrium constants have no units C Constants 15 3 Understanding and Working with Equilibrium 1 The Magnitude of Equilibrium Constants If K 1 Large K Equilibrium lies to the right products are predominate If K 1 Small K Equilibrium lies to the left reactants are predominate At equilibrium it is forward and reverse rate that are equal not concentrations The Direction of the Chemical Equations and K 2 The equilibrium constant expression for a reaction written in one direction is the reciprocal of the expression for the reaction written in the reverse direction Relating Chemical Equation Stoichiometry and Equilibrium 3 Constants A B C D K 1 then nA nB nC nD K K1 n You must relate each equilibrium constant you work with to a specific balanced chemical equation You can use Hess s Law to find the equilibrium constants for a reaction if you know the equilibrium constants for other reactions D 15 4 Heterogeneous Equilibria Homogeneous equilibria equilibria that involve substances that are all in the same phase usually gas or liquid Heterogeneous equilibria when substances in equilibria are in different phases Whenever a pure solid or a pure liquid is involved in a heterogeneous equilibrium its 15 5 Calculating Equilibrium Constants concentration is not included in the equilibrium constant expression E If we know the equilibrium concentration of at least one species we can generally use the stoichiometry of the reaction to deduce the equilibrium concentrations of the others For those species for which initial and equilibrium Tabulate all the initial and equilibrium concentration of the 1 species that appear in the equilibrium constant expression 2 concentrations are known calculate the change in concentration that occurs as the system reaches equilibrium 3 Use the stoichiometry of the reaction the coefficients in the balanced chemical equation to calculate the change in concentration for all other species in the equilibrium constant expression 4 the concentration from step3 to calculate any equilibrium concentration not tabulated in step 1 5 Use the initial concentrations from step 1 and changes in Determine the value of the equilibrium constant F 15 6 Applications of Equilibrium Constants 1 Predicting the Direction of Reaction The reaction quotient Q a number obtained by substituting reactant and product concentrations or partial pressure at any point during a reaction into an equilibrium constant expression For the general reaction aA bB dD eEthe reaction quotients in terms of molar concentrations is Qc D d E e A a B b To determine whether or not we are at equilibrium or in which direction the reaction proceeds to achieve equilibrium we compare the value of Qc and K c or Q p andK p Three possible situations arise o Q K the reaction quotient equals the equilibrium constant only if the system is at equilibrium o Q K the concentration of products is too large and that of the reactants is too small Substances on the right side of the chemical equation react to form substances on the left the reaction proceeds from right to left to approach equilibrium o Q K The concentrations of products is too small and that of reactants too large The reaction achieves equilibrium by forming more products it proceeds from left to right 2 Calculating Equilibrium Concentrations See Ice Table in notes G 15 7 Le Principle de Le Ch telier If a system at equilibrium is disturbed by a change in temperature pressure or a component concentration the system will shift its equilibrium position so as to counteract the effect of the disturbance 1 Change in Reactant or Product Concentration If a chemical system is already at equilibrium and the concentration of any substance in the mixture is increased either reactant or product the system reacts to consume some of that substance Conversely if the concentration of a substance is decreased the system reacts