Equilibrium Chapter 15 Chemical Equilibrium John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

Equilibrium The Concept of Equilibrium As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate.

Equilibrium A System at Equilibrium Once equilibrium is achieved, the amount of each reactant and product remains constant.

Equilibrium Depicting Equilibrium In a system at equilibrium, both the forward and reverse reactions are being carried out; as a result, we write its equation with a double arrow N 2 O 4 (g) 2 NO 2 (g)

Equilibrium The Equilibrium Constant

Equilibrium The Equilibrium Constant Consider the reaction The equilibrium expression for this reaction would be K c = [C] c [D] d [A] a [B] b aA + bBcC + dD

Equilibrium What Are the Equilibrium Expressions for These Equilibria?

Equilibrium The Equilibrium Constant Because pressure is proportional to concentration for gases in a closed system, the equilibrium expression can also be written K p = (P C ) c (P D ) d (P A ) a (P B ) b

Equilibrium Relationship between K c and K p Where K p = K c (RT)  n  n = (moles of gaseous product) − (moles of gaseous reactant)

Equilibrium Equilibrium Can Be Reached from Either Direction As you can see, the equilibrium constant remains constant at this temperature no matter what the initial concentrations of NO 2 and N 2 O 4 are.

Equilibrium What Does the Value of K Mean? If K >> 1, the reaction is product-favored; product predominates at equilibrium, equilibrium lies to the right. If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium, equilibrium lies to the left.

Equilibrium Manipulating Equilibrium Constants The equilibrium constant of a reaction in the reverse reaction is the reciprocal of the equilibrium constant of the forward reaction = K c = = at 100  C [NO 2 ] 2 [N 2 O 4 ] N2O4 (g)N2O4 (g) 2 NO 2 (g) K c = = 4.72 at 100  C [N 2 O 4 ] [NO 2 ] 2 N2O4 (g)N2O4 (g) 2 NO 2 (g)

Equilibrium Manipulating Equilibrium Constants The equilibrium constant of a reaction that has been multiplied by a number is the equilibrium constant raised to a power that is equal to that number. K c = = at 100  C [NO 2 ] 2 [N 2 O 4 ] N2O4 (g)N2O4 (g) 2 NO 2 (g) K c = = (0.212) 2 at 100  C [NO 2 ] 4 [N 2 O 4 ] 2 2 N 2 O 4 (g) 4 NO 2 (g)

Equilibrium Manipulating Equilibrium Constants The equilibrium constant for a net reaction made up of two or more steps is the PRODUCT of the equilibrium constants for the individual steps.

Equilibrium Equilibrium Calculations

Equilibrium Equilibrium Calculations A closed system initially containing x 10 −3 M H 2 and x 10 −3 M I 2 At 448  C is allowed to reach equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87 x 10 −3 M. Calculate K c at 448  C for the reaction taking place, which is H 2 (g) + I 2 (g) 2 HI (g)

Equilibrium What Do We Know? [H 2 ], M[I 2 ], M[HI], M Initially1.0 x x Change Equilibrium

Stoichiometry tells us [H 2 ] and [I 2 ] decrease by half as much [H 2 ], M[I 2 ], M[HI], M Initially1.0 x x Change- x + 2x Equilibrium

We can now calculate the equilibrium concentrations of all three compounds… [H 2 ], M[I 2 ], M[HI], M Initially1.0 x x Change- x + 2x Equilibrium1.0 x x2.0 x x2x = 1.87 x 10 -3

Equilibrium …and, therefore, the equilibrium constant Kc =Kc = [HI] 2 [H 2 ] [I 2 ] = 51 = (1.87 x ) 2 (6.5 x )(1.065 x )

Equilibrium The Reaction Quotient (Q) To calculate Q, one substitutes the INITIAL concentrations of reactants and products into the equilibrium expression. Q gives the same ratio the equilibrium expression gives, but for a system that is not at equilibrium.

Equilibrium If Q = K, the system is at equilibrium.

Equilibrium If Q > K, there is too much product and the equilibrium shifts to the left.

Equilibrium If Q < K, there is too much reactant, and the equilibrium shifts to the right.

Equilibrium Le Châtelier’s Principle

Equilibrium Le Châtelier’s Principle “If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.”

Equilibrium The Haber Process The transformation of nitrogen and hydrogen into ammonia (NH 3 ) is of tremendous significance in agriculture, where ammonia-based fertilizers are of utmost importance.

Equilibrium Change is Reactant or Product Concentrations Adding reactant – Shift right Removing reactant – Shift left Adding product – Shift left Removing product – Shift right

Equilibrium Effect of Volume and Pressure Changes Less Volume = More Pressure Equilibrium will shift towards the side with the least gas molecules (less gas = less pressure More Volume = Less Pressure Equilibrium will shift toward the side with the most gas molecules (more gas = more pressure)

Equilibrium Effect of Temperature Changes Changes in concentrations or partial pressures cause shift in equilibrium without changing the value of the equilibrium constant. In contrast, almost every equilibrium constant changes in value as the temperature changes.

Equilibrium Effect of Temperature Changes A simple way to do this is to treat heat as if it were a reactant or product. Endothermic: Reactants + heat  Products Exothermic: Reactants  Products + heat

Equilibrium Catalysts increase the rate of both the forward and reverse reactions.

Equilibrium Equilibrium is achieved faster, but the equilibrium composition remains unaltered.