Chapter 9: Chemical EquilibriumThe forward and reverse reaction are both taking place at the same rate

Production and Decomposition of AmmoniaForward Reaction: N2 (g) + 3H2 (g) 2NH3 (g)Reverse Reaction: 2NH3 (g) N2 (g) + 3H2 (g)Equilibrium Reaction: N2 (g) + 3H2 (g) 2NH3 (g)Note the double headed arrow!The ammonia is decomposing as fast as it is being made at equilibrium

Equilibrium and the Law of Mass Action2SO2 (g) + O2 (g) 2SO3 (g) 5 mixtures of different initial compositions of gases were made and allowed to reach equilibrium at 1000KAt first, you dont see a trend in the data

Equilibrium and the Law of Mass ActionNo trends, but if you calculate:You get the same value, regardless of initial concentrationNote: K is unitless!

The Equilibrium ConstantK is the equilibrium constant for the reaction

The Equilibrium ConstantAt equilibrium, the composition of the reaction mixture can be expressed in terms of an equilibrium constant where:For ideal gases, the concentrations are the partial pressures of the individual gasesFor solutions, the concentrations are the molar values of the individual atoms/ions/molecules

Examples of K setupaA (g) + bB (g) cC (g) + dD (g)

Units and Equilibrium ConstantsWhen working with equilibrium Constants, well use the following unit conventions:Gases: Units are barAqueous Solutions: Unit is MolaritySolids: The number 1Solids have a single value (1) because the concentration of a solid doesnt change.

Thermodynamic Origin of Equilibrium ConstantsThe Free Energy changes as the composition of the reaction mixture changesAll reactions will proceed towards equilibrium (by either forward or reverse reaction)G is the free energy difference b/w the pure products and pure reactants

Thermodynamic Origins of Equilibrium ConstantsWe can calculate the Free Energy change at any point along the reaction coordinate with the equation

aA (g) + bB (g) cC (g) + dD (g)Gr is the textbook Free Energy of reactionGr is the Free Energy of value when the reactants and products are at particular concentrations

Example:The standard free energy of reaction for:

2SO2 (g) + O2 (g) 2SO3 (g)

Is Gr= -141.74 kJ/mole at 25C. What is the Gibbs Free Energy of reaction when the partial pressure of each gas is 100.0 bar?

Example:The Standard Gibbs Free Energy of Reaction for

N2O4 (g) --> 2NO2 (g)

Is Gr = +4.73 kJ/mole at 298K. What is the value of Gr when the partial pressures are PN2O4 = 0.8 bar and PNO2 = 2.10 bar?

Free Energy of a Reaction at EquilibriumQ=K at equilibriumAt equilibrium, G=___Therefore,G = Gr + RTlnKGr = -RTlnK (only at equilibrium)

We can use this to compute equilibrium constants from Gr values

K and the Extent of ReactionsWhen K is very large, the reaction favors the productsWhen K is very small, the reaction favors the reactantsWhen K=1, the reaction is neither reactant nor product favored (Equilibrium)

The Direction of ReactionHow can we tell if a reaction will continue towards the products or back towards the reactants at a given point along the reaction coordinate?When QK, G is positive (reactant favored)Q = Reaction quotient used at any point in the coordinateK = Equilibrium constant

Equilibrium CalculationsToolbox 9.1: Know it. Love it. Use it.

Example:Under certain conditions, nitrogen and oxygen react to form dinitrogen oxide, N2O. Suppose that 0.482 moles of N2 and 0.933 moles of O2 are transferred to a reaction vessel of volume 10.0L and allowed to form N2O @ 800K. At this temperature, K=3.2x10-28 for the reaction:2N2 (g) + O2 (g) 2N2O (g)What are the partial pressures of the gases at equilibrium?

Example:Chlorine and fluorine react at 2500K to produce ClF and reach the equilibrium:

Cl2 + F2 2ClF

With an equilibrium constant value of 20. If a gaseous mixture of 0.2 bar Cl2, 0.1 bar F2 and 0.1 bar ClF is allowed to reach equilibrium, what is the partial pressure of ClF in the mixture?

LeChateliers PrincipleWhen the equilibrium composition is perturbed by adding or removing a reactant of product, the reaction tends to proceed in the direction that brings Q closer to that of K.

Consider the Equilibrium Reaction:4NH3 (g) + 3O2 (g) 2N2 (g) + 6H2O (g)What would result from the:Addition of N2Removal of NH3Removal of H2O

Effects of the Environment on EquilibriaCompressing a Gas Phase ReactionThe reaction shifts so as to decrease the pressureDecrease the number of gas moleculesChanging the Temperature of a ReactionFor exothermic reactions, lowering the temperature causes a shift towards the productsFor endothermic reactions, increasing the temperature causes a shift towards the products