2 At equilibrium: rateforward = ratereverse Equilibrium applies to the extent of a reaction, the concentration of product that has appeared after an unlimited time, or once no further change occurs.At equilibrium: rateforward = ratereverseA system at equilibrium is dynamic on the molecular level; no further net change is observed because changes in one direction are balanced by changes in the other.Kinetics applies to the speed of a reaction, the concentration of product that appears (or of reactant that disappears) per unit time.
3 Reversible reactions can proceed in either the forward or reverse direction. Equilibrium is a state in which there are no observable changes as time goes by.Chemical equilibrium is achieved when:the rates of the forward and reverse reactions are equal andthe concentrations of the reactants and products remain constantPhysical equilibriumH2O (l)H2O (g)Chemical equilibriumN2O4 (g)2NO2 (g)
4 N2O4 (g) 2NO2 (g) equilibrium equilibrium equilibrium Start with NO2 Start with NO2 & N2O4
6 Law of Mass Action N2O4 (g) 2NO2 (g) [NO2]2 K = = 4.63 x 10-3 [N2O4] aA + bB cC + dDai is called the activity of component i.Law of mass action: for a reversible reaction at equilibrium and at a constant temperature, a certain ratio of reactant and product concentrations has a constant value K (called the equilibrium constant).
7 ActivityThe activity (a) of a species in a reaction is generally a complex function of the pressures and concentrations of all the components present in the reaction mixture.Ideal gases:Solutes in dilute solution:Pure solids and liquids:
8 Significance of the Magnitude of K [C]c[D]d[A]a[B]baA + bB cC + dDany number greater than 10Equilibrium WillK >> 1Lie to the rightFavor productsK << 1Lie to the leftFavor reactantsany number less than 0.1
9 Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.
10 K using Partial Pressures gas activities as partial pressures,= moles of gaseous products - moles of gaseous reactantsconcentration equilibrium constantexcept when
17 Multiple EquilibriaK1 =PcPdPAPBA + B C + DK1K2 =PEPFPCPDC + D E + FK2K12 =PEPFPAPBA + B E + FK12If a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions.
18 Form of K and the Equilibrium Expression When the equation for a reversible reaction is written in the opposite direction, the equilibrium constant becomes the reciprocal of the original equilibrium constant.2. The value of K also depends upon how the equilibrium equation is balanced.
21 Predicting the Direction of a Reaction The reaction quotient (Qc ) is calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (Kc) expression.IFQc > Kc system proceeds from right to left to reach equilibriumQc = Kc the system is at equilibriumQc < Kc system proceeds from left to right to reach equilibrium
24 Calculating Equilibrium Concentrations Express the equilibrium concentrations of all species in terms of the initial concentrations and a single unknown x, which represents the change in concentration.Write the equilibrium constant expression in terms of the equilibrium concentrations. Knowing the value of the equilibrium constant, solve for x.3. Having solved for x, calculate the equilibrium concentrations of all species.
31 Gibbs Free Energy and Chemical Equilibrium DG = DG0 + RT lnQR is the gas constant (8.314 J/K•mol)T is the absolute temperature (K)Q is the reaction quotientAt EquilibriumDG = 0Q = K0 = DG0 + RT lnKDG0 = - RT lnK
32 Free Energy Versus Extent of Reaction DG0 < 0DG0 > 0
37 Le Châtelier’s Principle If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.Changes in ConcentrationN2 (g) + 3H2 (g) NH3 (g)AddNH3Equilibrium shifts left to offset stress
38 Le Châtelier’s Principle Changes in Concentration (continued)aA + bB cC + dDChangeShifts the EquilibriumIncrease concentration of product(s)leftDecrease concentration of product(s)rightIncrease concentration of reactant(s)rightDecrease concentration of reactant(s)left
40 Le Châtelier’s Principle Changes in Volume and Pressuregeneral ideal gas reaction,in terms of mole fractions.
41 Reaction shifts forward – toward products Reaction shifts in reverse – toward reactantsNo net change
42 Le Châtelier’s Principle Changes in Volume and Pressure: SummaryA (g) + B (g) C (g)ChangeShifts the EquilibriumIncrease pressureSide with fewest moles of gasDecrease pressureSide with most moles of gasIncrease volumeSide with most moles of gasDecrease volumeSide with fewest moles of gas