Kinetics applies to the speed of a reaction, the concentration of product that appears (or of reactant that disappears) per unit time. 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: rate forward = rate reverse A 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.
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 and the concentrations of the reactants and products remain constant Physical equilibrium H 2 O (l) Chemical equilibrium N2O4 (g)N2O4 (g) H 2 O (g) 2NO 2 (g) Reversible reactions can proceed in either the forward or reverse direction.
N 2 O 4 (g) 2NO 2 (g) Start with NO 2 Start with N 2 O 4 Start with NO 2 & N 2 O 4 equilibrium
N 2 O 4 (g) 2NO 2 (g) = 4.63 x 10 -3 K = [NO 2 ] 2 [N 2 O 4 ] aA + bB cC + dD Law of Mass Action 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). a i is called the activity of component i.
Activity The 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:
K >> 1 K << 1 Lie to the rightFavor products Lie to the leftFavor reactants Equilibrium Will K = [C] c [D] d [A] a [B] b aA + bB cC + dD any number less than 0.1 any number greater than 10 Significance of the Magnitude of K
Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.
K using Partial Pressures = moles of gaseous products - moles of gaseous reactants concentration equilibrium constant gas activities as partial pressures, except when
Heterogenous equilibrium applies to reactions in which reactants and products are in different phases. pure solids
Equilibrium pressure - independent of the amount of either solid. P P CO 2 = K p
A + B C + D C + D E + F A + B E + F K 1 = PcPdPcPd PAPBPAPB K1K1 K2K2 K 12 If 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. Multiple Equilibria K 2 = PEPFPEPF PCPDPCPD K 12 = PEPFPEPF PAPBPAPB
Form of K and the Equilibrium Expression 1.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.
The reaction quotient (Q c ) is calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (K c ) expression. IF Q c > K c system proceeds from right to left to reach equilibrium Q c = K c the system is at equilibrium Q c < K c system proceeds from left to right to reach equilibrium Predicting the Direction of a Reaction
Calculating Equilibrium Concentrations 1.Express the equilibrium concentrations of all species in terms of the initial concentrations and a single unknown x, which represents the change in concentration. 2.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.
Gibbs Free Energy and Chemical Equilibrium G = G 0 + RT lnQ R is the gas constant (8.314 J/K mol) T is the absolute temperature (K) Q is the reaction quotient At Equilibrium G = 0 Q = K 0 = G 0 + RT lnK G 0 = RT lnK
Free Energy Versus Extent of Reaction G 0 < 0 G 0 > 0
G 0 = RT lnK
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. Le Châtelier’s Principle Changes in Concentration N 2 (g) + 3H 2 (g) 2NH 3 (g) Add NH 3 Equilibrium shifts left to offset stress
Le Châtelier’s Principle Changes in Concentration (continued) ChangeShifts the Equilibrium Increase concentration of product(s)left Decrease concentration of product(s)right Decrease concentration of reactant(s) Increase concentration of reactant(s)right left aA + bB cC + dD
Le Châtelier’s Principle Changes in Volume and Pressure general ideal gas reaction, in terms of mole fractions.
Reaction shifts forward – toward products Reaction shifts in reverse – toward reactants No net change
Le Châtelier’s Principle Changes in Volume and Pressure: Summary A (g) + B (g) C (g) ChangeShifts the Equilibrium Increase pressureSide with fewest moles of gas Decrease pressureSide with most moles of gas Decrease volume Increase volumeSide with most moles of gas Side with fewest moles of gas
Changes in Temperature endothermic colorlessred-brown colderhotter exothermic
Quantifying Changes in Temperature van’t Hoff equation H o and S o are approximately constant between T 1 and T 2
Le Châtelier’s Principle Changes in Temperature: Qualitative Summary ChangeExothermic Increase temperatureK decreases Decrease temperatureK increases Endothermic K increases K decreases