Ch 15 - Chemical Equilibrium

equilibrium

when the rate of the reactants equals the rate of the products (NOT THE CONCENTRATION)

What is used to denote that an equilibrium reaction is occurring in both directions?

double arrow

equation for the equilibrium constant for the reaction aA → ← bB

K = [B]^b/[A]^a

law of mass action

regardless of the initial concentrations, at equilibrium, the ratio of concentrations raised to the appropriate powers will always equal K

What does the value of K depend on?

• specific chemical equations

• temperature

What does the value of K not depend on?

• concentration

• particle size

• catalyst

heterogenous equilibria

• equilibria that include species of different phases

• equilibrium constant expression doesn’t include solids or pure liquids

• only gas and aqueous species have concentrations that can change and are included in the equilibrium constant expression

What does the value of K for a reaction give quantitative information about?

the extent of the reaction

When are forward and reverse reactions equally favored?

• when the concentration of reactants and products are almost equal at equilibrium

• K is about 1

When is the forward reaction favored?

• when the concentration of reactants is less than the concentration of products at equilibrium

• K is less than 1

When is the reverse reaction favored?

• when the concentration of the reactants is more than the concentration of products at equilibrium

• K is more than 1

equilibrium constant in terms of concentration (aA + bB → ← cC + dD)

K_c = [C]^c[D]^d/[A]^a[B]^b

equilibrium constant in terms of partial pressure (aA (g) + bB (g) → ← cC (g) + dD (g))

K_p = (P_C)^c(P_D)^d / (P_A)^a(P_B)^b

K_c in terms of K_P

K_c = K_P (1/RT)^{(c+d) - (a+b)}

K_P in terms of K_c

K_P = K_c (RT)^{delta n}

What is delta n equal to?

(c+d) - (a+b)

What is the equilibrium constant of the overall equation when 2 or more equations are added together?

• it’s the product of the equilibrium constants of the individual equations

• K_overall = K₁ x K₂

What happens to the equilibrium constant when the coefficients of a reaction are multiplied by a factor?

the equilibrium constant is raised to the power of the same factor

What does the reverse equation yield?

the inverse (reciprocal) equilibrium constant

What does a large K value represent?

• equilibrium is shifted right

• there are more products

What does a small K value represent?

• equilibrium is shifted left

• there are more reactants

When does the K not apply?

when the system is not at equilibrium

reaction quotient (Q)

can be determined using the same ratio as K, but using concentrations or partial pressures when the system is NOT at equilibrium

reaction quotient in terms of concentration

Q_c = [C]^c[D]^d/[A]^a[B]^b

reaction quotient in terms of partial pressure

Q_p = (P_C)^c(P_D)^d / (P_A)^a(P_B)^b

What does it mean when Q = K?

• the reaction is at equilibrium

• no shift

What does it mean when Q > K?

• more products must be formed to achieve equilibrium

• shifts left

What does is mean when Q < K?

• more reactants must be formed to achieve equilibrium

• shifts right

What can you use to determine the equilibrium constant?

ICE table

Le Chatelier’s Principles

if stress is applied to a system at equilibrium, the reaction will shift to relieve stress

stress

change of conditions imposed on the system (i.e change in concentration or pressure on reactants or products, change in temperature of the system)

shift

• net forward reaction of reactants to form products

• net reverse reaction of products to form reactants

When delta n < 0, an increase in volume

causes a shift left

When delta n < 0, a decrease in volume

causes a shift right

When delta n =0, an increase or decrease in volume

causes no change

When delta n > 0, an increase in volume

causes a shift right

When delta n > 0, a decrease in volume

causes a shift left

In endothermic reactions, how are K and T related?

directly related

In exothermic reactions, how are K and T related?

inversely related

equation that shows relationship between K and T

ln K₂/K₁ = delta_r H/R (1/T₁ - 1/T₂)

Does a catalyst affect the equilibrium constant?

• no, it simply lowers the activation energy of both forward and reverse reactions

• equilibrium is established quicker, but position remains unchanged