Chapter 16

Chemical Equilibrium

most reactions stop when there is a

mixture of reactants and products

reversible process

the products of the reaction can react to form reactants

equilibrium

when the rates of the forward reaction and reverse reaction are the same

dynamic state

forward and reverse reactions continue to occur, although there is no net change in reactant and product concentration over time

equilibrium is a

dynamic state

What can equilibrium be established starting with?

only reactants, only products, or a mixture of products and reactants

equilibrium reactions are not required to have the same ___ of products and reactants

concentration

On a graph, the ___ is when an equilibrium occurs

plateau

reaction quotient

a fraction with product concentrations in the numerator and reactant concentrations in the denominator

the reaction quotient is not necessarily

at equilibrium

K_c

equilibrium constant, system is at equilibrium

with the equilibrium constant, each concentration is raised to a power equal to the

corresponding coefficient in the balanced equation

K_eq =

K_eq = [products]^y/[reactants]^x

A K_eq value > 1 means the reaction is

product favored (ΔG^o = -)

A K_eq value < 1 means the reaction is

reactant favored (ΔG^o = +)

if the reaction is going to the left, in order to find K_eq you must

rewrite the equation flipped

K_eq can be

• K_c = concentration

• K_p = pressure

• K_sp = solubility

• K_a = acid

• K_b = basic

heterogeneous equilibrium

when the species in a reversible chemical reaction are not all in the same phase

Phases that do not appear in equilibrium expressions

pure liquids and solids

the equilibrium constant gives the extent a reaction will proceed at a particular

temperature

when the reaction goes to completion, the equilibrium will consist predominantly of

products

when a reaction does not occur to any significant degree, the equilibrium mixture will consist predominantly of

reactants

when a reaction proceeds to a significant degree but will not go to completion, the equilibrium mixture will contain

comparable amounts of products and reactants

when a reversible chemical equation is manipulated, it is also necessary to make appropriate changes in the

equilibrium expression and constant

if the original reversible equation is reversed, the new constant is the

reciprocal of the original (1/K_c)

if the original reversible equation is multiplied by a number, the new constant is the

original constant raised to the same number (K_c)^y

if the original reversible equation is divided by two, the new constant is the

square root of the original constant (√K_C)

if two equations are added, the new constant is the

product of the two original constants (K_C1 x K_C2)

gaseous equilibrium definition

relationship between K_c and K_p for gases

gaseous equilibrium equation

K_p = K_c (RT)^Δn

Δn

moles of gaseous product - moles of gaseous reactant

Difference between Q_C and K_C

Q_C gives the ratio for a system not in equilibrium

How to calculate Q_C

substitute initial concentrations of products and reactants into equilibrium expression

What happens to the value of Q_C as the reaction progresses?

the value changes

Direction of reaction when Q > K

to the left

Direction of reaction when Q < K

to the right

When Q = K, the reaction is

at equilibrium and the initial concentrations are equilibrium concentrations

When Q < K, the system has too many ___ to be at equilibrium

reactants (concentration ratio is too small)

When Q > K, the system has too many ___ to be at equilibrium

products (concentration ratio is too large)

When Q < K, the reaction will proceed to make __ until the system achieves equilibrium

products

When Q > K, the reaction will proceed to make __ until the system achieves equilibrium

reactants

the sign of ___ determines spontaneity

ΔG

relationship between ΔG and ΔG^o equation

ΔG = ΔG^o + RT * ln(Q)

at equilibrium ΔG =

0

Equation incorporating ΔG^o at equilibrium

ΔG^o = -RT * ln(K)

Le Chatlier’s Principle

when stress is applied to a system at equilibrium, the system will respond by shifting in the direction that minimizes the effect of the stress

what is considered stress according to Le Chatlier’s Principle

addition/removal of reactant or product, change in temperature, or change in volume resulting in change in concentration or partial pressure of reactants/products

addition of reactant or removal of product will cause an equilibrium to shift to the

right

addition of product or removal of reactant will cause an equilibrium to shift to the

left

an equilibrium will shift in a direction that will partially consume

a reactant or product that is added

an equilibrium will shift in a direction that will partially replace

reactant or product that is removed

any change that has no effect on Q will

have no effect on K

any change that increases the value of Q will

cause the reaction to proceed to the left until Q=K

any change that decreases the value of Q will

cause the reaction to proceed to the right until Q=K

decreasing volume ___ pressure

increases

an increase in pressure will cause the reaction to shift in whichever direction that will

decrease the number of moles of gas

increasing volume ___ pressure

decreases pressure

a decrease in pressure will cause the reaction to shift in whichever direction that will

increase the number of moles of gas

changes in volume and concentration ___ the value of K_c

do not change

a change in ___ can alter the value of the equilibrium constant

temperature

for any exothermic reaction, heat is a

product

for any endothermic reaction, heat is a

reactant

for endothermic reactions ΔH^o is

positive

for exothermic reactions ΔH^o is

negative

adding heat to an endothermic reaction causes the reaction to shift toward the

products

adding heat to an endothermic reaction causes a ___ in K_C

increase

removing heat from an endothermic reaction shifts the reaction towards the

reactants

removing heat from an endothermic reaction causes a ___ in K_C

decrease

adding heat to an exothermic reaction causes the reaction to shift towards the

reactants

adding heat to an exothermic reaction causes a __ in K_C

decrease

removing heat from an exothermic reaction causes the reaction to shift towards the

products

removing heat from an exothermic reaction causes a ___ in K_C

increase

adding a catalyst ___ K

does not change

adding a catalyst ____ the direction of the reaction

doesn’t shift

adding a catalyst to a reaction means that it will

reach equilibrium sooner

catalysts lower ___ for both the forward and reverse reactions

the energy of activation (E_a)