19 Equilibrium

expression for K_c in N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

K_c = [NH_3(g)]² / [N_2(g)] [H_2(g)]³

how to work out units of K_c

What are the two main types of equilibria

homogeneous and heterogeneous

def homogeneous equilibria

A homogenous equilibrium contains equilibrium species that all have the same state or phase. e.g. an equilibrium is homogenous if all the equilibrium species have the same state and are gases.

e.g. N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

def heterogeneous equilibrium

A heterogeneous equilibrium contains equilibrium species that have different states or phases.

e.g. C_(s) + H₂O_(g) ⇌ CO_(g) + H_2(g)

Any species that are solids and liquids are omitted from the K_c expression, they’re automatically incorporated within the overall equilibrium constant. K_c only includes species that are (g) or (aq).

what is the K_c expression for C_(s) + H₂O_(g) ⇌ CO_(g) + H_2(g)

K_c = [CO_(g)] (H_2(g)] / [H₂O_(g)]

the method for calculating K_c from equilibrium amounts

1. make a table of the elements across the top then ‘ratio of n’, ‘initial n’, ‘change’ and ‘eqm n’ down the side

2. calculate [] elements

3. sub into K_c equation

how are equilibria involving gases usually expressed

K_p

what is the relationship between K_p and K_c

Concentration and pressure are proportional to each other and K_p and K_c have a direct relationship to ecah other

equation of a mole fraction of a gas A

mole fraction x(A) = (number of moles of A) / (total number of moles in gas mixture

what is the partial pressure p in a gas mixture

in a gas mixture, the partial pressure p of a gas is the contribution that the gas makes towards the total pressure P. The sum of the partial pressures of each gas equals the total pressure.

For gas A in a gas mixture:

partial pressure p(A) = mole fraction of A x total pressure P

p(A) - x(A) x P

the equilibrium constant

K_p

the K_p for H_2(g) + I_2(g) ⇌ 2HI_(g)

Kp = p(HI)² / p(H₂) x p(I₂)

p is the equilibrium partial pressure

K_p only includes gases because only gases have partial pressures, any other species must be ignored.

suitable units for partial pressures?

kPa or Pa or atm but same unit must be used for all gases

3 rules of le chatelier’s principle

1. if the concentration of a species is increased, the equilibrium shifts in the direction that reduces the concentration

2. if the pressure is increased, the equilibrium position shifts towards the side with fewer gaseous moles

3. if the temperature is increased, the equilibrium position shifts in the endothermic direction

why does le chatelier’s principle work? (2 words)

equilibrium constants

what does the value of K indicate

the exact position of an equilibrium

K = 1 indicates an eqm halfway between reactants and products

K = 100 indicates an eqm well in favour of the products

K = 1 × 10^-2 indicates an eqm well in favour of the products

when the reactants or products of a reversible reaction are mixed together, the rxn proceeds until the []s of the eqm species give the value of K when placed in the eqm constant expression.

At a set temperature, K is constant and doesn’t change despite any modifications to [], pressure, or the presence of a catalyst.

what is the only condition that will cause K to change its value

temperature

whether the value of K gets larger or smaller depends on whether the forward rxn is exothermic of endothermic

how is K affected if the forward rxn is exothermic w increasing temp

• the equilibrium constant decreases with increasing temperature

• raising the temperature decreases the equilibrium yield of products

explain the eqm shift when the forward rxn is exothermic w increasing temp

if the temp increases:

• K_p decreases

• system no longer in eqm

• the partial pressures of the product must DECREASE

• so the partial pressures of the reactants must INCREASE

• position of eqm shifts towards the reactants (LHS)

• so the ratio K_p = p(products) / p(reactants changes

how is K affected if the forward rxn is endothermic w increasing temp

• the eqm constant increases w increasing temperature

• raising the temp increases the eqm yield of products

explain the eqm shift when the forward rxn is endothermic w increasing temp

if the temp increases:

• K_p increases

• system no longer in eqm

• the partial pressure of the product must increase

• the partial pressures of the reactants must decrease

• position of eqm shifts to the products (RHS)

• so the ratio K_p = p(products) / p(reactants changes

how does a catalyst affect eqm constants

eqm constants are unaffected by the presence of a catalyst

• catalysts affect the rate of a chemical reaction but not the position of eqm

• they speed up both the forward and reverse rxns by the same factor

how do changes in [] and pressure affect equilibrium constants

the value of an eqm constant is unaffected by changes in [] or pressure.

the shift in eqm position learnt from le chatelier’s principle is a result from the fact the eqm constant doesn’t change.

explain the shift in eqm if concentration increases

if concentration of N₂O_4(g) increases

• the ratio of K_c changes, system no longer in eqm

• the concentrations must change to return the ratio back to the K_c value

• so the [product] must INCREASE

• and the [reactant] must decrease

Explain what would you see if more C were added at room temp? (3 marks)