Chapter 19 - Equilibrium

Question 1

what is a generalised equation for Kc

Answer 1

aA + bB –(REVERSIBLE)–> cC + dD

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

Question 2

when (in terms of Kc) is equilibrium to the left/right/centre

Answer 2

Kc < 1
equilibrium to the left

Kc > 1
equilibrium to the right

Kc = 1
equilibrium central

Question 3

how can we work out the units of Kc

Answer 3

write out the expression for Kc including only the units with powers
cancel accordingly

Question 4

what is a homogeneous equilibrium

Answer 4

“A homogeneous equilibrium contains equilibrium species that are all in the same state or phase”

Question 5

what is a heterogeneous equilibrium

Answer 5

“A heterogeneous equilibrium contains equilibrium species in different states or phases”

Question 6

what do we need to change about Kc if it’s a heterogeneous equilibrium

Answer 6

exclude any species in liquid or solid phase because their concentrations essentially remain constant

Question 7

what is the generalised method for calculating Kc where the system is not already at equilibrium

Answer 7

1) write out full equation and check for any solids/liquids to exclude from Kc
2) write out all known initial concentrations
3) use molar ratios and any information given about equilibrium moles to predict moles remaining/formed at equilibrium for all species
4) Find equilibrium concentrations using volume
5) calculate Kc

NOTE: be careful when doing moles formed, the molar ratios need to be in relation to the CHANGE in moles not just the moles

Question 8

how can we measure equilibria where all the species are in their gaseous phase

Answer 8

We can use Kp, this uses partial pressures instead of concentrations

as pressures and concentrations are very similar, Kc and Kp are directly linked

Question 9

what can we say about amounts of gases if they are under the same conditions

Answer 9

the same number of moles will take up the same volume

Question 10

how can we calculate the mole fraction of a gas

Answer 10

mole fraction(A) = moles of A / total moles

you can replace moles with volume because its a fraction and moles/volume are interchangeable

Question 11

what should all mol fractions sum to

Answer 11

1

Question 12

define partial pressure

Answer 12

“In a gas mixture the partial pressure, p, of a gas is the contribution it makes towards the total pressure, P”

Question 13

how can we calculate partial pressure

Answer 13

partial pressure = mole fraction of that gas x total pressure

p(A) = x(A) * P

Question 14

what should all partial pressures sum to

Answer 14

the total pressure

Question 15

how can we calculate Kp

Answer 15

for a reaction
aA(g) + bB(g) –(REVERSIBLE)–> cC(g) + dD(g)

Kp = (p(D))^d * (p(C))^c / (p(A))^a * (p(B))^b

calculate units as normal

Question 16

what are the 3 main points to remember from Le Chatalier’s principle

Answer 16

1) if the concentration of a species increases, the equilibrium position shifts to decrease that concentration
2) if pressure increases, equilibrium shifts to the side with fewer moles of gas
3) if temperature increases, equilibrium shifts to the endothermic side

Question 17

what is the only thing that changes an equilibrium constant

Answer 17

temperature

NOT pressure, concentration or catalyst

Question 18

what is the effect of temperature (in terms of Kp and Kc) on an equilibrium if the forwards reaction is exothermic

Answer 18

• equil constant decrease as temperature increases
• therefore yield of products decreases as temp increases

if Kc decreases then [C] and [D] MUST decrease and [A] and [B] MUST increase

• this makes equil shift left

Question 19

what is the effect of temperature (in terms of Kp and Kc) on an equilibrium if the forwards reaction is endothermic

Answer 19

• equil constants increase with increasing temperature
• therefore yield of products increase as temperature increases

as temp increases and Kc increases [C] and [D] MUST increase and [A] and [B] MUST decrease

Question 20

what is the effect of pressure and concentration of the equilibrium constants and why does the position of equilibrium move

Answer 20

• Kp and Kc is unaffected by changes in conc. and pressure

- position of equilibrium changes due to Le Chatalier’s BECAUSE Kc and Kp MUST stay the same

Question 21

for an equilibrium where Kc (tries to) decrease due to a concentration change, what happens to the concentrations and position of equilibrium

Answer 21

• if Kc would decrease when the concentration of a species is changed then to bring Kc back to its initial value, more products MUST be made and more reactants MUST be used up
• so equilibrium moves right, there are the same overall ratios, just more stuff

the same works vice versa

Question 22

how does an increase in pressure (in terms of Kp) affect a dynamic equilibrium where there are equal numbers of moles of gas on each side

Answer 22

all partial pressures increase by the same amount

but there are the same orders on the top and bottom of the equation so there is no overall change in equilibrium

Question 23

how does an increase in pressure (in terms of Kp) affect an equilibrium where there are different numbers of moles of gas on each side of the equation

Answer 23

• when total pressure increases, all partial pressures increase by the same amount
• but the orders on the top/bottom of the equation are different
• this would force Kp to change so the position of equilibrium moves to counter this and keep Kp constant

Question 24

what is the effect of a catalyst on Kc/Kp

Answer 24

• it has no effect

- a catalyst increases the rate of the forwards and backwards reactions by the same amount

Question 25

How do the rates of the forwards and backwards reactions change in an equilibrium where pressure increases (and uneven gas distribution)

Answer 25

• the rates of both reactions increase because concentration of reactant particles increase as pressure increases (in gases)
• so frequency of successful collisions increases
• however the reaction with more moles of gaseous reactants increases more because there are more reactant terms

Question 26

what to ALWAYS check and recheck when doing equilibrium calcs

Answer 26

that you don’t have a liquid or solid in the Kc expression

Question 27

what tend to be the units for kp

Answer 27

some power of (Pa) e.g. Pa^-1

or (atm)^n