3.1.10- Equilibrium constant Kp for homogeneous systems (PAPER 1)

Question 1

What is total pressure?

Answer 1

The sum of all the pressures of individual gases (partial pressures)

Question 2

SO2Cl2 (g) ⇌ SO2 (g) + Cl2 (g)

At equilibrium reaction vessel = 263kPa Cl2
Total pressure in vessel = 714kPa

Calculate partial pressure SO2Cl2 (pSO2Cl2) at equilibrium

Answer 2

Total pressure= p(SO2Cl2) + p(SO2) + (PCl2)

714 = p(SO2Cl2) + p(SO2) + 263

1:1 ratio SO2: Cl2 as both products, SO2= 263

714= p(SO2Cl2) + 263 + 263

(pSO2Cl2) = 188kPa

Question 3

How is partial pressure of a gas calculated?

Answer 3

Using mole fractions- a fraction of a gas in a gas mixture

Question 4

What is the equation for mole fraction of a gas?

Answer 4

number of moles of gas/ total n of moles of gas in the mixture

Question 5

What is the equation for partial pressure?

Answer 5

mole fraction of gas x total pressure in mixture

Question 6

What to use to calculate mole fractions?

Eg 3 moles SO2Cl2 heated, equilibrium mixture contains 1.75 moles Cl2

SO2Cl2 (g) ⇌ SO2 (g) + Cl2 (g)

Answer 6

ICE Table- initial, change, equilibrium moles

         SO2Cl2 (g) ⇌ SO2 (g) + Cl2 (g) initial     3                  0                  0 change  -1.75           +1.75         +1.75 equilibrium  1.25      1.75           1.75

Question 7

Use the ICE table to work out partial pressure SO2Cl2, total pressure= 714kPa

Answer 7

mole fraction SO2Cl2 = 1.25/ 1.25 + 1.75 + 1.75= 1.25/ 4.75= 0.263

0.263x 714= 188kPa

Question 8

Use this general equation to give expression for kp

aA (g) + bB (g) ⇌ cC(g) + dD (g)

Answer 8

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

We use partial pressures to find kp

Question 9

Find value of kp and units for

2A (g) + B(g) ⇌ C(g) + D(g)

Partial pressures are 324kPa for A, 162kPa for B, 182kPa for C, 182kPa for D at equilibrium

Answer 9

Write expression

kp = (pD) x (pC) / (pB) (pA)^2

kp= 182 x 182/ 162 x (324)^2
kp= 1.95 x10^-3

Units= kPa x kPa/ kPa x kPa x kPa
= 1/ kPa= kPa^-1

Question 10

How does temperature affect kp?

Answer 10

Changing the temperature changes equilibrium pressures, changing kp

If temperature change causes equilibrium to shift to right, kp INCREASES

If temperature change causes equilibrium to shift to left, kp DECREASES

Question 11

How will increasing and decreasing temperature affect kp of this reaction?

2SO2 (g) + O2(g) ⇌ 2SO3 (g) ΔH= -197kJmol-1

Answer 11

Reaction is exothermic to right

Increasing temperature causes equilibrium to shift to left decreasing kp
-Reactant partial pressure increases

Decreasing temperature causes equilibrium to shift to right increasing kp
-Product partial pressure increases

Question 12

How does pressure affect kp?

Answer 12

Does not affect kp- partial pressure ratio of reactants to products stays the same so Kp does not change.

Question 13

How do catalysts affect kp?

Answer 13

Do not affect kp- speeds up forward and reverse reaction so rate at which equilibrium is reached.