Topic 11 Equilibrium II

Question 1

How to write the units of a Kc expression?

Answer 1

Write the units top and bottom to cancel. Invert the powers if there is more units under the line.

Question 2

How to work out the conc of a species in equilibrium before working out Kc?

Answer 2

ICE method
Initial moles
Change in moles
Equilibrium moles

Use moles to calc concentration of reactants and products. Then put the concentrations into the KC equation. Then calc the units

Question 3

How to calculate an unknown concentration at equilibrium using Kc values?

Answer 3

Substitute values into the Kc equation. Rearrange to get the correct values alone. Square root to give final answer

Question 4

What is the total pressure?

Answer 4

The total pressure in a reaction is the sum of all the pressures of the individual gases (partial pressure)
Example:
SOCl2 <——> SO2 + Cl2
At equilibrium, the reaction vessel contains 263kPa of Cl2. The total pressure in the vessel is 714kPa. Calculate the partial pressure of SO2Cl2

We know the total pressure is the sum of all the partial pressures so
Total pressure = pSO2Cl2 + pSO2 + pCl2
714 = pSO2Cl2 + pSO2 + 263

We know that Cl2 has a partial pressure of 263kPa. There is a 1:1 molar ratio with SO2. As this is a product too the partial pressure for SO2 is the same as Cl2. So let’s complete the equation further
714 = pSO2Cl2 + 263 + 263
714 - 526 = pSO2Cl2
188kPa

Question 5

What are mole fractions?

Answer 5

The partial pressure of gas is calculated using mole fractions. A mole fraction is the proportion of a gas in a gas mixture

Mole fraction of a gas = Number of moles of gas / Total number of moles of gas in the mixture

Question 6

How to calc partial prssure?

Answer 6

Partial pressure = mole fraction of gas x total pressure in mixture

Use the ICE table. Then calc mole fraction then x by total pressure

Question 7

What is Kp?

Answer 7

It is the equilibrium constant for an equilibrium reaction involving gases. We use partial pressures instead of concentrations.

aA + bB <—–> cC + dD
SMALLER LETTERS ARE POWERS

Kp = (PC)(PD) / (PA)(PD)

Question 8

Kp Heterogeneous equilibria

Answer 8

Only includes gases

Question 9

What affects the value of Kp?

Answer 9

Temperature:
Kp is only valid for one temperature
Changing temperature will change equilibrium pressures, hence Kp will change too.

If temp change causes equilibrium to shift right, Kp will increase
If temp change causes equilibrium to shift left, Kp will decrease

Pressure - The value of Kp is unaffected by any changes in Pressure

Catalysts:
Adding a catalyst has no effect on Kp
A catalyst speeds up the forward and backward reaction equally
A catalyst speeds up the rate at which equilibrium is established

Question 10

What affects th evalue of Kc?

Answer 10

Temperature:
Temperature:
Kc is only valid for one temperature
Changing temperature will change equilibrium pressures, hence Kc will change too.

If temp change causes equilibrium to shift right, Kc will increase
If temp change causes equilibrium to shift left, Kc will decrease

Concentration - The value of Kc is unaffected by any changes in concentration

Catalysts:
Adding a catalyst has no effect on Kc
A catalyst speeds up the forward and backward reaction equally
A catalyst speeds up the rate at which equilibrium is established