1.10 Partial Pressure and Kp

Question 1

Give the definition of the partial pressure of a gas in a mixture.

Answer 1

The partial pressure of a gas in a mixture is the pressure that the gas would have if it alone occupied the volume occupied by the whole of the mixture.

If a mixture of gases contains 3 different gases then the total pressure will equal the 3 partial pressure added together.

Question 2

Give the equation for calculating the partial pressure of a gas in a mixture.

Answer 2

partial pressure = mole fraction x total pressure of gas 1

Question 3

Give the equation for calculating the mole fraction of a gas in a mixture.

Answer 3

mole fraction = no. moles of a gas / total no. moles of all gases

Question 4

Answer the example labelled 1 on page 1 in the chemrevise revision guide.

Answer 4

correct?

Question 5

Write a generalised expression for Kp

Answer 5

(see page 1 in the chemrevise revision guide)

Question 6

Answer the example labelled 2 on page 2 in the cherevise revision guide.

Answer 6

correct?

Question 7

If the Kp is larger, are there more reactants or products?

Answer 7

The larger the Kp the greater the amount of products.

If the Kp is small we say the equilibrium favours the reactants.

Question 8

Do these effect the value of Kp and Kc?

• temperature
• pressure
• concentration
• catalyst

Answer 8

Kc and Kp only change with temperature.

It does not change if pressure or concentration is altered.

A catalyst also has no effect on Kc or Kp.

Question 9

In this equilibrium which is exothermic in the forward direction, what effect does increasing the temperature have on the position of equilibrium and the value of Kp?

N2(g) + 3H2(g) 2NH3(g)

Answer 9

If the temperature is increased the reaction will shift to oppose the change and move in the backwards endothermic direction. The position of equilibrium shifts left. The value of Kp gets smaller as there are fewer products.

Question 10

In this equilibrium which is exothermic in the forward direction, what effect does increasing the pressure have on the position of equilibrium and the value of Kp?

N2(g) + 3H2(g) 2NH3(g)

Answer 10

If pressure is increased the reaction will shift to oppose the change and move in the forward direction to the side with fewer moles of gas. The position of equilibrium shifts right. The value of Kp stays the same though as only temperature changes the value of Kp.

Question 11

Read bottom of page 1 in the chemrevise revision guide below the example question.

Answer 11

understand?