2.3.1

# Properties of Kp (A2 Only)

Test yourself

## Partial Pressures of Gases

If you have a mixture of gases, you can split the total pressure into the pressures of each individual gas.

### Partial pressures

• Say you have a 50/50 mix of water vapour and nitrogen gas at a pressure of 1 bar.
• You can split this into a partial pressure for water vapour and nitrogen gas.
• This follows from the ideal gas law. This states that every gas occupies the same volume under the same conditions.
• We would say that the water vapour has a partial pressure of 0.5 bar and the nitrogen gas has a partial pressure of 0.5 bar.

### A different mixture

• If we had a 75/25 mixture of oxygen and argon at a pressure of 1 bar, we would say that:
• The partial pressure of oxygen is 0.75 bar.
• The partial pressure of argon is 0.25 bar.

### Mole fractions

• If you have 3 moles of oxygen gas and 2 moles of nitrogen gas, you can define a mole fraction of oxygen and nitrogen.
• The mole fraction of oxygen is ⅗.
• The mole fraction of nitrogen is ⅖.
• If you multiply the mole fraction by the total pressure, you get the partial pressures of the gases.

## What is Kp?

Kp is a special type of equilibrium constant that we use for reactions that are in the gas phase.

### Kp

• Where Kc uses concentrations, Kp uses partial pressures.
• The two approaches are equivalent, but it’s much easier to measure the partial pressure of a gas than the concentration of the gas.

### How is Kp defined?

• The equation for Kp is:
• $K_p = \frac{(p_D)^d(p_C)^c}{(p_A)^a(p_B)^b}$
• Where the reaction is: aA(g) + bB(g) ⇋ cC(g) + dD(g)

## Properties of Kp

Much like Kc, Kp has a set of properties you need to know.

### Le Chatelier

• Le Chatelier’s principle applies to Kp as much as it does to any aqueous equilibria.
• If you increase the pressure, the system will decrease the pressure by favouring the reaction that removes moles of gas.
• If you increase the temperature, the system will decrease the temperature by favouring the reaction that is endothermic.

### Temperature

• Just like Kc, Kp is temperature dependent.
• It will have different values at different temperatures.

### Catalysts

• Catalysts increase the rate of reaction by providing an alternate reaction pathway with a lower activation energy.
• They do not alter Kp.

### Pressure

• Changing pressure does not alter Kp.