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Properties of Kp (A2 Only)

Chemistry: AQA A Level, topic 2.3.1

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

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

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

Mole fractions

  • Another way to think about this is using 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

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?

How is Kp defined?

  • The equation for Kp is:
  • Kp=(pD)d(pC)c(pA)a(pB)bK_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

  • 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

Temperature

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

Catalysts

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

Pressure

  • Changing pressure does not alter Kp.

Test yourself on Properties of Kp (A2 Only)

  • Which of these is easier to measure?

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