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Kp

Chemistry: IB Diploma Higher Level, topic 5.7.4

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)
  • Kp only includes species in the gaseous phase.

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 favoring the reaction that removes moles of gas.
  • If you increase the temperature, the system will decrease the temperature by favoring 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 Kp

  • Which of these is easier to measure?

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