Intermediate Topic 486 of 1102 Kinetics & Equilibrium
Equilibrium in Gas Phase
Partial pressures replace concentrations in the equilibrium expression.
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In-depth explanation of Equilibrium in Gas Phase
For gas phase equilibria it is convenient to express the equilibrium constant in terms of partial pressures rather than concentrations. The partial pressure of each gas is its mole fraction times the total pressure, and the expression has the same form as for concentrations, with each pressure raised to its stoichiometric power. ICE tables can be applied to gas equilibria by working with initial pressures and calculating the changes, which is useful when the total number of moles changes during reaction. Such problems are often solved together with the ideal gas law, which links pressure, volume and temperature.
Going deeper: Equilibrium in Gas Phase
Key ideas: Use partial pressure Pi = xi P_total and Kp expressed in atm or bar. For a reaction starting with pure reactant at pressure P0, set up an ICE table in pressures; the total pressure changes by delta n x x. Dalton's law and the ideal gas law relate pressures to moles, and Kp = Kc(RT)^delta n converts to concentrations.
Common mistakes and connections: Students use total pressure instead of partial pressure in K or forget that the total pressure changes during the reaction at constant volume. Another mistake is using mole fraction in place of pressure without the proper reference. Adding an inert gas at constant volume has no effect on Kp. Related to Kc and Kp, to the effect of pressure on equilibrium and to degree of dissociation.
Visualization of Equilibrium in Gas Phase
Starting from 2.0 atm PCl5 with Kp = 1.8, the ICE solution gives x = 1.2 atm and these equilibrium partial pressures. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For PCl5 <-> PCl3 + Cl2 at 250 degrees C the equilibrium constant Kp is about 1.8, which can be used to find the partial pressures at equilibrium.
Frequently asked questions about Equilibrium in Gas Phase
What is Equilibrium in Gas Phase?
Partial pressures replace concentrations in the equilibrium expression.
Is Equilibrium in Gas Phase a beginner, intermediate or advanced topic?
Equilibrium in Gas Phase is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Equilibrium in Gas Phase?
Study these earlier topics in Kinetics & Equilibrium first: Selective Precipitation, Complex Ion Equilibria, Effect of Complexation on Solubility.
What comes after Equilibrium in Gas Phase?
The next topics in Kinetics & Equilibrium are Degree of Dissociation and Relationship of Rate Constants and K.
Can I practise Equilibrium in Gas Phase with quizzes?
Yes. The interactive lesson includes a self-check question on Equilibrium in Gas Phase, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Equilibrium in Gas Phase fits in Kinetics & Equilibrium
Equilibrium in Gas Phase is topic 67 of 91 in Kinetics & Equilibrium.