Gas-Phase Chemical Equilibrium: Summary
The answers to the ConcepTests are given below and will open in a separate window.
Key points from this module:
- The equilibrium constant is dimensionless and is independent of pressure, but not of temperature.
- The equilibrium composition is only independent of pressure if the number of gas-phase moles does not change in the reaction and the gases are ideal. If the number of gas-phase moles in the reaction increases, increasing the pressure at constant temperature decreases the equilibrium conversion. If the number of gas-phase moles in the reaction decreases, increasing the pressure at constant temperature increases the equilibrium conversion.
- An inert gas can change the equilibrium conversion if the reaction involves a change in the number of gas-phase moles. If the number of gas-phase moles in the reaction increases, adding an inert gas at constant temperature and pressure increases equilibrium conversion (the same as decreasing the pressure). If the number of gas-phase moles in the reaction decreases, adding an inert at constant temperature and pressure decreases equilibrium conversion (the same as increasing the pressure).
- For an exothermic reaction (ΔH < 0), as temperature increases, K decreases.
- For an endothermic reaction (ΔH > 0), as temperature increases, K increases.
From studying this module, you should now be able to:
- Express equilibrium constants in terms of thermodynamic activities.
- Explain why the chemical equilibrium constant is dimensionless.
- Explain why the equilibrium constant is independent of pressure.
- Calculate heat of reaction at 298 K from heats of formation.
- Calculate how the equilibrium constant changes with temperature.
- Calculate the standard Gibbs free energy change for a reaction at 298 K, and its equilibrium constant, from Gibbs free energies of formation.
- Calculate equilibrium conversions.
- Explain why pressures must be in bar in equilibrium calculations.
- Describe how equilibrium constants change with temperature for endothermic and exothermic reactions.
- Calculate equilibrium mole fraction for ideal gases and non-ideal gases.
- Predict how pressure changes will shift equilibrium conversion for a gas-phase reaction with a mole change.
- Predict how an addition of an inert will affect equilibrium conversion.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder