7: Chemical Equilibrium
- Page ID
- 453693
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- 7.1: Equilibrium Results when Gibbs Energy is Minimized
- For a chemical reaction, the system will reach equilibrium when the Gibbs energy is minimized as a function of the extent of reaction.
- 7.2: Reaction Quotient and Equilibrium Constant
- Let’s consider a prototypical reaction at constant T,P:
- 7.3: An Equilibrium Constant is a Function of Temperature Only
- The equilibrium constant, \(K\), is a function of temperature, \(T\), and not pressure, \(P\), or composition.
- 7.4: Pressure Dependence of Kp - Le Châtelier's Principle
- Since the equilibrium constant is a function of change in Gibbs energy, which is defined for a specific composition (all reactants in their standard states and at unit pressure (or fugacity), changes in pressure have no effect on equilibrium constants for a fixed temperature. However, changes in pressure can have profound effects on the compositions of equilibrium mixtures.
- 7.5: Degree of Dissociation
- Reactions such as the one in the previous example involve the dissociation of a molecule. Such reactions can be easily described in terms of the fraction of reactant molecules that actually dissociate to achieve equilibrium in a sample. This fraction is called the degree of dissociation.
- 7.6: The Dumas Bulb Method for Measuring Decomposition Equilibrium
- A classic example of an experiment that is employed in many physical chemistry laboratory courses uses a Dumas Bulb method to measure the dissociation of N2O4(g) as a function of temperature. In this experiment, a glass bulb is used to create a constant volume container in which a volatile substance can evaporate, or achieve equilibrium with other gases present.
- 7.7: Gibbs Energy of a Reaction vs. Extent of Reaction is a Minimum at Equilibrium
- We can relate thermodynamic quantities to concentrations of molecules and we can see that there will be a characteristic ratio of concentration of reactants and products that will exist for any reaction called the equilibrium constant.
- 7.8: The Sign of ΔG and not ΔG° Determines the Direction of Reaction Spontaneity
- \(\Delta G^\circ\) represents conditions only at standard state conditions, while \(\Delta G\) represents the Gibbs energy at any conditions. Therefore, it is \(\Delta G\), and not \(\Delta G^\circ\), that determines the direction of spontaneity.
- 7.9: Reaction Quotient and Equilibrium Constant Ratio Determines Reaction Direction
- The reaction quotient (\(Q\)) measures the relative amounts of products and reactants present during a reaction at a particular point in time. The reaction quotient aids in figuring out which direction a reaction is likely to proceed, given either the pressures or the concentrations of the reactants and the products. The \(Q\) value can be compared to the Equilibrium Constant, \(K\), to determine the direction of the reaction that is taking place.
- 7.10: The van 't Hoff Equation
- The expression for equilibrium constant is a rather sensitive function of temperature given its exponential dependence on the difference of stoichiometric coefficients. A linear relation between ln K and the standard enthalpies and entropies can be constructed and is known as the van’t Hoff equation.