| << Chapter < Page | Chapter >> Page > |
The free energy change for a process taking place with reactants and products present under nonstandard conditions, Δ G , is related to the standard free energy change, Δ G °, according to this equation:
R is the gas constant (8.314 J/K mol), T is the kelvin or absolute temperature, and Q is the reaction quotient. We may use this equation to predict the spontaneity for a process under any given set of conditions as illustrated in [link] .
T = 25 °C, and
Since the computed value for Δ G is positive, the reaction is nonspontaneous under these conditions.
Δ G = −136 kJ; yes
For a system at equilibrium, Q = K and Δ G = 0, and the previous equation may be written as
This form of the equation provides a useful link between these two essential thermodynamic properties, and it can be used to derive equilibrium constants from standard free energy changes and vice versa. The relations between standard free energy changes and equilibrium constants are summarized in [link] .
| Relations between Standard Free Energy Changes and Equilibrium Constants | ||
|---|---|---|
| K | Δ G ° | Comments |
| >1 | <0 | Products are more abundant at equilibrium. |
| <1 | >0 | Reactants are more abundant at equilibrium. |
| = 1 | = 0 | Reactants and products are equally abundant at equilibrium. |
The standard free energy change for this reaction is first computed using standard free energies of formation for its reactants and products:
The equilibrium constant for the reaction may then be derived from its standard free energy change:
This result is in reasonable agreement with the value provided in Appendix J .
K = 6.9
To further illustrate the relation between these two essential thermodynamic concepts, consider the observation that reactions spontaneously proceed in a direction that ultimately establishes equilibrium. As may be shown by plotting the free energy change versus the extent of the reaction (for example, as reflected in the value of Q ), equilibrium is established when the system’s free energy is minimized ( [link] ). If a system is present with reactants and products present in nonequilibrium amounts ( Q ≠ K ), the reaction will proceed spontaneously in the direction necessary to establish equilibrium.
Notification Switch
Would you like to follow the 'Ut austin - principles of chemistry' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|