How do various changes affect systems at equilibrium?
When reaction systems at equilibrium are subjected to a disturbance, the equilibrium adjusts to minimize the effect of the disturbance.
How do we know what "disturbs" the equilibrium? How does the equilibrium "adjust"?
The answers to these questions become apparent if one appreciates at equilibrium the magnitude of an expression known as the reaction quotient is equal to K, the equilibrium constant.
Disturbances to equilibrium systems result in a change in the magnitude of either the reaction quotient or K so that before the equilibrium is restored, the reaction quotient is not equal to K.
Changing a concentration in the reaction quotient is a disturbance because this change makes the reaction quotient unequal to K.
Changing all concentrations in the reaction quotient by the same factor (due to a pressure or volume change) is a disturbance IF the sum of the powers of concentration in the numerator is different to the sum of powers in the denominator.
Changing the temperature is a disturbance that changes K.
The rates of the forward and reverse reactions change by different amounts. The endothermic reaction direction changes to a greater extent.
How do equilibria adjust?
How can the direction of faster reaction during restoration of equilibrium be predicted?
How do we know what "disturbs" the equilibrium? How does the equilibrium "adjust"?
N2 + 3H2 2NH3 | |
| K = | [NH3]2 |
| [N2][H2]b | |
| reaction quotient | |
The form of the reaction quotient expression depends on the equation for the reaction. An example is shown at the right. [ ] are concentrations in .
Disturbances to equilibrium systems result in a change in the magnitude of either the reaction quotient or K so that before the equilibrium is restored, the reaction quotient is not equal to K.
| K > | [NH3]2 |
| [N2][H2]3 |
Adding N2 to a system at equilibrium makes the reaction quotient above less than K.
Changing all concentrations in the reaction quotient by the same factor (due to a pressure or volume change) is a disturbance IF the sum of the powers of concentration in the numerator is different to the sum of powers in the denominator.
| K > | 4 × [NH3]2 |
| 16 × [N2][H2]3 |
Doubling the overall pressure, and thus doubling the concentration of each component changes the reaction quotient for NH3 formation because there are two powers of concentration in the numerator and four in the denominator.
| K > | [NH3]2 |
| [N2][H2]3 |
The rates of the forward and reverse reactions change by different amounts. The endothermic reaction direction changes to a greater extent.
Reactions that are exothermic in the forward direction have larger K at lower temperatures.
Reactions that are endothermic in the forward direction have larger K at higher temperatures.
NH3 formation is exothermic and decreasing the temperature makes the reaction quotient unequal to K.
Reactions that are endothermic in the forward direction have larger K at higher temperatures.
NH3 formation is exothermic and decreasing the temperature makes the reaction quotient unequal to K.
How do equilibria adjust?
Reaction occurs faster in one direction than in the other to change the equilibrium composition (and thus the reaction quotient). Products of this reaction build up until the reaction quotient equals K.
How can the direction of faster reaction during restoration of equilibrium be predicted?
If the reaction quotient < K, the forward reaction must be faster so that product-buildup makes Q equal to K.
If the reaction quotient > K, the reverse reaction must be faster so that reactant-buildup makes Q equal to K.
For the case above:
N2 + 3H2
2NH3
increases NH3 (in numerator of the reaction quotient) relative to N2 and H2.
N2 + 3H2
2NH3increases NH3 (in numerator of the reaction quotient) relative to N2 and H2.
If the reaction quotient > K, the reverse reaction must be faster so that reactant-buildup makes Q equal to K.
For the case above:
N2 + 3H2
2NH3
increases N2 and H2 (in denominator of the reaction quotient) relative to NH3.
N2 + 3H2
2NH3 increases N2 and H2 (in denominator of the reaction quotient) relative to NH3.