Equilibrium and kinetic factors
The principles of equilibrium and kinetics detailed below can be applied in choosing conditions of pressure and temperature appropriate to obtain the maximum yield of the products of a gas phase reaction in the shortest time.
Kinetics is the study of rates of reaction. Reaction rates are faster
The yield of a reaction under specified conditions depends both on the magnitude of its equilibrium constant for the reaction and the expression (reaction quotient) that relates equilibrium concentrations to the equilibrium constant.
Increasing overall pressure results in all concentrations in the reaction quotient changing by the same factor
This can affect the relative amounts of products and reactants at equilibrium (and thus the yield). IF the sum of the powers of concentration in the numerator differ from the sum of the powers of concentration in the denominator.
Increasing the temperature
This increases the yield IF the reaction producing the desired product is endothermic because endothermic reactions have larger equilibrium constants at higher temperatures.
Using a catalyst
Catalysts increase the rates of forward and reverse reactions equally and do not appear in the reaction quotient expression. Therefore the use of a catalyst has no effect on the yield.
Kinetics is the study of rates of reaction. Reaction rates are faster
at higher temperatures
at higher reactant concentrations (higher pressures)
in the presence of a catalyst (if an appropriate one can be found).
at higher reactant concentrations (higher pressures)
in the presence of a catalyst (if an appropriate one can be found).
| K = | [NO2]2 |
| [NO]2[O2] | |
| equilibrium constant | reaction quotient |
These are shown for the reaction:
2NO(g) + O2(g)
2NO2(g)
NOTE: The equilibrium constant is a number. The reaction quotient (sometimes the symbol Q is used) is an expression.
K is calculated by substituting equilibrium concentrations into the reaction quotient expression.
2NO(g) + O2(g)
2NO2(g)NOTE: The equilibrium constant is a number. The reaction quotient (sometimes the symbol Q is used) is an expression.
K is calculated by substituting equilibrium concentrations into the reaction quotient expression.
Increasing overall pressure results in all concentrations in the reaction quotient changing by the same factor
This can affect the relative amounts of products and reactants at equilibrium (and thus the yield). IF the sum of the powers of concentration in the numerator differ from the sum of the powers of concentration in the denominator.
| K > | 4 × [NO2]2 |
| 8 × [NO]2[O2] |
The effect will be larger where more powers of concentrations are multiplied together!!
For example, doubling the overall pressure, and thus doubling each concentration changes the magnitude of reaction quotient for NO2 formation because there are two powers of concentration in the numerator and three in the denominator. Because the reaction quotient is smaller than K, reaction to restore equilibrium has NO2 as the product.
For example, doubling the overall pressure, and thus doubling each concentration changes the magnitude of reaction quotient for NO2 formation because there are two powers of concentration in the numerator and three in the denominator. Because the reaction quotient is smaller than K, reaction to restore equilibrium has NO2 as the product.
Increasing the temperature
| ΔH positive larger K at higher temperatures |
The enthalpy change for the reaction forming NO2 above is -114 kJ mol–1. Therefore the yield of NO2 (relative amount in the equilibrium mixture) is lower at higher temperatures due to K being lower.
Using a catalyst
Catalysts increase the rates of forward and reverse reactions equally and do not appear in the reaction quotient expression. Therefore the use of a catalyst has no effect on the yield.