CHMA11H3 Lecture Notes - Lecture 5: Thermodynamic Equilibrium, Equilibrium Constant, Partial Pressure
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16 Jan 2019
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CHMA11 Lecture 5: Understanding the Equilibrium Constant
- Significance of the Equilibrium Constant
o Large equilibrium constant: K >> 1
▪ Implies that the numerator in the equilibrium equation, which specifies
the amount of product at equilibrium, is larger than the denominator
which specifies the amount of reactant at equilibrium
▪ Therefore in this case at equilibrium the amount of products are favored
over the amount of reactants
▪ Indicates that the equilibrium point for the reaction is far to the right
• High partial pressure/concentration of products and low partial
pressure/concentration of reactants
o K does not tell us anything about the speed of a reaction
▪ It only tells us how far the reaction has proceeded once equilibrium is
reached
▪ A reaction with a large K may be very slow and take a fairly long time to
reach equilibrium
o Small equilibrium constant: K << 1
▪ Indicates that there will be more reactants than products at equilibrium
▪ Indicates that the equilibrium point for the reaction lies far to the left
• High partial pressures/concentration of reactants, low partial
pressures/concentration of products
o If K = 1
▪ Neither direction is favored
• The forward reaction proceeds about halfway
- Calculating Equilibrium Constant from Measured Quantities
o Most direct way to obtain experimental value for the equilibrium constant of a
reaction is to measure the partial pressures or concentrations of the reactants
and products in a reactant mix at equilibrium
o Many equilibrium problems will state the temp since K depends on temperature
▪ However temperature has no real formal part in the calculation
o Partial pressures within K should always be written in units of bar when we are
dealing with gas-phase reactions or mol/L when we are dealing with solution
phase reactions
▪ Units are not included when expressing the value of the thermodynamic
equilibrium constant, K
o For any reaction, the partial pressure or concentration of reactants and products
at equilibrium will depend on the initial values of said reactants and products
▪ The equilibrium partial pressure or concentration of reactants and
products will vary from one set of initial conditions to another