CHMA11H3 Lecture Notes - Lecture 5: Thermodynamic Equilibrium, Equilibrium Constant, Partial Pressure

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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
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
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