Calculating the Reaction Quotient,
The expression for the reaction quotient, Q, looks like that used to
calculate an equilibrium constant but Q can be calculated for any set of
conditions, not just for equilibrium.
Q can be used to determine which direction a reaction
will shift to reach equilibrium. If K > Q, a reaction will proceed
forward, converting reactants into products. If K < Q, the reaction
will proceed in the reverse direction, converting products into reactants.
If Q = K then the system is already at equilibrium.
In order to determine Q we need to know:
To calculate Q:
the equation for the reaction, including the physical
the quantities of each species (molarities and/or pressures), all measured
at the same moment in time.
Example: 0.035 moles of SO2, 0.500 moles of SO2Cl2,
and 0.080 moles of Cl2 are combined in an evacuated 5.00 L flask
and heated to 100oC. What is Q before the reaction begins?
Which direction will the reaction proceed in order to establish equilibrium?
Write the expression for the reaction quotient.
Find the molar concentrations or partial pressures of
each species involved.
Subsitute values into the expression and solve.
SO2(g) + Cl2(g)
Kc = 0.078 at 100oC
0.078 (K) > 0.011 (Q)
Write the expression to find the reaction quotient, Q.
Since Kc is given, the amounts must be expressed as moles per
The amounts are in moles so a conversion is required.
|0.500 mole SO2Cl2/5.00 L = 0.100 M SO2Cl2
|0.035 mole SO2/5.00 L = 0.070 M SO2
|0.080 mole Cl2/5.00 L = 0.016 M Cl2
Substitute the values in to the expression and solve
Compare the answer to the value for the equilibrium constant and predict
Since K >Q, the reaction will proceed in the forward direction in order
to increase the concentrations of both SO2 and Cl2
and decrease that of SO2Cl2 until Q = K.