Calculating the Reaction Quotient,
Q
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:

the equation for the reaction, including the physical
states,

the quantities of each species (molarities and/or pressures), all measured
at the same moment in time.
To calculate Q:

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.
Example: 0.035 moles of SO_{2}, 0.500 moles of SO_{2}Cl_{2},
and 0.080 moles of Cl_{2} are combined in an evacuated 5.00 L flask
and heated to 100^{o}C. What is Q before the reaction begins?
Which direction will the reaction proceed in order to establish equilibrium?
SO_{2}Cl_{2}(g)
SO_{2}(g) + Cl_{2}(g)
K_{c} = 0.078 at 100^{o}C

Write the expression to find the reaction quotient, Q.

Since K_{c} is given, the amounts must be expressed as moles per
liter (molarity).
The amounts are in moles so a conversion is required.
0.500 mole SO_{2}Cl_{2}/5.00 L = 0.100 M SO_{2}Cl_{2} 
0.035 mole SO_{2}/5.00 L = 0.070 M SO_{2} 
0.080 mole Cl_{2}/5.00 L = 0.016 M Cl_{2} 

Substitute the values in to the expression and solve
for Q.

Compare the answer to the value for the equilibrium constant and predict
the shift.
0.078 (K) > 0.011 (Q)
Since K >Q, the reaction will proceed in the forward direction in order
to increase the concentrations of both SO_{2} and Cl_{2}
and decrease that of SO_{2}Cl_{2} until Q = K.