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:

To calculate Q: 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?

SO2Cl2(g) double arrows SO2(g) + Cl2(g)       Kc = 0.078 at 100oC

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 SO2 and Cl2 and decrease that of SO2Cl2 until Q = K.

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