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For the general reaction
One use of Q is to determine which way a reaction will go. Simply compute Q using the current pressures or concentrations and compare it to K for the reaction.
Example: For the reaction below, K is 1.8 1/atm at a certain temperature. If the initial partial pressures of all species are 0.50 atm, which direction will the reaction move in?
Solution: The reaction quotient is computed the same way the equilibrium constant is, just using the current pressures. Thus:
Since Q = 2.00 1/atm which is more than K which is 1.8 1/atm, the reaction will move to the left