The Reaction Quotient (Q) expression of a reaction has the same equation as the equilibrium constant expression for that reaction. However, the reaction quotient is computed using the current concentrations or pressures, not the equilibrium ones.

For the general reaction

aA(g) + bB(g) < = > cC(g) + dD(g)

Q = (P_C)^c(P_D)^d/ (P_A)^a(P_B)^b

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.

• If Q < K, then the reaction will move to the right.
• If Q > K, then the reaction will move to the left.

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?

2NO(g) + O₂(g) < = > 2NO₂(g)

Solution: The reaction quotient is computed the same way the equilibrium constant is, just using the current pressures. Thus:

Q = (P_NO2)²/(P_NO)²P_O2

Q = (0.50 atm)²/(0.50 atm)²(0.50 atm) = 2.00 1/atm

Since Q = 2.00 1/atm which is more than K which is 1.8 1/atm, the reaction will move to the left

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