How is the reaction quotient (Qc) related to the equilibrium constant (Keq) for a chemical reaction?

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justaguide | College Teacher | (Level 2) Distinguished Educator

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The reaction quotient can be used with the equilibrium constant to determine if a chemical reaction is at equilibrium and in which direction it is likely to move.

Take a chemical reaction: A + B <--> C + D where the equilibrium constant is Keq.

The ratio [C][D]/[A][B] which is determined by using the concentrations at any moment of time is known as the reaction quotient Qc.

Comparing Qc and Keq, three situtions can arise:

  • Qc > Keq : This is the case when the concentration of the products is too high and the reaction moves towards the left with the formation of the reactants.
  • Qc < Keq: This is the case when the concentration of the reactants is too high and the reaction has to move in the forward direction with an increase in the concentration of the products.
  • Qc = Keq : When this is the case, the reaction is at equilibrium, and for other conditions remaining the same, the ratio of the concentration of reactants and products will not change.

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