Q.What is Reaction Quotient ?

The reaction quotient Q has the same expression as the equilibrium constant K but is evaluated at any moment during the reaction, and comparing Q to K tells you which direction the reaction will shift.

For a general reversible reaction

$aA + bB \rightleftharpoons cC + dD$

the reaction quotient $Q$ is defined as:

$Q = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

using the concentrations (or, for gases, the partial pressures, giving $Q_p$) of the species present at that particular instant, whether or not the system has actually reached equilibrium.

This is exactly the same expression used to define the equilibrium constant $K$, except that $K$ is calculated only from the concentrations once the system has reached equilibrium, whereas $Q$ can be calculated at any stage of the reaction - at the very start (using only initial reactant concentrations), partway through, or at equilibrium (at which point $Q$ becomes numerically equal to $K$).

Use of Q - predicting the direction of a reaction: comparing the calculated value of $Q$ with the known equilibrium constant $K$ at that temperature tells us which way the reaction will proceed to reach equilibrium:

• If $Q < K$: there is 'too little' product relative to equilibrium, so the reaction proceeds in the forward direction (net production of C and D) until $Q$ rises to equal $K$. …