Question

For a reaction,
$HCl\left(aq\right)+NaOH\left(aq\right)\rightleftharpoons NaCl\left(aq\right)+H_{2}O\left(l\right)Given{K}_c=0.5,\left[HCl\right]=3M,\left[NaOH\right]=4M,\left[NaCl\right]=6M$
Find $Q_c$ and the direction in which reaction will shift.

• A. $Q_c=0.5$ ; reaction shifts in backward direction
• B. $Q_c=0.5$ ; reaction shifts in forward direction
• C. $Q_c=0.5$ ; reaction is at equilibrium
• D. $Q_c=1$ ; reaction shifts in backward direction

Answer 1

The correct option is C $Q_c=0.5$ ; reaction is at equilibrium

Finding $Q_c:$

$Q_c=\frac{\left[NaCl\left(aq\right)\right]}{\left[HCl\left(aq\right)\right]\left[NaOH\left(aq\right)\right]}=\frac{6}{3\times 4}=0.5$

Given $K_c=0.5$

Comparing, we see that $Q_c=K_c=0.5$

The reaction is already at equilibrium, hence there is no shift in the direction.


Theory:

Difference between equilibrium constant $K_{eq}$ and reaction quotient $Q$ :


$K_{eq}$ (Equilibrium Constant)
The concentrations represented in the ratio are at equilibrium
Rate of forward Reaction = Rate of Backward Reaction.


$Q$ (Reaction Quotient)
The concentrations represented in the ratio may or may not be at equilibrium
Rate of forward Reaction may or may not be equal to Rate of Backward Reaction.