Based on Le Chatelier's principle, if the equilibrium of a system is disturbed by changing the temperature, pressure or concentration, then it will shift in a direction to undo the effect of the induced change.
The given equilibrium is:
A + B ↔ AB
Removal of the reactant A implies that the concentration of A has decreased, therefore the equilibrium will shift in a direction to produce more of A. Thus, it will shift to the left and the rate of the reverse or backward reaction will increase.
