Answer. Extent of a reaction means the extent to which reactants are converted into products. In fact, it measures how far a reaction proceeds before establishing equilibrium state.
Generally there are three possibilities of predicting extent of reactions; such as:
i. Almost all the amounts of reactants have been converted to products before establishing equilibrium state. Such types of reactions mostly go to completion.
ii. A negligible amount of products has been formed. It means that equilibrium state has established on very small conversion of reactants to products. Such types of reactions never go to completion.
iii. Reactions which have comparable amounts of reactants and products at equilibrium state. Such type of reactions neither completes nor remains incomplete.