Heat capacity at constant pressure is greater than heat capacity at constant volume. Why? 

At constant volume when the heat is added to a system, no work is done by the system. Thus the heat added to the system is used up completely to increase the internal energy of the system. Again if pressure is kept constant, during the addition of heat to a system, then heat absorbed also does some work for expansion in addition to the increase in internal energy. Thus if at constant pressure, the temperature of the system is to be raised through the same value as the constant volume, then some extra heat is required for doing the work of expansion. Hence C_p > C_v.