4B. Consider the following chemical reaction in an ADIABATIC CONTAINER WITH MOVABLE PISTON:

A (solid) + B (solid) -> C(solid) + D (gas)  + 20 kJ mol-1 of energy released (at 300K and 1 atm pressure)

One mole each of A and B are completely converted to 1 mole each of C and D in a piston at constant pressure, initially at 
300 K. The system  (contents of the piston) exchanges no heat with the surroundings, so the temperature of the system 
increases during the reaction. At the end of the reaction, the temperature of the system is higher by 10o C. What is the heat
capacity of the system (C+D) at constant pressure, in units of kJ mol-1 K-1?

Answer: I think the difficulty that you may face in conceptualising this problem is that we typically think of heat capacity= 
heat transfered/temperature change and here we have an adiabatic container possible heat transfer.

It would be clearer once you were to write a Hess cycle representation of how to use the information

(1)A+B (300K, 1bar, V1) ----(isothermal(Q=20KJ lost to heat bath))---> C+D (300K, 1bar, V')

(2)C+D (300K, 1bar, V')---(1bar, Q=20KJ from heat source)----> C+D (310K, 1bar, V2)

  --------------------------------------------------------
A+B (300K, 1bar, V1) --(no net heat exchange==adiabatic)---> C+D (310K, 1bar, V2)


We are hence able to calculate the Cp for C+D from the second step above as Q/dT

Note however for calculating the work done you would need the volume changes in both steps.