Question

Assertion :The heat energy $q$ absorbed by gas is $\Delta H$ if heat is absorbed in a constant pressure. Reason: The heat energy $q$ absorbed by gas is $\Delta U$ if heat is absorbed in a constant volume.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

Since absolute internal energies cannot be determine, $U_i$ are determined with respect to internal energy

of elements in their most stable state. The internal energies of elements in their most stable allotropic

modification is arbitary taken as zero at $298K$ and $1$ atm pressure.

${\Delta }_{r}U=q_v$ for reaction taking place under constant volume and temperature condition. For the

similar reaction occurring at constant pressure and temperature, enthalpy change , $\Delta H$ is given by

$\Delta H=c{H}_c+d{H}_d-a{H}_a-b{H}_b$

where H are enthalpies of respective species

however $\Delta H$ is equal to heat exchange during chemical process at constant pressure and temperature.

${\Delta }_{r}H=q_p$ for reaction taking place under constant pressure and temperature conditions.