Question

Which among the following equations represents the first law of thermodynamics under isobaric conditions ?

• A. $\ce{\Delta U = q_p - P_{ex} . \Delta V }$
• B. ${q_v = \Delta U }$
• C. $\ce{\Delta U = W }$
• D. $W =-q$

Answer 1

Answer: (A)

$\ce{\Delta U = q_p - P_{ex} . \Delta V }$

Answer 2

According to first law of thermodynamics, $\Delta U=q+W$ In case of isobaric conditions, change of state is brought about at constant pressure, i.e. $dp= Q$ Moreover, work done for a process is given as $W=-p_{\text {ext. }} \Delta V$. Substituting the value of 'W' in first law of themodynamics $\Delta U=q_{p}-p_{\text {ext }} \cdot \Delta V$