Concept:The first law of thermodynamics restricts where heat energy can go depending on the physical boundaries of the system (constant volume vs. constant pressure).
Formula:$$ q = \Delta E + P\Delta V $$
Solution:- When the volume is rigidly held constant, the system cannot expand or contract, meaning \( \Delta V = 0 \).
- Consequently, the pressure-volume work term (\( P\Delta V \)) becomes exactly zero.
- The entire first law equation collapses to: \( q_v = \Delta E \).
- Thus, heat supplied at constant volume perfectly equals the Internal energy change.
Why other options are incorrect:Enthalpy change equals heat at constant
pressure (\( q_p \)). Bond energy and heat of sublimation are specific thermochemical constants, not the generalized outcome of constant-volume heating.
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