Chemistry · Thermodynamics · NEET
Internal energy is easy to use only at constant volume, where ΔU = qV. But real reactions in a lab happen in open test tubes and flasks, so the pressure stays constant (atmospheric) while the volume can change. Chemists needed a quantity whose change equals the heat measured at constant pressure. That quantity is enthalpy, H.
Start from the first law at constant pressure: ΔU = qp − pΔV, so qp = ΔU + pΔV. Write it out: qp = (U2 + pV2) − (U1 + pV1). The bracket (U + pV) appears naturally. We name it H = U + pV. Then qp = H2 − H1 = ΔH. So the definition is not random; it is the exact combination that makes heat at constant pressure come out clean.
The pV term accounts for the work the system does by pushing back the atmosphere when it expands (or the work done on it when it shrinks). Internal energy U is the energy stored inside the substance. Adding pV bundles in that push-the-surroundings energy, so H is like 'total heat content' available at constant pressure.
Enthalpy is a state function. It is built from U, p and V, and all three are state functions (they depend only on the current state, not the path). So H, and its change ΔH, depend only on start and end states. Note: heat q is normally path-dependent, but at constant pressure qp equals ΔH, so in that special case qp becomes path-independent too.
They are just labels for the condition. At constant volume no expansion work is done (ΔV = 0), so all heat changes U: qV = ΔU. At constant pressure the system can expand, so some heat also does work; the leftover bookkeeping is handled by enthalpy: qp = ΔH. Two conditions, two clean results.
Not always. Since H = U + pV and pV is positive for gases, H is usually larger than U for gaseous systems. But for solids and liquids pV is very small, so H is almost equal to U. The size of the gap depends on the pV term, mainly on how many gas moles are present.
Which amongst the following options is the correct relation between change in enthalpy and change in internal energy?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Enthalpy is defined as H = U + pV, where U is internal energy, p is pressure and V is volume. It is a state function whose change equals the heat absorbed at constant pressure.
Almost all reactions in the NCERT and in exams are run at constant atmospheric pressure. There, the measured heat of reaction is exactly ΔH, so enthalpy is the quantity you actually calculate for combustion, formation and neutralisation problems.
Yes, the definition H = U + pV holds for all states. But for solids and liquids the pV term is tiny, so H is nearly equal to U. The difference matters mainly when gases are involved.
qV is heat at constant volume and equals ΔU. qp is heat at constant pressure and equals ΔH. The gap between them is the expansion work pΔV that the system does against the surroundings.
