Chemistry · Thermodynamics · NEET
Yes, they are the same idea. The law of conservation of energy is the general statement: energy cannot be created or destroyed. The first law of thermodynamics is this same law written for a thermodynamic system as ΔU = q + w. So the first law is just the conservation law applied to heat (q), work (w) and internal energy (U). NEET asks it both ways, so treat them as one concept.
No. The energy is not destroyed, it only moves out to the surroundings. If the system loses heat, the surroundings gain exactly that much. The total energy of the universe (system + surroundings) stays the same. This is why an isolated system, which cannot exchange heat or matter, has constant internal energy (ΔU = 0).
NCERT and NEET use the modern IUPAC convention: ΔU = q + w. Here work done ON the system is positive (+w) and work done BY the system is negative (−w). The old physics form ΔU = q − W uses W as work done BY the system. Both give the same final answer; just do not mix them. For NEET always start from ΔU = q + w and put the correct sign on w.
Two simple rules. Heat: absorbed BY the system is +q, released by the system is −q. Work: done ON the system (compression) is +w, done BY the system (expansion) is −w. Example: if a gas absorbs 500 J and does 200 J of work, then q = +500, w = −200, so ΔU = 500 − 200 = +300 J.
Internal energy is the total of all molecular energies (kinetic, potential, bond, electronic). We cannot count all of these, so we never know the true value of U. But we can measure the CHANGE, ΔU, from heat and work using the first law. This is why every conservation-of-energy problem asks for ΔU, not U itself.
An isolated system exchanges neither heat nor matter with the surroundings, so q = 0 and no work crosses the boundary. Then ΔU = 0, meaning its internal energy is constant. NCERT states this directly: the energy of an isolated system is constant. This is the purest form of the conservation law.
At a certain temperature T (K), during a process, 500 J is absorbed by the system and work of 200 J is done by the system. Then the change in internal energy of the system is:
A gas is allowed to expand in a well insulated container against a constant external pressure of 2.5 atm from an initial volume of 2.50 L to a final volume of 4.50 L. The change in internal energy ΔU of the gas in joules will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Energy can neither be created nor destroyed; it can only be transformed from one form to another or transferred from one body to another, so the total energy stays constant.
ΔU = q + w, where ΔU is the change in internal energy, q is heat added to the system, and w is work done on the system.
For NEET-level chemistry, yes. It holds for all ordinary chemical and physical processes. It is one of the most tested and reliable laws in the syllabus.
It stays constant. An isolated system cannot exchange heat or matter, so ΔU = 0. NCERT states the energy of an isolated system is constant.
NCERT follows the modern IUPAC sign convention where +w means work done ON the system. Using ΔU = q + w with correct signs avoids confusion, so stick to it for every NEET problem.
