Chemistry · Thermodynamics · NEET
Zeroth law = it lets us define temperature. If body A is in thermal equilibrium with body C, and body B is also in equilibrium with C, then A and B are in equilibrium with each other. This is why a thermometer works. First law = energy is conserved: ΔU = q + w (change in internal energy = heat given to the system + work done on the system). Second law = every spontaneous (natural) change increases the total disorder, so the entropy of the universe keeps rising. Third law = the entropy of a perfect crystal at 0 K (absolute zero) is zero. For NEET, you mostly use the First and Second laws in numericals.
The First, Second and Third laws were named first (in the 1800s). Later, scientists realised there was an even more basic idea that all of them silently assumed: that temperature is a real, comparable quantity. Because this idea is more fundamental than the First law, it had to come before it. But renaming the older laws would confuse everyone, so they numbered it zero. So 'zeroth' just means 'even more basic than the first'. It defines temperature using thermal equilibrium.
Yes, they are the same idea. The law of conservation of energy says energy can neither be created nor destroyed. The first law is this same rule written for a thermodynamic system, adding the two ways energy crosses the boundary: heat (q) and work (w). So ΔU = q + w. NCERT states the first law as: the internal energy change of an isolated system is zero, and for a closed system ΔU = q + w. Both are just conservation of energy in different words.
The Second law. The First law only says the total energy is fixed; it does NOT tell you which direction a change will go. The Second law fixes the direction: a change is spontaneous only if the entropy of the universe (system + surroundings) increases, ΔS(universe) > 0. This is also why we use Gibbs energy ΔG = ΔH − TΔS. A negative ΔG (spontaneous) is just a shortcut for 'total entropy went up'. Almost every NEET spontaneity question is a Second-law question.
The Third law says the entropy of a pure, perfectly ordered crystal is exactly zero at absolute zero (0 K). Because we know this fixed starting point, we can measure absolute entropy (S°) of substances by heating them from 0 K and adding up the entropy gained. This is why data tables give a real value for S° (not just ΔS). Contrast this with internal energy and enthalpy, where only changes (ΔU, ΔH) can be found, never an absolute value.
NEET Chemistry Thermodynamics focuses on the First law (ΔU = q + w, enthalpy, calorimetry) and the Second law (entropy, spontaneity, Gibbs energy ΔG = ΔH − TΔS). The Zeroth law is background (it defines temperature and equilibrium). The Third law is used lightly, mainly to explain why absolute entropy S° has a real value. Most calculation PYQs come from the First and Second laws, so master those two first.
The correct thermodynamic conditions for the spontaneous reaction at all temperatures is:
For a given reaction, ΔH = 35.5 kJ mol⁻¹ and ΔS = 83.6 J K⁻¹ mol⁻¹. The reaction is spontaneous at (assume ΔH and ΔS do not vary with temperature):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
You should know what each law states in one line, but NEET calculations mainly test the First law (ΔU = q + w, enthalpy) and the Second law (entropy and spontaneity via ΔG = ΔH − TΔS). Learn those two deeply.
Almost never as a calculation. It is conceptual: it explains why temperature can be defined and why a thermometer reading is trustworthy. Just understand thermal equilibrium.
The Third law gives a fixed zero point for entropy (a perfect crystal at 0 K has S = 0). Internal energy has no such natural zero, so only ΔU can be measured, not U itself.
No. The Second law only says a spontaneous change has the potential to happen (ΔG < 0). It says nothing about speed. Some spontaneous reactions are extremely slow, like H₂ and O₂ sitting together without reacting.
