Chemistry · Thermodynamics · NEET
The superscript circle (°, sometimes written as a plimsoll ⦵) means "standard state." So ΔrH° is the enthalpy change of the reaction when all reactants and products are in their standard states — pure form at 1 bar. If you see ΔrH with no circle, it just means the reaction ran under whatever conditions were given, not the special standard conditions. For NEET, always read "°" as "1 bar, pure form."
For NEET and the current NCERT, the standard pressure is 1 bar. Older books used 1 atm. They are very close (1 atm = 1.01325 bar), but the modern IUPAC and NCERT choice is 1 bar. If a question mentions "standard state," answer with 1 bar unless the problem clearly says 1 atm.
No — this is the most common mistake. The standard state only fixes the pressure at 1 bar and the pure form. Temperature is NOT part of the standard state. You can have a standard state at any temperature: pure liquid ethanol at 298 K and 1 bar, or pure solid iron at 500 K and 1 bar. Because ΔH changes with temperature, data are usually reported at 298 K, but that is a reporting choice, not a rule of the standard state.
Enthalpy of reaction (ΔrH) is the heat change under any conditions the reaction happens in. Standard enthalpy of reaction (ΔrH°) is that same heat change but with a strict rule: every substance must be pure and at 1 bar. Because the conditions are fixed, ΔrH° values from different books can be compared and added — that is why NEET calculations (Hess's law, formation enthalpies) all use the standard values.
The standard state of a substance at a chosen temperature is its pure form at 1 bar. Example: standard state of ethanol at 298 K is pure liquid ethanol at 1 bar; standard state of iron at 500 K is pure solid iron at 1 bar. For a gas it is the pure gas behaving ideally at 1 bar. It is a fixed reference so everyone measures ΔH the same way.
Because the enthalpy change of a reaction depends on the conditions (pressure, physical form). If two students measured ΔH under different pressures, their numbers would not match or add up. Fixing a common reference — pure form at 1 bar — lets us tabulate values, compare them, and combine them with Hess's law. For NEET, all standard-value data (ΔfH°, ΔcH°) rely on this shared rule.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is written ΔrH° (or ΔrH⦵). The subscript r means "reaction" and the superscript circle ° means "standard state" (pure form at 1 bar).
1 bar (100,000 Pa). This is the modern NCERT/IUPAC standard. Older texts used 1 atm (≈1.013 bar).
No. Values are usually tabulated at 298 K (25 °C) for convenience, but the standard state itself does not fix temperature — only pressure (1 bar) and pure form.
The pure gas behaving as an ideal gas at 1 bar pressure, at the specified temperature.
Because ΔrH° values are fixed under common conditions, they can be added, reversed, and combined using Hess's law and standard formation enthalpies — which is how most thermochemistry NEET problems are solved.
