Chemistry · Thermodynamics · NEET
It is always positive. Melting means breaking the fixed, ordered arrangement of a solid so particles can move as a liquid. Breaking those forces needs energy, so heat is absorbed (endothermic), and any absorbed heat is written with a plus sign. That is why every ΔfusH° value in your NCERT table is positive. Freezing (the reverse) releases the same amount of heat, so freezing is negative, but fusion itself is positive.
During melting, all the heat you supply is used to break the forces holding the solid together, not to speed up the particles. Since temperature depends on how fast particles move, and their speed does not increase yet, the temperature stays fixed at the melting point. For ice this is 273 K. Only after the whole solid has melted does the temperature start to rise again. NEET may test this as 'at phase change temperature remains constant.'
They mean almost the same thing. 'Heat of fusion' is the older name; 'enthalpy of fusion' is the modern term used in NCERT because melting happens at constant pressure, where heat absorbed equals the enthalpy change (qp = ΔH). The word 'standard' (the ° symbol) just adds one condition: pressure is exactly 1 bar. So standard molar enthalpy of fusion = heat to melt 1 mole at 1 bar.
The ° (theta) symbol means the process is measured in the standard state: pressure fixed at 1 bar, and the substance in its pure form. 'Molar' means we take exactly 1 mole. So ΔfusH° for ice = 6.00 kJ/mol tells you: melting 1 mole (18 g) of ice at 1 bar and 273 K needs 6.00 kJ of heat.
Melting only loosens the particles so they can slide past each other; they still stay close together as a liquid. Vaporization must pull the particles completely apart into a gas, which needs far more energy. That is why ΔvapH° is much larger than ΔfusH° for the same substance. For water, fusion is 6.00 kJ/mol but vaporization is about 40.79 kJ/mol.
Yes. Fusion (solid → liquid) absorbs heat, so ΔfusH° is positive. Freezing (liquid → solid) is the exact reverse, so it releases the same amount of heat, and its value has the same size but a negative sign. For water: melting is +6.00 kJ/mol, freezing is −6.00 kJ/mol.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
ΔfusH° for water is 6.00 kJ/mol. This is the heat needed to melt 1 mole of ice at 273 K and 1 bar pressure, as given in NCERT.
Melting is endothermic. It absorbs heat from the surroundings to break the solid structure, so enthalpy of fusion is positive.
kJ/mol (kilojoules per mole), because it is the heat change for melting exactly 1 mole of substance.
It is a common phase-change topic. NEET can ask its sign (always positive), why temperature stays constant during melting, and Hess's-law problems that add fusion, vaporization and sublimation enthalpies (ΔsubH° = ΔfusH° + ΔvapH°).
Sublimation (solid → gas) can be split into two steps: fusion (solid → liquid) then vaporization (liquid → gas). So ΔsubH° = ΔfusH° + ΔvapH°.
