Enthalpy of fusion (ΔHfus) is the amount of energy required to change one mole of a substance from solid to liquid at its melting point. In AP Chem Unit 6, it explains why temperature stays flat during melting and shows up in q = nΔHfus calculations.
Enthalpy of fusion (ΔHfus) is the energy you have to add to melt a substance at its melting point. Usually it's reported per mole, so a value like 6.01 kJ/mol for ice means every mole of ice absorbs 6.01 kJ to become liquid water at 0°C.
Here's the part that trips people up. While a solid melts, all that incoming energy goes into breaking apart the intermolecular forces holding particles in place, not into making them move faster. That's why the temperature does not change during melting, even though heat is pouring in. Melting is always endothermic, so ΔHfus is always positive. You're paying an energy cost to pull particles out of their rigid arrangement.
Enthalpy of fusion lives in Unit 6: Thermochemistry, where the CED treats phase changes as physical processes with their own enthalpy values. It connects to learning objective 6.8.A, which asks you to calculate the enthalpy change for a chemical or physical process. Melting is the classic physical process. The exam loves pairing ΔHfus with heating curves, where you have to recognize that the flat plateau at the melting point is where q = nΔHfus applies instead of q = mcΔT. It's also a clean test of whether you understand that energy can change a substance's structure without changing its temperature, which is one of the central ideas of the whole unit.
Keep studying AP Chemistry Unit 6
Heat of Solidification (Unit 6)
Solidification is melting run in reverse, so the heat of solidification is the same number as the enthalpy of fusion with the sign flipped. If ice absorbs 6.01 kJ/mol to melt, water releases 6.01 kJ/mol when it freezes. One value, two directions.
Latent Heat (Unit 6)
Enthalpy of fusion is one specific type of latent heat, which is the umbrella term for energy absorbed or released during any phase change without a temperature change. On a heating curve, every flat segment is latent heat doing its thing.
Melting Point (Unit 3)
The melting point tells you the temperature where fusion happens, and ΔHfus tells you the energy cost. Both trace back to intermolecular forces from Unit 3. Stronger IMFs mean both a higher melting point and a larger enthalpy of fusion, because there's more attraction to break.
Standard Conditions (Unit 6)
Enthalpy values come with a degree symbol (like ΔH°) when measured at standard conditions. The same bookkeeping that lets you use tables of standard enthalpies of formation under 6.8.A.1 applies to phase-change enthalpies too, so always check the conditions a table assumes.
Multiple-choice questions usually hand you a heating curve and ask what's happening at the first plateau, or give you a ΔHfus value and ask how much energy melts a given number of moles (q = nΔHfus, watch your mole conversion). You also need to keep your delta-H symbols straight. A practice question staple asks what ΔHf represents, and the answer is enthalpy of formation, not fusion, so read subscripts carefully. On free-response questions, phase-change enthalpies show up inside multi-step calculations where you combine q = mcΔT for the temperature-change segments with q = nΔHfus for the melting segment. The classic conceptual FRQ move is explaining why temperature stays constant during melting: the energy goes into overcoming intermolecular forces, not into kinetic energy.
These two share an unfortunate subscript. Enthalpy of fusion (ΔHfus) is the energy to melt a substance, a physical change. Enthalpy of formation (ΔHf or ΔH°f) is the enthalpy change when one mole of a compound forms from its elements in their standard states, and it's the value you plug into ΔH°reaction = ΣΔH°f products − ΣΔH°f reactants. If the problem involves melting, you want fusion. If it involves a reaction and a table of values, you want formation.
Enthalpy of fusion (ΔHfus) is the energy needed to melt one mole of a substance at its melting point, and it is always positive because melting is endothermic.
Temperature stays constant during melting because the added energy breaks intermolecular forces instead of increasing the particles' kinetic energy.
On a heating curve, use q = nΔHfus at the melting plateau and q = mcΔT on the sloped segments, never both at once.
Freezing releases exactly the same amount of energy that melting absorbs, so the heat of solidification equals negative ΔHfus.
Stronger intermolecular forces mean a larger enthalpy of fusion, which links this Unit 6 concept directly back to Unit 3.
Don't confuse ΔHfus (fusion, melting) with ΔHf (formation, building a compound from its elements); the subscripts look similar but the processes are completely different.
It's the energy required to change a substance from solid to liquid at its melting point, usually given in kJ/mol. For ice, it's 6.01 kJ/mol at 0°C, and you use it in the formula q = nΔHfus.
No. Fusion (ΔHfus) is the energy to melt a substance, a physical change. Formation (ΔH°f) is the enthalpy change when one mole of a compound forms from its elements, and it's what you use in ΔH°reaction = ΣΔH°f products − ΣΔH°f reactants.
Because all the incoming energy goes into breaking intermolecular forces that hold the solid together, not into speeding up the particles. Temperature measures average kinetic energy, and that isn't changing during the phase change.
Yes. Melting always absorbs energy, so ΔHfus is always positive. The reverse process, freezing, releases that same energy, so the heat of solidification is the negative of ΔHfus.
Latent heat is the general term for energy absorbed or released during any phase change without a temperature change. Enthalpy of fusion is the specific latent heat for the solid-to-liquid transition.