Enthalpy of Fusion / Melting
Also: Heat of Fusion, Latent Heat of Melting
The enthalpy of fusion is the amount of energy a substance must absorb in order to change from the solid to the liquid state at its melting point, without changing its temperature.
The enthalpy of fusion (also called heat of fusion or latent heat of melting) describes the energy that must be supplied to a substance in order to convert it completely from the solid to the liquid state at constant pressure and constant temperature - namely at the melting point. This energy does not change the temperature but breaks the crystal lattice bonds in the solid. The process is endothermic; on solidification the same energy is released again as the enthalpy of crystallisation.
Physical basis
When a metal is heated, the temperature initially rises linearly. At the melting point it remains constant despite further heat supply, until the entire substance has melted - only then does the temperature of the melt continue to rise. The supplied energy goes into overcoming the intermolecular bonding forces.
The specific enthalpy of fusion $\Delta H_\text{fus}$ is given by:
Q = m x DeltaH_fus
where Q is the required heat quantity (in J), m the mass (in kg) and DeltaH_fus the specific enthalpy of fusion (in kJ/kg) of the respective material.
Enthalpies of fusion of important precious metals compared
| Metal | Melting point | Enthalpy of fusion (kJ/mol) | Enthalpy of fusion (kJ/kg) |
|---|---|---|---|
| Gold | 1,064 degC | 12.55 | 63.7 |
| Silver | 961 degC | 11.28 | 104.6 |
| Platinum | 1,768 degC | 22.17 | 113.6 |
| Palladium | 1,555 degC | 16.74 | 157.3 |
| Copper | 1,085 degC | 13.05 | 205.4 |
Platinum and palladium have significantly higher enthalpies of fusion, both per mole and per kilogram, than gold, which explains the increased energy expenditure in their processing at the refinery.
Relevance for precious metal processing and melt value
For buying, recycling and affinage of scrap gold, broken gold or dental gold, the enthalpy of fusion plays a practical role: the melt value of an object describes the pure material value in relation to the current gold price or silver price - independent of form or minting. Refineries and recycling operations must calculate the thermal energy input during melting, which depends directly on the enthalpy of fusion of the alloy mixture.
The fineness of an alloy also affects its effective enthalpy of fusion: alloys such as 585 gold (14 carat) melt over a temperature range (solidus to liquidus), not at a defined point like the pure metal fine gold (999).
In brief
The enthalpy of fusion is a key quantity of precious metal physics: it determines the energy requirement during melting, explains the plateau in the heating curve and is the basis for the melt value calculator. A higher enthalpy of fusion means more energy expenditure per kilogram - with a direct influence on processing costs at the refinery and in recycling.