Melting is a process that has a positive enthalpy change.
When a substance undergoes melting, it transitions from a solid state to a liquid state. This process requires energy to overcome the intermolecular forces holding the solid structure together. The change in enthalpy (ΔH) for melting, often called the heat of fusion, reflects the energy absorbed during this transition.
As confirmed by the reference: "When a substance converts from a solid state to a liquid state, the change in enthalpy (ΔH) is positive." This indicates that melting is an endothermic process, meaning it absorbs heat from the surroundings.
Understanding Enthalpy Change (ΔH)
Enthalpy (H) is a thermodynamic property that represents the total heat content of a system at constant pressure. The change in enthalpy (ΔH) measures the heat absorbed or released during a process.
- Positive ΔH: Indicates that heat is absorbed by the system (endothermic process).
- Negative ΔH: Indicates that heat is released by the system (exothermic process).
For melting to occur, energy must be supplied to the solid to break down its rigid structure and allow molecules to move more freely, characteristic of a liquid. This energy input results in a positive change in enthalpy.
Key Points about Melting Enthalpy
- State Change: Solid → Liquid
- Energy Flow: Heat is absorbed
- Enthalpy Change (ΔH): Positive (ΔH > 0)
- Process Type: Endothermic
Comparing Melting and Freezing
Melting and freezing are opposite processes. While melting (solid to liquid) requires energy input (positive ΔH), freezing (liquid to solid) releases energy (negative ΔH).
| Process | State Change | Energy Flow | Enthalpy Change (ΔH) |
|---|---|---|---|
| Melting | Solid → Liquid | Absorbed | Positive (+) |
| Freezing | Liquid → Solid | Released | Negative (-) |
Understanding the sign of enthalpy change is crucial in thermodynamics and chemistry as it tells us whether a process requires energy input or releases energy.
