Question

What is the equation for the calculation of heat of fusion?

Answer 1

The enthalpy of fusion, `DeltaH_"fus"`, is a measured quantity, also known as the latent heat of fusion, but it is not the same as `q_"fus"`, the heat flow that causes melting or freezing to occur.

The only "equation" that utilizes `q_"fus"` in general chemistry is:

`q_"fus" = pmnDeltaH_"fus"`

• `q_"fus"` is the heat flow at constant atmospheric pressure, that pertains to either melting or freezing. Here it is in units of `"kJ"`.
• `n` is the mols of liquid being freezed, or the mols of solid being melted.
• `DeltaH_"fus"` is the enthalpy of fusion, and is a positive quantity usually reported in `"kJ/mol"`.

The `pm` is assigned depending on the direction of heat flow... if one wishes to melt a solid,

`q_"fus" -= q_"melt" = nDeltaH_"fus"`

If one wishes to freeze a liquid...

`q_"fus" -= q_"freeze" = -nDeltaH_"fus"`

And this is because the object is the system... so

• heat flowing in to melt it must be given a positive sign, for the system gains heat to melt.
• heat flowing out to freeze it must be given a negative sign, for the system loses heat to freeze.