Phase change math

Since the temperature doesn’t change (at least not usually) during a phase change we cannot use the formula q=mcΔT to calculate the heat involved in that phase change. However the calculation is quite simple. If the difference between any two phases is simply the potential energy difference, all we need to know is that amount to calculate the heat involved in the process.

The formula that we use is

 

where q =heat, m=amount (sometimes in grams and sometimes in moles) and ΔH is the heat required for the phase change (actually the amount of heat required for one gram or one mole of the substance to undergo the phase change).

If the phase change is between solid and liquid the formula looks like q= mΔH fus and ΔH fus is called the heat of fusion. If the phase change is between liquid and gas the formula looks like q=mΔ vap and ΔH vap is called the heat of vaporization.

For any given substance the ΔH vap is always bigger than the ΔH fus because the potential energy change is always larger when going from liquid to gas than from solid to liquid.

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