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•The heat energy required for a state change is given by the formula:
q= n x L
Where q is the heat energy (kJ) n is the amount of substance (mol)
L is the latent heat (kJmol-1)
•Calculate the heat energy, in kJ, required to evaporate 100g of water at 100oC.
n= m/M, n= 100/18 = 5.56 mol. L = 44.0 kJmol-1 (heat of vaporisation- since temperature is 100oC- boiling point of water) q = n x L
q = 5.56 x 44.0 = 244kJ.
Specific latent heat is energy hidden in the internal structure of a substance. As we saw in the sensible heat calculator, if we transfer heat to water, its temperature increases according to its specific heat.
Once the temperature reaches the boiling point, something interesting happens. The temperature stops increasing, and instead, the water vaporizes. This process of turning water into vapor takes some energy. This is the latent heat. Once all the water is turned into vapor, a further transfer of heat will simply increase the temperature again.
Condensation is the reverse process of vaporization, where gas turns into liquid. The latent heat released during condensation is equal to the latent heat absorbed during vaporization.
A change from a liquid to a gaseous phase is an example of a phase transition. Another common phase transition is from a solid to a liquid phase. Specific latent heat is the amount of energy absorbed or released during the phase transition per 1 kg of the substance.