Week 7 Lesson 2 Latent Heat

Latent Heat When a solid melts or a liquid boils, energy must be added but the temperature remains constant! (This can be explained by considering that it takes energy to break the bonds holding the material together.) The amount of energy it takes to melt or boil a certain amount of material is called a latent heat.

Latent Heat For water, the latent heat of fusion (heat needed to melt ice to water) is 79.7 cal/gm For water, the latent heat of vaporization (heat needed to boil water) is 540 cal/gm. For alcohol, the latent heat of vaporization is less at 204 cal/gm. Note: These values are huge compared to the heat capacity of water which is 1 cal/gm o C. (1 calorie = Joules).

Latent Heat - Example Example: how much energy does it take to vaporize 1 liter of water if the water is initially at a temperature of 98 o F (36.66 o C) ?

Latent Heat - Example cont. First we need to find the energy to raise the temperature of the water up to boiling: this involves the heat capacity (which for water is 1 cal/gm* o C) (density of water is 1 gm/cc, 1 liter = 1000 cc): C = Q/(m*  T), with  T = (212-98) o F *5 o C//9 o F =63 o C Q = (1 cal/gm* o C)*(1 gm/1cc)*1000 cc/ l *63 o C = 63,333 cal/ l * (4.186 J/cal) = 265,000 J/ l.

Latent Heat - Example cont. Now we add in the latent heat: (for water, this is 540 cal/gm) Q = L*m = (540 cal/gm)*(1 gm/cc)*(1000 cc/ l ) = 540,000 cal/ l * (4.186 J/cal) = 2,260,000 J/ l Total energy required is: 265,000 J/ l + 2,260,000 J/ l = 2,525,000 J /l. Note: the latent heat component is much larger than the heat capacity component.

Plenary Define Latent heat ! What is the latent heat of fusion? What is the latent heat of vaporization?