Latent heat
is the heat released or absorbed during a phase change, where the
temperature does not change. If we are talking specifically about the
heat of vaporization, it is the heat that must be absorbed to change
liquid water to water vapor or the heat that must be released to
condense water vapor to liquid water.
ch301.cm.utexas.edu
Notice that we are on the horizontal liq/gas equilibrium line above.
Example: The latent heat of vaporization for water is 40.68 kJ/mol.
How much heat would be needed to completely vaporize 36.0g of water at
100 deg C?
ANS: Since the water is already at its boiling point, there will be
no heat for changing temperature (the diagonals on the diagram). All of
the heat will go directly to changing the phase.
This becomes a dimensional analysis problem with the given of 36.0g
36.0gH2O1⋅1molH2O18.0gH2O⋅40.68kJ1molH2O=81.4kJ
Note:
Latent heat of vaporization is actually the total amount of enthalpy (a kind of energy/heat) necessary to accomplish a phase change for a liquid or gas at the boiling/condensation point.
Phase changes are generally considered at constant pressure, rather than constant volume. Because a kg of say, 100°C steam, occupies a much greater volume than a kg of 100°C water, a lot of work
has to be done to push the environment out of the way as that water
expands to become steam. (The fact that vaporization does a lot of work
is why we use steam to power a large number of the turbines in the world.)
Enthalpy H includes the internal energy U that goes into the new vapor, plus the work, W=PΔV, to expand the liquid into a gas at the same pressure:
H=U+PΔV,
where P is the ambient pressure, ΔV is the change in volume, and U
is the internal energy of the substance in its new phase. Latent heat
of vaporization is typically much bigger than latent heat of fusion
because of the much larger change in volume involved
Friday, 31 March 2017
Latent heat is the heat released or absorbed during a phase change, where the temperature does not change. If we are talking specifically about the heat of vaporization, it is the heat that must be absorbed to change liquid water to water vapor or the heat that must be released to condense water vapor to liquid water
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