Final NB 2016-2017 Turley

Step Two: solid ice melts
Now, we continue to add energy and the ice begins to
melt.
However, the temperature DOES NOT CHANGE. It
remains at zero during the time the ice melts.
Each mole of water will require a constant amount of
energy to melt. That amount is named the molar heat of
fusion and its symbol is ΔHf. The molar heat of fusion is
the energy required to melt one mole of a substance at its
normal melting point. One mole of solid water, one mole
of solid benzene, one mole of solid lead. It does not
matter. Each substance has its own value.
During this time, the energy is being used to overcome water molecules' attraction for each other,
destroying the three-dimensional structure of the ice.
The unit for this is kJ/mol. Sometimes you see older references that use kcal/mol. The conversion
between calories and Joules is 4.184 J = 1.000 cal.
Sometimes you also see this number expressed "per gram" rather than "per mole." For example,
water's molar heat of fusion is 6.02 kJ/mol. Expressed per gram, it is 334.16 J/g.
Typically, the term "heat of fusion" is used with the "per gram" value.
72.0 grams of solid water is 0.0 °C. It is going to melt AND stay at zero degrees. This is an important
point. While the ice melts, its temperature will remain the same. We need to calculate the energy
needed to do this.
This summarizes the information needed:
ΔHf = 6.02 kJ/mol
The mass = 72.0 g
The molar mass of H2O = 18.0 gram/mol
The calculation needed, using words & symbols is:
q = (moles of water) (ΔHf)
We can rewrite the moles of water portion and make the equation like this:
q = (grams water / molar mass of water) (ΔHf)
Why is this equation the way it is?
Think about one mole of ice. That amount of ice (one mole or 18.0 grams) needs 6.02 kilojoules of
energy to melt. Each mole of ice needs 6.02 kilojoules. So the (grams water / molar mass of water) in
the above equation calculates the amount of moles.
With the numbers in place, we have:
q = (72.0 g / 18.0 g mol¯1 ) (6.02 kJ / mol)
So we calculate and get 24.08 kJ. We won't bother to round off right now since there are three more
calculations to go. We're doing the second step now. When all five are done, we'll sum them all up.
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