17.3. Heat and Changes of State http://www.ck12.org
heat, it follows that the freezing of any substance releases heat. Themolar heat of solidification(∆Hsolid)of a
substance is the heat released by one mole of that substance as it is converted from a liquid to a solid. Since fusion
and solidification of a given substance are the exact opposite processes, the numerical value of the molar heat of
fusion is the same as the numerical value of the molar heat of solidification, but opposite in sign. In other words,
∆Hf us=−∆Hsolid. Shown below (Figure17.7) are all of the possible changes of state along with the direction of
heat flow during each process.
FIGURE 17.7
From left to right, heat is absorbed from
the surroundings during melting, evapo-
ration, and sublimation. From right to
left, heat is released to the surroundings
during freezing, condensation, and depo-
sition.Every substance has a unique value for its molar heat of fusion, depending on the amount of energy required to
disrupt the intermolecular forces present in the solid. When 1 mol of ice at 0°C is converted to 1 mol of liquid water
at 0°C, 6.01 kJ of heat are absorbed from the surroundings. When 1 mol of water at 0°C freezes to ice at 0°C, 6.01
kJ of heat are released into the surroundings.
H 2 O(s)→H 2 O(l) ∆Hfus= 6 .01 kJ/mol
H 2 O(l)→H 2 O(s) ∆Hsolid=− 6 .01 kJ/molThe molar heats of fusion and solidification of a given substance can be used to calculate the heat absorbed or
released when various amounts are melted or frozen.
Sample Problem 17.4: Heat of Fusion
Calculate the heat absorbed when 31.6 g of ice at 0°C is completely melted.
Step 1: List the known quantities and plan the problem.
Known
- mass of ice = 31.6 g
- molar mass of H 2 O = 18.02 g/mol
- molar heat of fusion (H 2 O) = 6.01 kJ/mol
Unknown