Thermochemistry 4
INFORMATION
If you were to place a thermometer in a beaker of water and boil the water vigorously with a Bunsen burner, you would notice that the temperature of the water would never rise above the boiling point the entire time the water boiled. The water would continue to boil, however, and the energy being provided to the beaker would certainly be consumed by the liquid.
So where is the energy going? Any change of state in matter requires energy, energy that goes towards changing the state rather than increasing (or decreasing) the temperature. The amount of heat necessary to change state is, like standard enthalpies of formation, reported on a per mole basis.
The amount of energy required to be absorbed by a mole of solid to change to a liquid without changing its temperature is called the molar heat of fusion (∆H fusion
). A negative sign of the molar heat of fusion indicates the molar heat of solidification (-∆H solid
), the heat released by 1 mole of gas when it changes from liquid to solid.
The amount of energy required to be absorbed by a mole of liquid to change to a gas without changing its temperature is called the molar heat of vaporization (∆H vap
). A negative sign of the molar heat of vaporization indicates the molar heat of condensation (-∆H cond
), the heat released by 1 mole of gas when it changes from gas to liquid.
Like standard enthalpies, molar heats of fusion or vaporization must be measured. For example, for water:
H
2
O
(s)
H
2
O
(l)
∆H fusion
= 333 J/g
H
2
O
(l)
H
2
O
(s)
∆H solid
= -333 J/g
H
2
O
(l)
H
2
O
(g)
∆H vap
= 2261 J/g
H
2
O
(g)
H
2
O
(l)
∆H cond
= -2261 J/g
Heats of fusion and vaporization vary from substance to substance.
If the amount of energy absorbed is graphed versus temperature, the profile will appear as follows for any given substance:
Changes of State / 1

Key Question
1.
On the graph above, point A represents the substance in the solid state. What is represented by points B, C, D, and E?
2.
Why is a burn from steam potentially much more dangerous than a burn from hot water?
INFORMATION
Heats of Solution
During the formation of a solution (that is, when a solute dissolves in a solvent), heat is either absorbed or released. The enthalpy change observed by the dissolution of one mole of a substance is called the molar heat of solution (∆H soln
).
The dissolution of sodium hydroxide in water, for example, is exothermic:
NaOH
(s)
+ H
2
O
(l)
Na
+
(aq)
+ OH
-
(aq)
∆H soln
= -445.1 kJ/mol
The dissolution of ammonium nitrate in water, on the other hand, is endothermic:
NH
4
NO
3(s)
+ H
2
O
(l)
NH
4
+
(aq)
+ NO
3
-
(aq)
∆H soln
= 82.8 kJ/mol
Key Question
3.
What practical, real-life applications can you think of that might arise from the enthalpy change that results from the dissolution of solids in water?
Changes of State / 2