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CHEMISTRY 122
HEAT OF SOLUTION
HEAT OF SOLUTION (∆Hsoln)
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During the formation of a solution, heat is either
released or absorbed
The enthalpy change by dissolution of one mole of
substance is the molar heat of solution
Remember, a reaction can be either exothermic or
endothermic.
Exothermic Reactions
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The heat of solution is written as a negative number
to indicate that heat is being released from the
system.
In the case of examples given in the textbook, both
reactions are the result of heat being released from
a heat pack once the substance comes into contact
with water.
Endothermic Reactions
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The heat of reaction is positive. This indicates heat
being absorbed by the system from the
surroundings.
In the example given in the textbook, a cold pack,
when activated by a chemical reaction with water,
becomes extremely cold. Heat is moving from the
surroundings to the system.
Figure 17.11
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Is the process that occurs in the cold pack
endothermic or exothermic? How do you know?
In which direction is the heat flowing?
Is the process in a hot pack endothermic or
exothermic? How do you know?
In which direction is heat flowing?
For tomorrow…
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Answer the following questions:
 How
many kilojoules of heat are released when 25.0g
of NaOH are dissolved in water?
 Sketch a heating curve for 1 mole of ice being heated
from -10°C to 110°C. Write a brief explanation of
the graph. Calculate how much energy would be
required to change 54.0g of ice at -10°C to water
vapour at 110°C.
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Questions 27 – 31, page 526