Chemistry 17.4
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17.4
Calculating Heats of Reaction
Emeralds are composed of the
elements chromium, aluminum,
silicon, oxygen, and beryllium. What
if you wanted to determine the heat
of reaction without actually breaking
the gems down to their component
elements? You will see how you can
calculate heats of reaction from
known thermochemical equations
and enthalpy data.
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17.4
Calculating Heats of Reaction
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Hess’s Law
Hess’s Law
What are two ways that you can
determine the heat of reaction when it
cannot be directly measured?
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17.4
Calculating Heats of Reaction
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Hess’s Law
Hess’s law allows you to determine the
heat of reaction indirectly.
Hess’s law of heat summation states that
if you add two or more thermochemical
equations to give a final equation, then you
can also add the heats of reaction to give
the final heat of reaction.
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17.4
Calculating Heats of Reaction
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Hess’s Law
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17.4
Calculating Heats of Reaction
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Hess’s Law
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17.4
Calculating Heats of Reaction
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Standard Heats of Formation
Standard Heats of Formation
For a reaction that occurs at standard
conditions, you can calculate the heat of
reaction by using standard heats of
formation.
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17.4
Calculating Heats of Reaction
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Standard Heats of Formation
The standard heat of formation (∆Hf0) of a
compound is the change in enthalpy that
accompanies the formation of one mole of a
compound from its elements with all substances
in their standard states at 25°C.
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17.4
Calculating Heats of Reaction
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Standard Heats of Formation
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17.4
Calculating Heats of Reaction
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Standard Heats of Formation
The Standard Heat of Formation of Water
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SAMPLE PROBLEM 17.7
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SAMPLE PROBLEM 17.7
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SAMPLE PROBLEM 17.7
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SAMPLE PROBLEM 17.7
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Practice Problems for Sample Problem 17.7
Problem Solving 17.32 Solve
Problem 32 with the help of an
interactive guided tutorial.
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17.4
Calculating Heats of Reaction
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Standard Heats of Formation
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17.4 Section Quiz.
Assess students’ understanding
of the concepts in Section 17.4.
Continue to:
-or-
Launch:
Section Quiz
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17.4 Section Quiz.
1. According to Hess’s law, it is possible to
calculate an unknown heat of reaction by
using
a. heats of fusion for each of the compounds
in the reaction.
b. two other reactions with known heats of
reaction.
c. specific heat capacities for each compound
in the reaction.
d. density for each compound in the reaction.
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17.4 Section Quiz.
2. The heat of formation of Cl2(g) at 25°C is
a. the same as that of H2O at 25°C.
b. larger than that of Fe(s) at 25°C.
c. undefined.
d. zero.
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17.4 Section Quiz.
3. Calculate  H0 for
NH3(g) + HCl(g)  NH4Cl(s).
Standard heats of formation:
NH3(g) = 45.9 kJ/mol, HCl(g) = 92.3 kJ/mol,
NH4Cl(s) = 314.4 kJ/mol
a.
176.2 kJ
b.
360.8 kJ
c.
176.2 kJ
d.
268 kJ
.
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Calculating Heats of Reaction
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Concept Map 17
Solve the Concept Map with the help of an
interactive guided tutorial.
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END OF SHOW