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Chemistry 17.4
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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.
17.4
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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?
17.4
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>Calculating Heats of Reaction 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.
17.4
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Calculating Heats of Reaction > Hess’s Law17.4
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Calculating Heats of Reaction > Hess’s Law17.4
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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.
17.4
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>Calculating Heats of Reaction 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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>Calculating Heats of Reaction Standard Heats of Formation17.4
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>Calculating Heats of Reaction Standard Heats of Formation
The Standard Heat of Formation of Water
17.4
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SAMPLE PROBLEM
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17.7
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SAMPLE PROBLEM
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17.7
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SAMPLE PROBLEM
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17.7
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SAMPLE PROBLEM
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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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>Calculating Heats of Reaction Standard Heats of Formation17.4
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Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
17.4 Section Quiz.
17.4.
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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.
17.4 Section Quiz.
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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 forNH3(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
¡ 176.2 kJ
¡ 360.8 kJ
¡ 176.2 kJ
¡ 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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