Jump to a New ChapterIntroduction to the SAT IIIntroduction to the SAT II Chemistry TestStrategies for Taking the SAT II Chemistry TestThe Structure of MatterThe States of MatterReaction TypesStoichiometryEquilibrium and Reaction RatesThermodynamicsDescriptive ChemistryLaboratoryBasic Measurement and Calculation ReviewChemical Formulas Review: Nomenclature and Formula WritingPractice Tests Are Your Best Friends
 9.1 Enthalpy 9.2 Spontaneous Reactions 9.3 Heat Capacity and Specific Heat 9.4 Enthalpies of Reactions 9.5 Hess’s Law

 9.6 More About Entropy 9.7 Gibb’s Free Energy 9.8 Practice Questions 9.9 Explanations
Enthalpies of Reactions
You can find the overall enthalpy of a reaction by subtracting the enthalpy at the beginning of the reaction from the enthalpy at the end of the reaction:
DH = Hfinal - Hinitial
which is virtually the same as saying
DH = Hproducts - Hreactants
under the thermodynamic standard states 25ºC (298K), 1 atm, and 1 M. Try using this equation in a problem.
Example
Calculate the DH for the following:
3Al(s) + 3NH4ClO4(s)Al2O3(s) + AlCl3(s) + 3NO(g) + 6H2O(g)
given the following values:
 Substance (kJ/mol) NH4ClO4(s) -295 Al2O3(s) -1676 AlCl3(s) -704 NO(g) 90 H2O(g) -242 Al(x) 0 (since it’s an element)
Explanation
[1(-1676) + 1(-704) + 3(90) + 6(-242)] - [3(0) + 3(-295)] = -2677 kJ
This is an exothermic reaction since DH is negative. Don’t forget to multiply values by coefficients since each coefficient represents the number of moles of each substance!
Try another one:
Example
Find the DHf of C6H12O6(s) using the following information:
C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(l) + 2800 kJ
 Substance (kJ/mol) CO2(g) -393.5 H2O(l) -285.8
Explanation
[6(-393.5) + 6(-285.8)] - [1(x) + 6(0)] = -2800 kJ
Now solve the above for x and you have your answer! The value 2800 is negative because this reaction is exothermic. Oxygen is considered an element in its free state, so it is assigned a value of zero. (All diatomic molecules are assigned zeros for the same reason.) After solving for x, you get for glucose.
 Jump to a New ChapterIntroduction to the SAT IIIntroduction to the SAT II Chemistry TestStrategies for Taking the SAT II Chemistry TestThe Structure of MatterThe States of MatterReaction TypesStoichiometryEquilibrium and Reaction RatesThermodynamicsDescriptive ChemistryLaboratoryBasic Measurement and Calculation ReviewChemical Formulas Review: Nomenclature and Formula WritingPractice Tests Are Your Best Friends
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