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158 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
Substituting the values yields the following.
⌬H° rxn ϭ Ϫ986.09 kJ Ϫ 2(Ϫ285.830 kJ)
⌬H° rxn ϭ Ϫ986.09 kJ ϩ 571.660 kJ ϭ Ϫ414.43 kJ
The reaction of one mole of calcium metal with two moles of water
releases 414 kJ.
Practice Problems
12. Use standard enthalpies of formation from Table A-8 in the
back of this book to calculate ⌬H° rxn for each of these
reactions.
a. Hg 2 Cl 2 (s) 0 HgCl 2 (s) ϩ Hg(l)
b. 2HCl(aq) ϩ Zn(s) 0 ZnCl 2 (aq) ϩ H 2 (g)
c. C 2 H 4 (g) ϩ 3O 2 (g) 0 2CO 2 (g) ϩ 2H 2 O(g)
d. HCl(aq) ϩ AgCN(s) 0 HCN(aq) ϩ AgCl(s)
15.5 Reaction Spontaneity
Entropy (S) is a measure of the disorder or randomness of the particles that make up a system. Spontaneous processes always result in
an increase in the entropy of the universe. The change in the entropy
of a system is given by the following equation.
⌬S system ϭ S products Ϫ S reactants
Whether ⌬S system is positive or negative can be predicted in some
cases by examining the reaction or process. Several factors affect the
change in entropy of a system.
• Changes of state Entropy increases when a solid changes to a
liquid and when a liquid changes to a gas because these
changes of state result in freer movement of the particles.
• Dissolving of a gas in a solvent When a gas is dissolved in
a liquid or solid solvent, the motion and randomness of the
particles are limited and the entropy of the gas decreases.
• Change in the number of gaseous particles When the
number of gaseous particles increases, the entropy of the
system usually increases because more random arrangements
are possible.
158 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
Substituting the values yields the following.
⌬H° rxn ϭ Ϫ986.09 kJ Ϫ 2(Ϫ285.830 kJ)
⌬H° rxn ϭ Ϫ986.09 kJ ϩ 571.660 kJ ϭ Ϫ414.43 kJ
The reaction of one mole of calcium metal with two moles of water
releases 414 kJ.
Practice Problems
12. Use standard enthalpies of formation from Table A-8 in the
back of this book to calculate ⌬H° rxn for each of these
reactions.
a. Hg 2 Cl 2 (s) 0 HgCl 2 (s) ϩ Hg(l)
b. 2HCl(aq) ϩ Zn(s) 0 ZnCl 2 (aq) ϩ H 2 (g)
c. C 2 H 4 (g) ϩ 3O 2 (g) 0 2CO 2 (g) ϩ 2H 2 O(g)
d. HCl(aq) ϩ AgCN(s) 0 HCN(aq) ϩ AgCl(s)
15.5 Reaction Spontaneity
Entropy (S) is a measure of the disorder or randomness of the particles that make up a system. Spontaneous processes always result in
an increase in the entropy of the universe. The change in the entropy
of a system is given by the following equation.
⌬S system ϭ S products Ϫ S reactants
Whether ⌬S system is positive or negative can be predicted in some
cases by examining the reaction or process. Several factors affect the
change in entropy of a system.
• Changes of state Entropy increases when a solid changes to a
liquid and when a liquid changes to a gas because these
changes of state result in freer movement of the particles.
• Dissolving of a gas in a solvent When a gas is dissolved in
a liquid or solid solvent, the motion and randomness of the
particles are limited and the entropy of the gas decreases.
• Change in the number of gaseous particles When the
number of gaseous particles increases, the entropy of the
system usually increases because more random arrangements
are possible.