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CHAPTER 6 163 
 
 Bonds Broken kJ/mol 
 C=C (just the  component) +251 
 H-Br +368 
 
 Bonds Formed kJ/mol 
primary C-Br 285 
 (approximated by CH3CH2-Br) 
 secondary C-H 397 
 (approximated by H-CH(CH3)2) 
 
The net sum is 63 kJ/mol. Since H is predicted to be 
negative, we can also predict that this reaction will be 
exothermic. 
 
 
6.3. 
(a) ΔSsys is expected to be negative (a decrease in 
entropy) because two molecules are converted into one 
molecule. 
(b) ΔSsys is expected to be negative (a decrease in 
entropy) because an acylic compound is converted into a 
cyclic compound. 
(c) ΔSsys is expected to be positive (an increase in 
entropy) because one molecule is converted into two 
molecules. 
(d) ΔSsys is expected to be positive (an increase in 
entropy) because one molecule is converted into two 
ions. 
(e) ΔSsys is expected to be negative (a decrease in 
entropy) because two chemical entities are converted 
into one. 
(f) ΔSsys is expected to be positive (an increase in 
entropy) because a cyclic compound is converted into an 
acyclic compound. 
 
6.4. 
(a) There is a competition between the two terms 
contributing to ΔG. In this case, the reaction is 
endothermic, which contributes to a positive value for 
ΔG, but the second term contributes to a negative value 
for ΔG: 
 
 
The sign of ΔG will therefore depend on the competition 
between these two terms, which is affected by 
temperature. A high temperature will cause the second 
term to dominate, giving rise to a negative value of ΔG. 
A low temperature will render the second term 
insignificant, and the first term will dominate, giving rise 
to a positive value of ΔG. 
 
(b) In this case, both terms contribute to a negative value 
for ΔG, so ΔG will definitely be negative (the process 
will be spontaneous). 
 
(c) In this case, both terms contribute to a positive value 
for ΔG, so ΔG will definitely be positive (the process 
will not be spontaneous). 
 
(d) There is a competition between the two terms 
contributing to ΔG. In this case, the reaction is 
exothermic, which contributes to a negative value for 
ΔG, but the second term contributes to a positive value 
for ΔG: 
 
 
The sign of ΔG will therefore depend on the competition 
between these two terms, which is affected by 
temperature. A high temperature will cause the second 
term to dominate, giving rise to a positive value of ΔG. 
A low temperature will render the second term 
insignificant, and the first term will dominate, giving rise 
to a negative value of ΔG. 
 
6.5. A system can only achieve a lower energy state by 
transferring energy to its surroundings (conservation of 
energy). This increases the entropy of the surroundings, 
which more than offsets the decrease in entropy of the 
system. As a result, ΔStot increases. 
 
6.6. 
(a) A positive value of ΔG favors reactants. 
(b) A reaction for which Keq