CHAPTER 6
THERMOCHEMISTRY
Questions
13. Path-dependent functions for a trip from Chicago to Denver are those quantities that depend
on the route taken. One can fly directly from Chicago to Denver, or one could fly from
Chicago to Atlanta to Los Angeles and then to Denver. Some path-dependent quantities are
miles traveled, fuel consumption of the airplane, time traveling, airplane snacks eaten, etc.
State functions are path-independent; they only depend on the initial and final states. Some
state functions for an airplane trip from Chicago to Denver would be longitude change,
latitude change, elevation change, and overall time zone change.
14. Products have a lower potential energy than reactants when the bonds in the products are
stronger (on average) than in the reactants. This occurs generally in exothermic processes.
Products have a higher potential energy than reactants when the reactants have the stronger
bonds (on average). This is typified by endothermic reactions.
15. 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g); the combustion of gasoline is exothermic
(as is typical of combustion reactions). For exothermic reactions, heat is released into the
surroundings giving a negative q value. To determine the sign of w, concentrate on the moles
of gaseous reactants versus the moles of gaseous products. In this reaction, we go from 25
moles of reactant gas molecules to 16 + 18 = 34 moles of product gas molecules. As reactants
are converted to products, an expansion will occur because the moles of gas increase. When a
gas expands, the system does work on the surroundings, and w is a negative value.
16. ∆H = ∆E + P∆V at constant P; from the definition of enthalpy, the difference between ∆H
and ∆E, at constant P, is the quantity P∆V. Thus, when a system at constant P can do
pressure-volume work, then ∆H ≠ ∆E. When the system cannot do PV work, then ∆H = ∆E at
constant pressure. An important way to differentiate ∆H from ∆E is to concentrate on q, the
heat flow; the heat flow by a system at constant pressure equals ∆H, and the heat flow by a
system at constant volume equals ∆E.
17. a. The ∆H value for a reaction is specific to the coefficients in the balanced equation. Be-
cause the coefficient in front of H2O is a two, 891 kJ of heat is released when 2 mol of
H2O are produced. For 1 mol of H2O formed, 891/2 = 446 kJ of heat is released.
b. 891/2 = 446 kJ of heat released for each mol of O2 reacted.
18. Water has a relatively large heat capacity, so it takes a lot of energy to increase the
temperature of a large body of water. Because of this, the temperature fluctuations of a large
body of water (oceans) are small compared to the temperatures fluctuations of air. Hence,
oceans act as a heat reservoir for areas close to them which results in smaller temperature
changes as compared to areas farther away from the oceans
