Chapter 8 Source Answers and Examples
Chapter problems → · Original reading →
This chapter has no answer key. Its worked examples follow.
Worked examples and reviewed revisions
Example Problem 8.1.1
Worked example
Example Problem 8.1.1 — AP edition reviewed replacement.
Reviewed same-composition ideal-gas model: compare mean molecular speeds at −5 °C and 25 °C. Find the percent increase on the warmer day. Mean speed is a magnitude; it is distinct from the zero vector average velocity of a gas at rest.
Convert temperatures to kelvin: 268.15 K and 298.15 K. At fixed gas composition, mean speed is proportional to √T.
v̄(warm)/v̄(cold)=√(298.15/268.15)=1.054456. Percent increase=(1.054456−1)×100%=5.45%.
The molar mass cancels in this ratio. Individual speeds or a mean vector velocity are not required.
Example Problem 8.1.2
Worked example
Example Problem 8.1.2 — AP edition reviewed replacement.
Reviewed conceptual task: why is a comparison of “enthalpy per mole of gasoline” and carbon dioxide incomplete without a named fuel composition, specified physical states and thermochemical reference?
Gasoline is a mixture. A mole of an unspecified mixture does not identify one chemical substance or one molar enthalpy. Enthalpy is a state function relative to a stated reference.
Choose a named fuel, a balanced combustion reaction, physical states and data. Breaking fuel and oxygen bonds requires energy; forming product bonds releases energy. Exothermic combustion has a net negative ΔH.
No numerical molar-enthalpy ordering follows from the incomplete source comparison.