Chapter 9, Problem 9.10P

The standard heat of combustion (AH°) of liquid 2,3,3-trimethylpentane [C₈H₁₈] is reported in a table of physical properties to be -4850 kJ/mol. A footnote indicates that the reference temperature for the reported value is 25°C and the presumed combustion products are CO₂(g) and H₂O(g).

1. In your own words, briefly explain what all that means.

There is some question about the accuracy of the reported value, and you have been asked to determine the heat of combustion experimentally. You burn 2.010 grams of the hydrocarbon with pure oxygen in a constant-volume calorimeter and find that the net heat released when the reactants and products [CO₂(g) and H₂O(g)] are all at 25°C is sufficient to raise the temperature of 1.00 kg of liquid water by 21.34°C. Write an energy balance to show that the heat released in the calorimeter equals $n_{C_8{H}_{18}}\Delta {\hat{U}}_c^{°}$, and calculate $\Delta {\hat{U}}_c^{°}$

, (kJ/mol). Then calculate AH°. (See Example 9.1-2.) By what percentage of the measured value does the tabulated value differ from the measured one?

Use the result of Part (b) to estimate $\Delta {\stackrel{\dot{}}{H}}_r^{°}$for 2,3,3-trimethylpentane. Why would the heat of formation of 2,3,3-trimethylpentane probably be determined this way rather than directly from the formation reaction?