# u3c6yourturn

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UNIT 3 C.6 YOUR TURN Heats of Combustion
To better understand the energy involved in burning hydrocarbon fuels, use Table 5 (page 201) to answer the
following questions. The first one is already worked out as an example.

1. How much energy (in kilojoules) is released by completely burning 25.0 mol of hexane, C6H14?

Table 5 indicates, that the molar heat of combustion of hexane is 4,141 kJ. Putting it another way, 4,141 kJ
of energy are released when one mole of hexane burns. Thus, burning 25 times more fuel will produce 25
times more energy:
25.0 mol C6H14          x       4141 kJ       =      104000kJ
1mol C6H14

Burning 25.0 mol of hexane would thus release 104,000 kJ of thermal energy.

2. Write a chemical equation, including thermal energy, for the complete combustion of the following
alkanes:
a. Methane (main component of natural gas)

b. Butane (variety of uses, including lighter fluid)

3. Examine the data summarized in Table 5.
a. How does the trend in heats of combustion for hydrocarbons expressed in kJ/g compare with that
expressed in kJ/mol?

b. Assuming the trend continues to larger hydrocarbons, predict the heat of combustion for decane,
C10H22, in units of kJ/g and kJ/mo1.

c. Which prediction in question 3b was easier? Why?

4. a. How much thermal energy is produced by burning two moles of octane?

b. How much thermal energy is produced by burning one gallon of octane? (One gallon of octane has a
mass of 2,660 g.)

c. Suppose your car operates so inefficiently that only 16% of the thermal energy from burning fuel is
converted to useful "wheel-turning"(mechanical) energy. How many kilojoules of useful energy would
be stored in a 20.0-gallon tank of gasoline? (Assume that octane burning and gasoline burning produce
the same results.)

5. The heat of combustion of carbon as coal is 394 kJ per mole.
a. Write a chemical equation for burning carbon. Include the quantity of thermal energy produced.

b. Gram for gram, which is the better fuel, carbon or octane? Explain.

c. In what applications might carbon as coal replace petroleum based fuel?

d. Describe one application in which coal would be a poor substitute for a petroleum product.

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