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General Chemistry Principles, Patterns, and Applications, 2011

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A Write a balanced chemical equation for the conversion of coal to methane. Referring to Table 5.8<br />

"Enthalpies of Combustion of Common Fuels <strong>and</strong> Selected Organic Compounds", calculate the ΔH comb of<br />

methane <strong>and</strong> carbon.<br />

B Calculate the ratio of the energy released by combustion of the methane to the energy released by<br />

combustion of the carbon.<br />

Solution:<br />

A The balanced chemical equation for the conversion of coal to methane is as follows:<br />

2C(s) + 2H 2 O(g) → CH 4 (g) + CO 2 (g)<br />

Thus 1 mol of methane is produced for every 2 mol of carbon consumed. The ΔH combof 1 mol of methane is<br />

= -890kJ<br />

The ΔH comb of 2 mol of carbon (as coal) is<br />

= -680 kJ<br />

B The ratio of the energy released from the combustion of methane to the energy released from the combustion<br />

of carbon is<br />

= 1.31<br />

The energy released from the combustion of the product (methane) is 131% of that of the reactant (coal).<br />

The fuel value of coal is actually increased by the process!<br />

How is this possible when the law of conservation of energy states that energy cannot be created? The<br />

reaction consumes 2 mol of water ( DHo f = -285.8 kJ / mol ) but produces only 1 mol of CO 2 (<br />

DHo f = -393.5 kJ / mol ). Part of the difference in potential energy between the two<br />

(approximately 180 kJ/mol) is stored in CH 4 <strong>and</strong> can be released during combustion.<br />

Exercise<br />

Using the data in Table 5.8 "Enthalpies of Combustion of Common Fuels <strong>and</strong> Selected Organic<br />

Compounds", calculate the mass of hydrogen necessary to provide as much energy during combustion as 1<br />

bbl of crude oil (density approximately 0.75 g/mL).<br />

Answer: 36 kg<br />

The Carbon Cycle <strong>and</strong> the Greenhouse Effect<br />

Even if carbon-based fuels could be burned with 100% efficiency, producing only CO2(g) <strong>and</strong> H2O(g),<br />

doing so could still potentially damage the environment when carried out on the vast scale required by an<br />

industrial society. The amount of CO2released is so large <strong>and</strong> is increasing so rapidly that it is apparently<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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