# Take Only Pictures, Leave Only Footprints

Chemistry Level 3

Natural gas ($$\ce{CH_4}$$—methane) is often praised for being a low-footprint fuel source because it contributes less carbon dioxide to the atmosphere than gasoline does in exchange for the same amount of energy. Seeing as natural gas relies on the same combustion reaction as traditional fossil fuels, it would be remarkable if it were significantly different than other hydrocarbons like gas or coal.

Using the given bond enthalpies and the balanced hydrocarbon combustion mechanisms for methane and octane, approximate the amount of energy released per mole of carbon dioxide produced ($$E_C$$) for natural gas (methane) and gasoline ($$\ce{C8 H_{18}}$$—octane).

How much can we reduce our footprint by switching to natural gas as a fuel source relative to plain old gasoline (octane)?

I.e. find

$\frac{E_\textrm{C}(\textrm{natural gas})}{E_\textrm{C}(\textrm{octane})}-1$

 Bond $$\ce{C-C}$$ $$\ce{C-H}$$ $$\ce{C=O}$$ $$\ce{O=O}$$ $$\ce{O-H}$$ Bond energy (kJ/mol) 347 414 803 498 464

Details and Assumptions:

• Assume that the principal isomer of octane in gas has no carbon-carbon double bonds, i.e. only single carbon-carbon bonds, and carbon-hydrogen bonds.

• The generic, unbalanced combustion mechanism for hydrocarbons is given by $\ce{C_{n}H_{m} + O2 ->CO2 + H2O}$

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