Also, petroleum is considered a finite resource, and many scientists
say the point at which our consumption outstrips our ability to find new
reserves will happen within 40 to 50 years.
The main difference between petroleum and ethanol is that petroleum
is produced from remnants of plants that died millions of years ago,
while ethanol comes from existing plants.
As with petroleum, when we burn ethanol (CH3CH2OH) (Fig. 1),
carbon dioxide is also produced:
CH3CH2OH + 3 O2 ➔ 2 CO2 + 3 H2O
But this time, the carbon dioxide is
recycled. The plants used to produce ethanol
absorb carbon dioxide from the air to make
the chemical compounds they need to grow.
So when we use these plants to produce ethanol and then burn that
ethanol, we generate the same amount of carbon dioxide that was initially taken up by the plants. The carbon dioxide is continuously recycled
from the atmosphere, as new plants grow each season and are used to
make more ethanol.
In contrast, the carbon dioxide released by burning fossil fuels is
not part of the natural carbon cycle, because it is only added to the
atmosphere, causing a net increase of atmospheric carbon dioxide. The
reason is that millions of years ago, plants pulled carbon dioxide from
the air, just as they do today. Normally when a plant dies, it releases the
carbon dioxide back to the environment, keeping the amount of carbon
By Jonathan Sherwood
YOU HAVE PROBABLY SEEN A STICKER THAT SAYS “Contains 10% Ethanol” on gas pumps when you or your parents fill the gas tank, and you may have won- dered if it is the same stuff that is in alcoholic beverages—and what the heck is it doing in my gas tank?
You know that gasoline is extremely flammable, and you may
also know that foods and drinks containing alcohol are flammable.
So it may make sense that they would both go in a car engine, where
small, controlled explosions provide the car’s power. But why would
some gas stations in the United States go to the trouble of adding alcohol to gasoline?
Hydrocarbons versus ethanol
To understand why ethanol may be a promising source of energy
in the future, let’s first look at today’s main source of energy: petroleum. Petroleum consists of the decomposed remnants of ancient
organic matter that has been mixed and buried underground and has
been under high pressure and high temperature for millions of years.
Petroleum provides raw materials for everything from plastics to
fuel to medicines, but it is also widely regarded as the primary cause
of worldwide climate change. That is because the burning of petroleum
generates carbon dioxide. For example, methane, which is the simplest
of the molecules present in petroleum, burns as follows:
CH4 + 2 O2 ➔ CO2 + 2 H2O + energy
16 ChemMatters | APRIL/MAY 2016 www.acs.org/chemmatters
Fig 1. Structure of