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Posted by on Jul 25, 2013 in General Chemistry, Physical Chemistry | 0 comments

Physical chemistry-Where is chemical energy stored?

One question I pose every year to my physical chemistry class of life science students in the first lecture is “Where is chemical energy stored?”  Almost all of them say in chemical bonds. Ask how the energy is released, and they say “When bonds are broken.”

Wrong! 

It is a huge myth that energy is stored in chemical bonds. All I have to ask is:

“To break a chemical bond, is energy added or removed?”

“Added!” they realize. So they get it.

You have to put energy into a bond to break it.  You do not get energy out of a chemical bond, so bonds do not store energy.  Rather energy is lost to the surroundings as a chemical bond forms.  A bond only forms if it is more stable than its composite atoms, so energy is released as the lower energy chemical bond is formed.  See the example (click image to enlarge):

How, one then asks the students, does a liter of gasoline release energy to move your car? Befuddled silence.  Few students in a class of 150 want to answer   I tell them: when hydrocarbons burn they form molecules with stronger bonds.

C8H18 + 25/2 O –>  8CO2 + 9H2O  +  energy

So the C-O bonds of CO2 and the H-O bonds of H2O are stronger than the weaker H-C  bonds of hydrocarbons.  Since the energy difference must go to the surroundings, we harness it in the car engine, and away we go (see the figure above).

This system is exothermic because stronger bonds are formed in combustion. Here at constant temperature and pressure, the difference between the bonds broken (C8H18) and bonds formed (CO2 , H2O) is the enthalpy, ΔH.

We are into chemical thermodynamics and a major myth is dispelled.

I make this point in chapter 12 of our text book, Physical Chemistry by Laidler, Meiser and Sanctuary (LMS)

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