Potential Energy In Chemical Bonds

Suppose I accept some hydrogen gas (H₂) and mix information technology with oxygen gas (O₂). What happens? Cypher. Nada happens unless y'all add a bit of energy—from a spark, mayhap. Add energy and Smash: You lot become an explosion, and the hydrogen and oxygen (some of it) make water (H_iiO). Conspicuously this reaction releases energy, but where did the free energy come from?

Hither is a surprisingly common reply:

"The energy is stored in the chemical bonds. When you lot break the bonds, y'all get energy."

As Derek Muller (from Veritasium) notes, this thought of free energy stored in the chemical bonds is very wrong. To get a ameliorate understanding of energy in chemical bonds, permit'south consider a simplified model.

Atomic Bond Model

When ane hydrogen atom interacts with another hydrogen to class molecular hydrogen (H₂), many things are going on. Still, one of the cardinal interactions is due to the electrostatic strength between protons and electrons. Yes, in that location are quantum mechanical effects too—but let me stick with a elementary model. In this model, I accept ii hydrogen atoms that experience some blazon of electrical force attracting them. When they get really shut, there is another force repelling the 2 atoms. To go along things calm, I add together a elevate force. Here is what it looks like when these two atoms interact.

What should you notice?

At that place is an attractive force between the two atoms.
Every bit they go closer, the two atoms increase in kinetic energy.
There is something that prevents the 2 atoms from crashing into each other.
The atoms don't keep oscillating considering they lose energy (this would be like heating up their surroundings).
Hydrogen atoms are yellowish (but yous should have already known that).

If yous desire to think of this system in terms of free energy, information technology might be useful to look at a sketch of the potential free energy for these two hydrogen atoms. It would sort of expect like this (simply a sketch).

Sketches Spring 2015 key

Nosotros can imagine the hydrogen atoms are similar a ball rolling on a hill shaped like the potential bend. You can see that it would increase in speed as information technology goes downward the hill, and so slow downwards and move back as it went up the "hill". But here is the of import indicate: If the brawl was at the lesser of the bend, you would have to add energy to move information technology up the hill. Yous would take to add energy to suspension this chemical bond.

Where Does the Energy Come From?

Permit'southward go back to the example of hydrogen and oxygen. If you become this reaction started, you lot do indeed get a lot of energy. But this energy does not come from the hydrogen-hydrogen bail, nor does information technology comes from the oxygen-oxygen bond. The energy comes from the germination of the hydrogen-oxygen bonds in the water. Peradventure another energy sketch would help. Suppose I correspond the energy of the gases and the water with the following:

Sketches Spring 2015 key

Moving that ball to the lower part of the bend (the water part) requires a little fleck of energy, just you get a lot back. But there is still not energy stored in the bonds of the water. Instead you go energy by forming the bond.

Another Molecular Model

The defoliation over energy in chemical bonds is office of the reason that Derek Muller is working on a new molecular model—the Snatoms.

You probably think those ball and stick molecular models from higher or high school. The Snatoms are like, but instead of using sticks they utilize magnets. At that place are two advantages of magnetic connections. Offset, the are quicker to assemble since the atoms snap together. Second (and more than of import), students can feel that there is a strength pulling the atoms together. They can also experience the force needed to pull them apart. This will aid build the thought that it takes energy to break bonds.

Of course the Snatoms are still just a model. They don't fully represent everything about molecules, but at least they should help with the energy-bond thing.