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Chemical Potential Energy: Definition & Examples

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  • 0:00 What is Potential Energy?
  • 1:17 Chemical Reactions
  • 1:53 Forming a Chemical Bond
  • 3:59 Releasing Potential Energy
  • 5:38 Net Energy of a Reaction
  • 6:40 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth Basa
Every single bite of food you will ever eat taps into chemical potential energy. Learn, in this lesson, exactly what chemical energy is and why it has potential.

What is Potential Energy?

You probably already know that without eating, your body becomes weak from lack of energy. Take a few bites of a turkey sandwich, and moments later, you feel much better. That's because food molecules contain potential energy, or stored energy, that can do work in the future.

When your body breaks down these molecules, energy is released that it can use to do work, like walk or think. But food molecules aren't the only source of potential energy. Every physical thing in the world is made of molecules, and all of them have potential energy within their bonds.

To understand why chemical energy has the potential to do work, let's walk through a few stepping-stones. Our discussion will touch on basic ideas about molecules and chemical bonding in general, as well as some basic rules about chemical reactions. When we put all of this information together, you'll see how the net overall energy balance of a chemical reaction determines the potential energy available for use. Use by what? Whatever happens to be present in that space at that moment. For example, in a chemical reaction that breaks down car fuel, the potential energy that comes out is used to run the engine.

Chemical Reactions

A molecule is any combination of atoms that are bonded together. Molecules can also be called chemicals since they form through chemical reactions. Therefore, the energy in any molecule is called chemical energy.

We tend to associate the word chemical with something artificial that people make in a laboratory, which might be true for everyday use of the word. But, scientifically, this isn't accurate. Absolutely everything around us, both living and non-living, natural and altered, is made of chemical molecules of one kind or another that formed from chemical reactions.

Forming a Chemical Bond

For two atoms to bond together there must be a stable fit. Not all types of atoms can bond with each other. For covalent bonds, this means sharing electrons in such a way that both atoms achieve a more balanced outer, or valence, shell. But, even if the two atoms are more stable bonded together, to form that bond, an initial quantity of energy must be absorbed.

You might wonder why two atoms that are more stable together than apart need extra energy to hold in a bond. This is actually a complex topic, but to put it simply, all atoms have a positively charged nucleus and negatively charged electron shells or clouds. Therefore, when atoms come together, there are conflicting forces at work. The positively charged nucleus of one atom is attracted to the negatively charged electrons of the other. But at the same time, the two nuclei repel each other. The energy in the bond helps overcome these repulsive forces.

Additionally, energy comes in many forms, and only certain forms can be captured and used to make chemical bonds. Scientists call energy available for use in forming bonds free energy.

Once contained within the bond, the energy participates in holding the atoms together but does not get 'used up' by changing to heat. The same amount that was absorbed to make the bond remains in the bond. This is why many people describe bonds as a storage place for energy.

This notion is a little misleading, because the bonds don't store energy in the way we tend to think of stored items, like a stack of photographs in a box or sports equipment in the garage just sitting around waiting to be used. The energy in a chemical bond is active, yet the quantity remains constant. In this way, energy in a bond does differ from other ways energy is used. For example, when you use energy to move your legs, the energy transforms into heat and dissipates. Your legs can't access that energy again, which is why you need to eat every day - to get more useable, or free, energy.

Releasing Potential Energy

So, just how does free energy, or potential energy, come out of a bond? The bond is broken, using more energy. Sound confusing? Think of it this way: imagine blowing up a balloon and tying it closed. Let's say the air in the balloon represents energy, and the balloon itself is the bond. The air is trapped inside. To get it back out, what do you need to do? Pop the balloon! To do this, you need just a bit more energy, like a poke.

Bonds work the same way. A bit of energy is needed to break them, and then, like the balloon, what was inside rushes out. For a chemical bond, free energy rushes out.

However, this is only half of the story. In the world of chemical reactions, there are always two sides: the breaking of bonds, which occurs in the reactants, and the forming of new bonds, which are called the products. Let's take a moment to point out another basic fact about bonds: they're not all the same. Some bonds are weaker than others and therefore require more energy to hold them together. What makes a bond weak? The combination of the two atoms simply isn't that great of a fit. It's kind of like wearing a pair of shoes that are just a bit too small. You can still wear them, but they aren't exactly comfortable.

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