Chemical Potential Energy: Definition, Examples & Quiz
Every single bite of food you will ever eat taps into chemical potential energy. Learn exactly what chemical energy is and why it has potential.
Chemical 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. Clearly, food molecules have potential energy in them. When your body breaks down the molecules, energy is released that your body can use to do work, like walk or think. Any stored energy that can do work in the future is called potential energy. In addition to food, every physical thing in the world is made of molecules, and all of them have potential energy within their bonds.
The Big Picture
To understand why chemical bond energy has the potential to do work, this lesson will walk you through a few stepping-stones. Our discussion will touch on basic ideas about molecules and chemical bonding in general. This will include some general rules about chemical reactions as well. When we put all of this information together, you will 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 the chemical reaction that breaks down car fuel, the potential energy that comes out is used to run the engine.
Molecules Come From 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 as something artificial people make in a laboratory, which may be true for everyday use of the word. However, scientifically, this is not accurate. Absolutely every thing 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 of electrons in such a way so 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 may 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 do not store energy in the sense of how 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, that energy transforms into heat and dissipates. Your legs cannot access that energy again, which is why we need to eat everyday--to get more useable or free energy.
Releasing Potential Energy From Bonds
So just how does the free energy (potential energy) come out of the bond? Answer: 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 the bond. The air is trapped inside, and to get it back out, what would you need to do? Pop the balloon! To do this, you would need just a tiny more bit of energy, like a poke. Bonds work the same way. A bit of energy must be used to break them, and then like with 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 ones, which are called the products. You may be familiar with these basic chemical terms.
Strong Bonds vs. Weak Bonds
Let's take a moment here to point out another basic fact about bonds: they are 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 is not that great of a fit, kind of like wearing a pair of shoes that are just a bit too small. You can wear them, but it isn't wonderful. On the other hand, a strong bond is one where the combination of the atoms is extremely stabilizing for them and being together is so easy. In the bond world this means less energy is required to hold the atoms together. In short: strong bonds require less energy to form than weaker bonds (This means they contain less energy too).
The Net Energy Release of a Reaction
Now, back to the second part of our reaction. Almost instantaneously after bonds break, new ones form using some of the free energy that the reactants released. (Remember, bonds require energy to form). If atoms naturally move to a more stable state, and given our recent discussion on weak and strong bonds, what type of bonds will atoms likely form when left on their own without outside intervention? Answer: Strong ones of course! The most stable bond and strongest bond will form, and since strong bonds require less energy than weaker bonds, overall after the reaction extra energy remains.
This is why the chemical energy in bonds can be described as potential energy. The energy left over from the formation of more stable bonds from weaker ones has the potential to do work when it comes out.
It is worth mentioning that with every reaction some energy does transform into heat and dissipates into the surrounding environment, which explains where much of your body heat comes from.
Let's summarize why chemical energy is potential energy:
1. All molecules absorb energy into their bonds upon formation.
2. This energy holds atoms together and does not get used up. It remains a constant quantity. This energy therefore has potential to do work when it leaves the bond.
3. Additional energy is used to break a bond (not the energy in the bond). The free energy inside the bond is released upon breakage of the bond.
4. New bonds form (products), absorbing some of the free energy from the broken bonds (reactants).
5. Every chemical reaction involves the breaking and forming of bonds.
6. Spontaneous reactions result in the breaking of weaker bonds to form stronger ones, which results in a net release of energy overall.
7. The extra free energy not used to form the products can be utilized to do other work.
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