Potential energy is an extremely abstract concept that many students have trouble wrapping their heads around. However, having a solid understanding of potential energy is crucial if a student is going to understand chemistry, chemical bonding, and chemical reactions. For now, we will be focusing on potential energy as it relates to intermolecular forces and bonds, but everything that you will be studying applies equally to forces within molecules and chemical bonds.
You have learned that water molecules behave like small magnets and are attracted to each other. What we have not discussed yet is the energy it takes to separate two water molecules. If you have two refrigerator magnets attached to each other, it takes some effort to pull them apart. This effort (or work) is energy. It is the same way with two water molecules.
If two water molecules are held together by intermolecular forces, they will stay together unless one or both of those water molecules gains enough kinetic energy (and speed) to break away. This additional kinetic energy can come from the absorption of a photon from radiation by one of the molecules, or more likely, a collision with a third molecule that knocks one of the molecules away. In this simulation, we simply give the moving water molecule a little boost of kinetic energy at just the right moment to send it off by itself. What happens to that kinetic energy as the water molecule moves away from the other water molecule? The kinetic energy decreases as the water molecule slows down. The reason why the water molecule slows down is because the force of attraction between the two molecules is still pulling on it (this pull gets weaker the farther away the water molecule gets).
But how can the kinetic energy of the water molecule decrease as it moves away? Haven’t we been saying that energy is conserved? If the water molecule is losing kinetic energy, where is that energy going? The kinetic energy of a molecule depends on the speed of the molecule. The force of attraction between two molecules will cause the molecules to accelerate (speed up) when moving toward each other and to decelerate (slow down) when moving away from each other. Potential energy is the energy stored in a body due to its position in a force field. So when a molecule is moving away from another molecule and slowing down, it is transferring kinetic energy into potential energy. And when a molecule is moving toward another molecule and speeding up, it is transferring potential energy into kinetic energy.
If potential energy seems like a bit of an accounting trick, in some ways it is. By factoring in potential energy, the total energy of the molecule (kinetic energy + potential energy) is conserved. Potential energy is created whenever an object exists in a force field. When the force field is gravitational, such as the field generated by the planet Earth, then an object has gravitational potential energy. When the force field is electromagnetic, such as the field generated by atoms and molecules, then an object has electric potential energy. The “chemical” potential energy stored in the chemical bonds within molecules is simply the electric potential energy created by rearranging the physical positions of atoms and subatomic particles (electrons, protons, and neutrons). The electric potential energy of a molecule will be at its maximum when the molecule is completely separated from other molecules.
Most students first encounter the concept of potential energy when studying projectile motion in physics. In projectile motion, objects move within a gravitational field. It takes work (energy) to lift an object in a gravitational field, and this work gets stored in the object as gravitational potential energy. The higher an object, the more gravitational potential energy it has. An object’s gravitational potential energy can be calculated using the following formula: U = mgh, where m is the mass of the object, g is the acceleration due to gravity (near the surface of the Earth, g ≈ 9.81 m/s2), and h is the height of the object.
When the ball is rolling downhill, the ball speeds up and the gravitational potential energy stored inside of the ball is being released as kinetic energy. When the ball is rolling uphill, the ball slows down and kinetic energy is being stored inside the ball as gravitational potential energy. The kinetic energy of the ball is greatest when the ball is on the bottom of the ramp and moving at its fastest. The gravitational potential energy of the ball is greatest when the ball is at the top of the ramp on the right and the ball has completely stopped moving and is about to turn around. The total energy of the ball (kinetic energy + potential energy) is conserved throughout its journey.
Consider our box of molecules once again. When the attraction between molecules is very low (or nonexistent), the kinetic energy and temperature of this system is also low. But once I flip on molecular attraction, the molecules get pulled closer together and the force of attraction causes the molecules to speed up. This increases the kinetic energy, and therefore, the temperature inside the box. It would take quite a bit of energy to pull these molecules apart once molecular attraction has been turned on.
You have actually seen this phenomenon before… weakly attracted molecules heating up when the molecular attraction between them increases. Do you remember mixing 50 mL of 91% isopropyl alcohol with 50 mL of water earlier in this unit? What happened? Isopropyl alcohol molecules are weakly attracted to each other, but they are strongly attracted to water molecules. So when the isopropyl alcohol gets mixed with the water, the water molecules pull the isopropyl alcohol molecules closer together. This is why (combined with the size difference between water molecules and isopropyl alcohol molecules) the volume of the isopropyl alcohol and water mixture was less than 100 mL. It is also why the temperature of the mixture increased, heating up enough to melt cheap plastic graduated cylinders. This heat came from potential energy turning into kinetic energy, a real temperature increase that you can measure on a thermometer.
Earlier in this unit, you learned that the gram was first defined as the mass of 1 cm3 of liquid water at 0 °C. To measure heat, the calorie (cal) was defined in 1824 as the amount of energy needed to increase the temperature of 1 g of liquid water by 1 °C. In most scientific fields, the calorie has been replaced by the joule (J), the standard unit of energy in the International System of Units (SI). However, the calorie is still commonly used as a unit of food energy. Unfortunately, a food calorie (Cal) is not the same as a calorie (cal)… 1 food calorie = 1000 calories.
Later in this unit, you will study chemical reactions. Chemical reactions occur when the chemical bonds within molecules break and re-form to create new molecules. Some chemical reactions release energy and other chemical reactions store energy. The way that molecules store and release energy is by rearranging the physical positions of atoms and subatomic particles to convert energy between potential energy and kinetic energy. Turning kinetic energy into potential energy stores energy and turning potential energy into kinetic energy releases energy (as heat).