Investigation 5: Energy and Motion

So far in this CELL, students have investigated chemical, heat, electrical, mechanical, and sound energy. Students have discovered that mechanical energy can be either potential or kinetic energy. Students have found that the Law of Conservation of Energy holds true in that energy can be transformed from one form to another but is not created or destroyed. For example, students observed the conversion of the gravitational potential energy of the steel marble when at the top of the ramp, into kinetic energy as it rolled down the ramp and collided with the flower pot. The collision demonstrated a principle of mechanical energy in that the energy did work – the collision resulted in a force being exerted on the flower pot, causing it to travel over a distance.

Mechanical energy, regardless of how it is generated, is able to do work. In Investigation Five, students will discuss examples of mechanical devices that use mechanical energy to do work, such as vacuum cleaners, bicycles, and springs. All of these devices convert one type of energy into mechanical energy which allows them to do work, to perform their functions. The vacuum cleaner converts electrical energy into mechanical energy. The bicycle converts the kinetic energy of the rider’s moving legs into mechanical energy. When a compressed spring is released, the stored potential energy is converted to mechanical energy.

One conversion that students may not have previously thought about is the conversion of chemical potential energy to mechanical energy. In Investigation Three, students manipulated a battery to discover that stored, chemical potential energy can result in electrical energy and therefore light energy. In Investigation Five, students will discover that chemical energy, the energy contained in chemical reactions, can be converted to mechanical energy that causes objects to move.

In Investigation Five, students will inflate a balloon by blowing into it and then sealing it, thereby trapping the molecules of gas, including carbon dioxide, inside. The gas molecules possess kinetic energy. They move around the inside of the balloon. Creating a path for the gas molecules to be released from the balloon allows for a demonstration of how the kinetic energy of the gas can be transformed into mechanical energy required to move a model car.

Molecules, Atoms, and Chemical Bonds

As the molecular model of acetic acid (vinegar) below shows, molecules are made up of atoms, and the atoms within a molecule are held together by chemical bonds. In such “Ball and stick” models of molecules, the balls represent the atoms while the sticks that join them together represent the chemical bonds.

A chemical reaction often involves two or more different types of molecules interacting with each other. This typically results in the rearranging of the atoms of the molecules. Such a rearrangement obviously requires the breaking of chemical bonds followed by the making of new chemical bonds. As we discussed in Investigation Four, if the newly formed molecules (the products) contain less energy than the molecules from which they came (the reactants), energy in the for of heat is released, the temperature increases, and we call the reaction exothermic. On the other hand, if the products contain more energy in their chemical bonds than was present in the reactants, then heat is absorbed to supply this energy, the temperature decreases, and we call the reaction endothermic. Below is a chemical reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) that you will perform in Lab:

The chemical reaction of the baking soda (sodium bicarbonate) with the vinegar (acetic) results in the formation of a chemical called sodium acetate and the formation of a gas, carbon dioxide. A molecule of water (H₂O) is also formed.  In a sealed container, the rapid rate of the reaction quickly causes a large amount of pressure to build up as more carbon dioxide gas is produced.

Students will realize that some of the chemical potential energy of the two reactants is converted to the kinetic energy of the expanding gas which is responsible for the pressure. The pressure of the gas causes the rubber stopper to pop forcefully from the end of the container. Hence, the chemical potential energy of the reactants allows students to witness the mechanical energy necessary to forcefully move the stopper.

How does the vinegar/baking soda reaction produce mechanical energy? The energy of the bonds between the atoms of a molecule is known as chemical potential energy. Remember from the previous Investigation concerning endothermic and exothermic reactions, that energy is used to break and create bonds between atoms. In the case of the vinegar and baking soda reaction, the acetic acid (vinegar) and sodium bicarbonate (baking soda) produce sodium acetate plus water and carbon dioxide. In this reaction, the chemical potential energy in the reactants is greater than the chemical potential energy of the products. The reaction, therefore, results in energy that is both converted into the energy of the new bonds between the carbon and oxygen atoms of the carbon dioxide molecules, and the kinetic energy that allows carbon dioxide molecules to move rapidly in the air inside the sealed container. The net result is that carbon dioxide gas molecules begin to fill the chamber of the container. Because the reaction continues to take place, carbon dioxide molecules become progressively more concentrated until sufficient pressure and kinetic energy of the molecules’ motion has built up to overcome friction and release the stopper from the centrifuge tube.