Kinetic Molecular Theory (KMT) describes the experimentally discovered behavior of particles. KMT is most often referenced in relation to the behavior of gases, but it could also be applied to solids and liquids.
Molecules that make up a solid are thought to be very relatively tightly packed molecules. These molecules will gently sway in place without changing physical locations. In general, they are not affected by the shape or volume of the container that is holding them.
Liquids have a bit more space in-between each molecule than solids. Liquid molecules also move a bit faster than solids, and they will simply flow past other molecules within the container. As a result, liquids will tend to take the shape of the container which holds them, but the volume of a liquid is not typically affected by the container.
Gas molecules have the most space in-between molecules. These molecules also move the fastest out of the three phases (the three phases being solid, liquid, and gas). With the speed at which gas molecules move, they are able to overcome any attractive forces that would hold the molecules together in close proximity. As a result, gases move about wildly and take on both the shape and volume of the container that holds them.
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As a result of the molecules' movement in each phase, a substance is able to undergo a change from one phase to another by overcoming the intermolecular dipole-dipole forces which hold them together (these dipole-dipole forces occur at oppositely charged poles between the molecules). Kinetic energy weakens the dipole-dipole forces to the point where molecules at certain temperatures can break those bonds, thereby changing some of their properties. The breaking of these bonds from the dipole interactions is what causes a substance to change from one phase into a new phase.
There are several basic assumptions that have been made about gases (through experimentation). The following list details each of the five assumptions which are commonly accepted. These five assumptions may sometimes vary in number depending on the source of the list, because a few of these assumptions can be further consolidated and combined with some of the others.
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Through the use of KMT, a few different laws have been created to help quantify the relationship between the measurable characteristics of gases. The three laws are called the ideal gas law, Charles' law, and Boyle's law. These three laws relate the pressure, volume, temperature, and amount of a gas.
The ideal gas law finds a relationship between the product of the pressure (P) and volume (V) of a gas; and the product of the amount of moles of gas (n), the temperature of that gas (T), and the ideal gas constant, known as an R value. The R value is typically represented with the number 8.314 J/(K * mol).
This math equation would be written out as:
PV = nRT.
An ideal gas assumes that particles (a.) do not attract or repel one another and (b.) take up no space (in other words, have no volume). No gas is truly ideal, but the ideal gas law does provide a good approximation of real gas behavior under many conditions.
Boyle's Law focuses on the relationship between only the pressure and volume of a gas when all other factors are held constant. Through experimentation, it can be determined that the pressure and volume of a gas in a closed container are inversely related. This means that as the pressure of a gas increases, the volume of the gas decreases proportionally. In an equation format, this appears as:
P1V1 = P2V2
P1 is the starting pressure, and V1 is the starting volume; P2 is the ending pressure, and V2 is the ending volume.
Charles' Law focuses on the relationship between the temperature and the volume of a gas. This relationship has been found to be a direct relationship when all other factors are held constant. The equation is as follows:
T1/V1 = T2/V2
T1 = the starting temperature of the gas, measured in Kelvin
V1 = the starting volume of the gas
T2 = the ending temperature of the gas, measured in Kelvin
V2 = The ending volume of the gas
KMT is also able to make a prediction between the pressure of a gas and the quantity (amount of moles) of that gas. The more moles of gas that are present in a container, the higher the pressure. This can best be visualized when pressure is thought of as the number of collisions that the gas particles have with the walls of the container. As more gas is put into a container, those molecules experience more collisions, and therefore the pressure in the container would be higher.
Kinetic Energy is the measurement of the magnitude of the motion of an object. It's usually recorded in Joules. This amount of motion of an ideal gas can be calculated with the equation:
KE = (3/2) RT
KE stands for Kinetic Energy, R stands for the gas constant (mentioned previously in the ideal gas law), and T stands for the temperature (in Kelvin).
For each of the three phases of matter, the amount of kinetic energy would vary greatly. The amount of kinetic energy in a solid will be the lowest value since the particles are moving the slowest. Liquids will have a moderate amount of kinetic energy due to the average movement speed of the molecules. Gases will always have the highest amount of kinetic energy due to the rapid random movements of the particles.
KMT plays an important role in our everyday lives. Here are a few real world examples of KMT acting around us:
Example #1
Our first example can be seen in how car tires inflate/deflate depending on the temperature outside. A tire's air pressure will fluctuate greatly depending on the weather. If it is cold, molecules tend to move slower, and create less air pressure. If it is hot outside, however, molecules will move faster and therefore create more air pressure.
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Example #2
Another example of KMT playing a role in everyday life can be found within the lungs of a person who is breathing out. The involuntary decrease in the volume of the lungs results in an increase in the pressure of the lungs. The high pressure air that is created as a result then flows out of the lungs and back into the atmosphere. In the reverse process, air is forced back into the lungs when a person is breathing in. Specifically, this is an example of KMT through the use of Boyle's Law.
Kinetic Molecular Theory has several key points which describe the motion of molecules. Solid particles are most often pictured as tightly compacted molecules with strong attractive forces between each atom. These molecules would slowly sway in place. They also contain the least amount of kinetic energy.
Liquid molecules have an average amount of intermolecular attraction. These molecules will flow past one another due to the reduction in attractive forces.
Gas molecules contain the greatest amount of kinetic energy among the three phases and therefore contain little to no attraction between its particles. Gas molecules play such a large role in everyday life that, through experimentation, scientists have formed the five key assumptions presented below:
KMT can also be described mathematically through the use of the ideal gas law (PV=nRT), Boyle's law (P1V1=P2V2), and Charles' law(T1/V1=T2/V2). The kinetic energy of the molecules can also be calculated if necessary.
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In this activity, students are going to be looking for substances in their normal environment and describing how the kinetic molecular theory would apply. Students should choose two solids and two liquids, describe the motion of the particles in each and draw a picture.
For example, students might choose Kool-Aid as a liquid example for this project. They would describe the motion of the particles, moving quickly with some space between the particles, and draw this as a diagram of the molecules. As a solid, a student might choose a metal pot. Students would describe the motion of the particles, moving slowly, and draw a picture of orderly molecules that don't have much motion.
In this activity, you're going to be applying the kinetic molecular theory to materials you find in your everyday life. To complete this activity, you'll be finding two solids and two liquids in your life. For each one, you'll describe how you think the molecules are moving, draw a sketch of their movement and then describe which intermolecular forces might be at work. For the last part, you can use the internet to see which intermolecular forces are at play for which type of substances.
| Substance | State of Matter | Description of Molecular Motion | Picture of Molecular Motion |
|---|---|---|---|
The kinetic molecular theory states that the motion of molecules is predictable based upon measurable traits such as the temperature, volume, and pressure of the atmosphere. There are between 4-6 key thoughts used to describe the motion of molecules depending on how the statements are listed.
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