Kinetic Molecular Theory | Properties, Laws, & Examples

chase smith, Elizabeth (Nikki) Wyman, Amanda Robb
  • Author
    chase smith

    Chase is a 14 year veteran science teacher with a specialization is chemistry/STEM. He has a Bachelors Degree in Chemistry and a Master's Degree in Instructional Media with a specialization in STEM.

  • Instructor
    Elizabeth (Nikki) Wyman

    Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

  • Expert Contributor
    Amanda Robb

    Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

Understand kinetic molecular theory and see how it explains the kinetic energy in solids, liquids, & gases. Study how it interacts with gas laws, and view examples. Updated: 11/24/2021

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Kinetic Molecular Theory Definition (KMT)

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.

The three states of matter

Flowchart depicting the three states of matter and the changes between them.

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.

What are the Assumptions of Kinetic Molecular Theory?

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.

  • Gases are composed of particles that are in random, constant motion.
  • Gases move in a straight line until they collide with something.
  • Gas molecules are not attracted to one another or to the container.
  • Collisions that occur between gas molecules are thought of as being perfectly elastic.
  • The average kinetic energy of a collection of gas particles depends only upon the temperature of the gas.

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  • 0:03 Kinetic Molecular Theory
  • 1:23 KMT and Properties of Liquids
  • 2:49 KMT and Solids
  • 3:54 Intermolecular Forces
  • 6:17 Phase Changes
  • 7:09 Lesson Summary
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Kinetic Molecular Theory and Gas Laws

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

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Additional Activities

Kinetic Molecular Theory in Real Life

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.

Examples

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.

Directions

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.

SubstanceState of MatterDescription of Molecular MotionPicture of Molecular Motion




Frequently Asked Questions

What are the five main points of the kinetic molecular theory?

  • Gases are composed of particles that are in random, constant motion.
  • Gases move in a straight line until they collide with something.
  • Gas molecules are not attracted to one another or the container.
  • Collisions that occur between gas molecules are thought of as being perfectly elastic.
  • The average kinetic energy of a collection of gas particles depends only upon the temperature of the gas.

What does the kinetic molecular theory state?

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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