State the significant concepts behind the kinetic molecular theory of gases.Demonstrate the relationship in between kinetic energy and molecular speed.Apply the kinetic molecular theory to explain and predict the gas laws.

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Gases were amongst the first substances studied using the modern scientific method, i beg your pardon was arisen in the 1600s. The did no take lengthy to acknowledge that gases every shared details physical behaviours, suggesting that gases might be explained by one all-encompassing theory. The kinetic molecular theory of gases is a design that helps us understand the physics properties of gases in ~ the molecule level. The is based on the adhering to concepts:

Gases consist of particles (molecules or atoms) that are in consistent random motion.Gas particles space constantly colliding v each other and also the walls of your container. These collisions are elastic; the is, there is no network loss of power from the collisions.Gas particles are tiny and the full volume populated by gas molecule is negligible loved one to the full volume of their container.There space no interactive forces (i.e., attraction or repulsion) between the corpuscle of a gas.The typical kinetic energy of gas particles is proportional come the absolute temperature that the gas, and all gases in ~ the very same temperature have the same median kinetic energy.

Figure 6.6 “The Kinetic Molecular concept of Gases” mirrors a depiction of just how we mentally picture the gas phase.

Figure 6.6 “The Kinetic Molecular concept of Gases.” The kinetic molecular theory of gases describes this state of matter as composed of small particles in constant motion with a lot of distance between the particles.

Because most of the volume populated by a gas is empty space, a gas has actually a low density and can increase or contract under the ideal influence. The fact that gas particles room in consistent motion way that two or an ext gases will always mix together the corpuscle from the separation, personal, instance gases move and collide with each other. The variety of collisions the gas particles make v the wall surfaces of their container and also the force with which lock collide determine the magnitude of the gas pressure.

Kinetic Energy and Molecular Speed

Gas particles space in consistent motion, and also any object in motion has kinetic energy (Ek). Kinetic energy, for an individual atom, have the right to be calculate by the adhering to equation where m is the mass, and u is the speed.


Overall, the molecule in a sample that a gas share an median kinetic energy; however, individual molecule exhibit a distribution of kinetic energies because of having a circulation of speed (Figure 6.7 “Stylized Molecular speed Distribution”). This distribution of speeds arises from the collisions the occur in between molecules in the gas phase. Although this collisions room elastic (there is no network loss the energy), the individual speeds of each molecule connected in the collision may change. For example, in the collision of two molecules, one molecule may be deflected at a slightly higher speed and the other at a slightly lower speed, however the median kinetic energy does not change.

Figure 6.7 “Stylized Molecular rate Distribution.”

When analyzing a chart of the circulation of molecular speeds, there room several generally used terms to be familiar with. The most probable speed (ump) is the speed of the largest number of molecules, and also corresponds come the height of the distribution. The typical speed (uav) is the mean speed of every gas molecules in the sample. The root-mean-square (rms) rate (urms) synchronizes to the speed of molecule having specifically the exact same kinetic energy as the average kinetic power of the sample.

Figure 6.8 “Distribution of the Molecular speeds of Oxygen Gas in ~ −100, 20, and 600°C.”

According to the kinetic molecular theory, the median kinetic energy of gas corpuscle is proportional to the absolute temperature that the gas. This have the right to be expressed through the adhering to equation where k represents the Boltzmann constant. The Boltzmann constant is just the gas constant R separated by the Avogadro’s constant (NA). The bar above certain terms suggests they are average values.


Since average kinetic energy is related both come the absolute temperature and the molecular speed, us can integrate the equation over with the vault one to determine the rms speed.


This demonstrates that the rms rate is concerned the temperature. We can more manipulate this equation by multiplying the numerator and also denominator by Avogadro’s continuous (NA) to provide us a form using the gas continuous (R) and molar massive (M).


This form of the equation demonstrates the the rms speed of gas molecule is also related to the molar mass of the substance. Comparing 2 gases of various molar mass in ~ the same temperature, we watch that despite having the same typical kinetic energy, the gas with the smaller molar fixed will have a greater rms speed.

Figure 6.9 “Molecular Speed circulation of Noble Gases.” (Source: adjusted from MaxwellBoltzmann-en.svg by Pdbailey/Public Domain.)

Calculate the rms rate of nitrogen molecule at 25ºC.



Knowing that

, we can convert to metres every second:


Applying the Kinetic Molecular theory to the Gas Laws

The kinetic molecule theory can be offered to explain or suspect the speculative trends the were used to create the gas laws. Let’s work-related through a couple of scenarios to demonstrate this point.

What will take place to the pressure of a mechanism where the volume is diminished at continuous temperature?

This problem can it is in approached in two ways:

The ideal gas law can be rearranged to resolve for pressure and estimate the change in pressure:


Volume is located in the denominator the the equation, and it is being decreased. This way the rest of the equation is being separated by a smaller number, for this reason that must make the press larger.The kinetic molecular theory deserve to be used. Since the temperature is continuing to be constant, the typical kinetic energy and the rms speed remain the same as well. The volume that the container has actually decreased, which method that the gas molecules need to move a shorter distance to have actually a collision. Over there will as such be an ext collisions every second, causing rise in pressure.

What will take place to the pressure of a device where the temperature is increased and the volume remains constant?

Again, this form of difficulty can be approached in 2 ways:

The best gas law have the right to be rearranged to resolve for pressure and also estimate the adjust in pressure.


Temperature is situated in the numerator; there is a straight relationship between temperature and also pressure. Therefore an increase in temperature need to cause rise in pressure.The kinetic molecular theory deserve to be used. Temperature is increased, for this reason the average kinetic energy and the rms rate should additionally increase. This means that the gas molecules will hit the container walls more frequently and with greater force because they are all moving faster. This should boost the pressure.
The physics behaviour the gases is defined by the kinetic molecular theory of gases.The variety of collisions that gas corpuscle make with the walls of their container and the force at which they collide recognize the magnitude of the gas pressure.Temperature is proportional to mean kinetic energy.

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QuestionsState the concepts of the kinetic molecular theory of gases.Calculate the rms rate of CO2 at 40°C.Using the kinetic molecule theory, explain how rise in the number of moles of gas at constant volume and also temperature affects the pressure.AnswersGases consist of small particles of matter that room in continuous motion. Gas particles space constantly colliding through each other and also the walls of a container. This collisions room elastic; that is, there is no network loss of power from the collisions. Gas particles space separated by big distances. The dimension of gas corpuscle is tiny compared to the distances that different them and also the volume of the container. There are no interactive forces (i.e., attraction or repulsion) in between the corpuscle of a gas. The average kinetic energy of gas corpuscle is dependency on the temperature that the gas.421 m/sTemperature continues to be the same, therefore the median kinetic energy and the rms speed need to remain the same. Boosting the number of moles the gas method there are an ext molecules of gas accessible to collide v the walls of the container at any given time. Because of this pressure should increase.