Difference Between Effusion and Diffusion

Introduction

The Scottish chemist Thomas Graham first put forward the systematic study of diffusion of gas molecules. He made his contributions to diffusion in the year 1828 to 1833. He observed that when different gases are brought together there is a spreading of gases instead of their arrangement in the terms of their density. So there is a spontaneous intermixing of gases taking place. And this property is diffusion. In the year 1848, Thomas Graham itself discovered the property of gas, effusion too. The escape of gas molecules irrespective of their nature happens when they are brought into small holes. He then formulated the law Graham law of effusion and diffusion.

What are Effusion and Diffusion?

The escape or a rapid movement of gas molecules when they see very small holes or regions is called an effusion. So when a container having a small pore is introduced with a particular gas the gas molecules will spread or escape through the small openings and the gas with the small molecular mass will spread or escape first from the holes than the gases with larger molecular mass. So Graham formulated a law relating to this effusion. And is called Graham effusion which states that the rate of effusion is inversely proportional to the molecular mass.

$$\mathrm{Rate\:of\:effusion\:\alpha\:\frac{1}{\sqrt{M}}}$$

The spreading of gaseous particles from a region of high concentration to low concentration is diffusion. When gas is opened from a container it will spread. And is because of the difference in concentration between the container and the surroundings. But gas particles are unaware of this concentration. This is because when gas particles are bound in a particular container they are in a very congested form but when an open surrounding appears they will just randomly spread out. So the molecules present in the container rapidly spread out when they see an open environment. This phenomenon of spreading gas particles is diffusion and it takes place from a region of high concentration to a low concentration region. And this motion of gases does not require any medium for its spreading. The rate of diffusion of gas molecules depends on the factors such as the concentration gradient and also the only surface area available for them to spread. The process of diffusion is also found in other states like a liquid state but it is weaker in a solid-state.

By relating effusion and diffusion with the kinetic theory of gases it is obtained that the rate of effusion and diffusion depends on the value of speed at which they move. The equation connecting this relation is,

$$\mathrm{\frac{V_{rms},B}{V_{rms},A}\:=\:\sqrt{\frac{M_{A}}{M_{B}}}}$$

Difference Between Diffusion and Effusion

Even though these two terms sound similar they are different properties of gas molecules. The major difference between diffusion and effusion is that the term diffusion is associated with the concentration gradient of class molecules while that of effusion is associated with the escape of gas molecules when they appear in a region that has a small pore. Some of the important differences between these answers have been considered in the following table.

Diffusion	Effusion
There is a movement of gas molecules from a region of high concentration to low concentration.	Here only the movement of gas molecules through small pores or holes is present.
This phenomenon is less efficient.	But this is an efficient method as it involves some amount of pressure variations.
It happens in other states of matter such as liquid and solids.	This happens only for gas molecules.
During the moment of a gas molecule,some amount of collision takes place.	There is no collision between molecules present
The driving force for this phenomenon is the concentration gradient.	The pressure variation developed due to the concentration gradient is the driving force for this process.
The process of diffusion is limited as it depends on the size of particles.	As it involves any type of coalition between gas molecules it is not limited to any extent.
An example of diffusion is − The smell of flowers is due to the diffusion process.	An example of effusion is The escape of gas molecules from tires when they are punctured.

Similarities Between Diffusion and Effusion

Ask these two terms deals with the gaseous particles it has some similarities too. Some of these similarities that are present between diffusion and diffusion are discussed in the following points.

• These two are mechanisms associated with gaseous molecules.

• The movement or flow of gaseous molecules is random and in a constant way.

• These two phenomena involve the use of concentration gradient.

• No energy is needed for both processes.

• The process of diffusion is associated with the effusion process as it may happen after the effusion process, that is, the spreading of gases may happen after it gets escaped from the particular hole or small area.

How to Eliminate the Misconceptions About Effusion and Diffusion?

There are some misconceptions associated with these terms. Especially in the diffusion process we always learn or study that diffusion is associated with or closely related to the term concentration gradient. So when particular gas molecules are present or allowed to spread in a region where there are no other gas molecules it happens that the spreading of gas molecules will take place even though it reaches an equilibrium between the low concentration and high concentration region. That is even though equilibrium is established between the low concentration and high concentration region, the process of diffusion will not stop that is it will continue even though there is no concentration gradient found. So the term diffusion is not strictly bound to concentration gradient it happens when there is dynamic nature of the gas molecules is present. This is the reason why self-diffusion also happens in water molecules. And it involves the movement of water molecules from one place to another even though there is no concentration gradient only water molecules are present. So the common misconception associated with the process of diffusion is the concentration gradient.

Another misconception associated with effusion is that the hole or small pores are necessary for the process of effusion. But in another way, the pressure of the gas molecules present in a particular object may develop the holes or pores and cause the effusion process. This may increase with increasing temperature too. Some internal pressure of gas molecules is also associated with the process of effusion.

Conclusion

Effusion and diffusion are two times associated with gas molecules. The term diffusion may also deal with the liquid and some extended to solid particles too. Effusion is the process of escaping gas molecules by small pores or holes. While diffusion is the spreading of gas molecules from a region of a lower concentration to a region of higher concentrations. Even though they are closely associated with the gas particles they have differences too. The significant difference between these two terms is that the driving force for the process of diffusion is the concentration gradient while the driving force for the process of effusion is the pressure gradient. These two terms are closely related too.

FAQs

1. Does temperature affect diffusion?

The process of diffusion is associated with the temperature too. As we know that the increased temperature can increase the kinetic energy of gas molecules thereby it will increase the rate of diffusion since the molecules can move faster with more kinetic energy they acquired.

2. Does heat diffuse?

Heat can also diffuse or spread from molecules into molecules. But the heat diffusion needs a medium for its spreading. The process that deals with the rate of spreading of gas molecules are thermal diffusivity.

3. Can a gas diffuse in liquid?

Yes, gas molecules can also diffuse in water. An important example of this process is the dissolution of oxygen in the water.

4. Does effusion depend on temperature?

The rate of effusion of gas molecules is inversely proportional to the square root of temperature according to the Graham law. That is the rate of effusion decreases when we raise the temperature.

5. Why do liquids diffuse faster than solids?

As liquids are arranged in a way that there is a space present in between two atoms but there is no such space present in solids. So the particles present in liquid diffuse faster than solid particles. So they diffuse faster than solids.