[Chemistry Class Notes] on Brownian Motion Pdf for Exam

“Brownian motion in chemistry is a random movement. It can also be displayed by the smaller particles that are suspended in fluids. And, commonly, it can be referred to as “Brownian movement”- the Brownian motion results from the particle’s collisions with the other fast-moving particles present in the fluid.

When two particles collide, the path of one particle will be changed. A further collision also causes the particle to follow a random motion, which is called zigzagging. Momentum and energy are exchanged between the particles during this process.

An illustration that describes the random movement of the fluid particles can be given as follows.

Note: The Brownian motion was named after the Scottish Botanist Robert Brown, who first observed that when placed in water, pollen grains move in random directions.

Brownian Movement

Biologically the Brownian Movement occurs when a particle moves randomly in a zigzag pattern, which can be observed under a high-power microscope. A similar motion is described by Robert Brown as the Brownian movement and resembles how pollen grains move in the water.

The Brownian movement of pollen was later clarified by Albert Einstein in his paper, explaining that the pollen was moved by water molecules. Molecular and atomic existence has been strengthened with this discovery.

Modern atomic theory is based on the Brownian movement, which is imperative to comprehend. Also, the kinetic theory of gasses is based on the Brownian motion model of particles. The mathematical models that describe Brownian motion are used in various disciplines such as Physics, Maths, Economics, Chemistry, and more.

What is the Brownian Movement in Chemistry?

The Brownian movement in chemistry, which is also called Brownian motion, can be defined as the erratic or uncontrolled movement of particles in fluid because of their constant collision with other fast-moving molecules.

In general, this random movement of a particle can be observed to be stronger in the less viscous liquid, smaller sized particles, and at a higher temperature. There also exist other factors that affect the movement of particles in a fluid.

One of such most common examples of the Brownian motion can be given as diffusion. The cases where calcium is diffused in bones or pollutants are diffused in the air can be considered examples of this effect.

Brownian Movement in Colloids

We can see the Brownian motion effect in all types of colloidal sol. On the other hand, this phenomenon explains the sol particles’ random motion clearly and indicates that these particles are not static. Nevertheless, the major reason for this type of motion in the sol particles is the unequal bombardment of the depressed phase particle, leading to a non-uniform movement in native because of the particle’s size difference.

Meanwhile, the Brownian movement cannot be seen in the true solution because it is homogeneous, and there lies a uniform bombardment. However, considering colloids, the system is heterogeneous, and the bombardments are non-uniform, leading to a random measurement.

One of the major advantages of this effect is that it keeps the sol particles in continuous motion so the particles do not settle at the bottom by further preventing the lyophobic sols’ coagulation. So, this type of motion increases the stability of a sol. Brownian motion can also be observed in the cell’s plasma, where the particles in the cell also exist in random motion without making the plasma in the cell dry.

Cause of Brownian Motion

The primary causes of the Brownian Motion can be listed as follows:

• The particle’s size is inversely proportional to the motion’s speed, which means the small particles exhibit faster movements.

• This is due to the momentum transfer being inversely proportional to the particles’ mass. At the same time, lighter particles obtain greater speeds from collisions.

• The Brownian motion’s speed is inversely proportional to the viscosity of the fluid: the lower the fluid’s viscosity, the faster the Brownian movement.

• Viscosity can be given as a quantity that expresses the internal friction magnitude in a liquid. It is the resistance’s measure for a fluid’s flow.

Effects of Brownian Motion

• The Brownian movement causes fluid particles to be in constant motion.

• This prevents the particles from settling down, leading to the colloidal sol’s stability.

• We can distinguish a true sol from a colloid with the help of this motion.

Albert Einstein’s paper on Brownian motion provides significant evidence that molecules and atoms exist. In the kinetic theory of gases, the particles of the Brownian motion model are responsible for describing temperature, volume, and pressure.