Kinetic theory (or the kinetic or kinetic-molecular theory of gases) is the theory that all matter is made up of a large number of small particles (atoms or molecules), all of which are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container.
Kinetic theory explains macroscopic properties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion. Essentially, the theory posits that pressure is due not to static repulsion between molecules, as was Isaac Newton's conjecture, but due to collisions between molecules moving at different velocities.
In summary, The theory helps to explain the behavior of matter. Two important areas explained are the flow or transfer of heat and the relationship between pressure, temperature and volume properties of gases.
Solid
Inter-Molecular Forces
# In a solid the strong attractions between the particles hold them tightly packed together. Even though they are vibrating this is not enough to disrupt the structure.
Hence, a lot of thermal energy is required to pull apart the particles.
This explains why Solids have High Melting Point.
Process of Melting of solid:
When a solid is heated the particles gain energy and start to vibrate faster and faster. Initially the structure is gradually weakened which has the effect of expanding the solid. Further heating provides more energy until the particles start to break free of the structure. Although the particles are still loosely connected they are able to move around.
At this point the solid is melting to form a liquid. The particles in the liquid are the same as in the solid but they have more energy. To melt a solid, energy is required to overcome the attractions between the particles and allow them to pull them apart.So, the solid is heated up.
(The temperature at which something melts is called its "melting point" or melting temperature)
# Solids have a fixed surface and volume (at a particular temperature) because of the strong particle attraction.
# Solids will expand a little on heating but nothing like as much as liquids because of the greater particle attraction restricting the expansion and contraction occurs on cooling.
* The expansion is caused by the increased energy of particle vibration, forcing them further apart causing an increase in volume and corresponding decrease in density.
Arrangement of Particles
# Solids cannot be compressed because the molecules are arranged close together in a regular pattern and there is little space between them.
# With increase in temperature, the molecules gain kinetic energy and vibrate more. The separation between molecules increases slightly and the solid expands.
# Solids have the greatest density (‘heaviest’) because the particles are closest together.
# Solids are extremely difficult to compress because there is no real ‘empty’ space between the particles.
Motion
# Individual molecules are locked in position near each other, and cannot move past one another. The atoms or molecules of solids remain in motion.
However,
# Particles of solids vibrate constantly in their fixed positions due to their internal energy but they cannot move from one place to another. Particles of solids possess only vibrational energy.
# Solids cannot flow freely like gases or liquids because the particles are strongly held in fixed positions.
# Diffusion is almost impossible in solids because the particles are too closely packed and strongly held together with no ‘empty space’ for particles to move through.
Liquids
Inter Molecular Forces
# Particles of liquids are kept together by forces of attraction that are weaker than those of solid particles.
# Often in liquids, intermolecular forces (such as the hydrogen bonds shown in the animation below) pull molecules together and are quickly broken.
# Liquids will expand on heating but nothing like as much as gases because of the greater particle attraction restricting the expansion (will contract on cooling).
* Note: When heated, the liquid particles gain kinetic energy and hit the sides of the container more frequently, and more significantly, they hit with a greater force, so in a sealed container the pressure produced can be considerable!
# Liquid also cannot be compressed as the molecules are still close together and there is little space between them. When a liquid is heated, the molecules vibrate and move about more vigorously. Thus the liquid expands, but only very slightly. Hence, more thermal energy is required to pull particles of liquid apart.
This results in Liquids having a lower Melting point and boiling point than that of a solid
Liquids have a surface, and a fixed volume (at a particular temperature) because of the increased particle attraction, but the shape is not fixed and is merely that of the container itself.
* Liquids seem to have a very weak 'skin' surface effect which is caused by the bulk molecules attracting the surface molecules disproportionately.
Arrangement of particles
# The molecules are not arranged in a regular pattern; randomly arranged and are slightly farther apart than in solids.
# Though there are still forces between the molecules, they are not held in fixed positions.
Hence,
# Liquids have a much greater density than gases (‘heavier’) because the particles are much closer together because of the attractive forces.
# Liquids are not readily compressed because of the lack of ‘empty’ space between the particles and as the molecules are still close together and there is little space between them
# When a liquid is heated, the molecules vibrate and move about more vigorously. Thus the liquid expands, but only very slightly.
Motion
# In liquids, molecules can move past one another and bump into other molecules; however, they remain relatively close to each other like solids
# Liquids usually flow freely despite the forces of attraction between the particles but liquids are not as ‘fluid’ as gases.
* Note 'sticky' or viscous liquids have much stronger attractive forces between the molecules BUT not strong enough to form a solid.
# Liquids have a surface, and a fixed volume (at a particular temperature) because of the increased particle attraction and , but the shape is not fixed and is merely that of the container itself.
(Within the walls of the container they can move from place to place bumping into the sides of the container and into other particles. This type of energy is called translational energy. This energy gives a liquid the ability to flow and be poured and to spread when a liquid is spilled. Liquid particles also have vibrational energy.)
Click Here to see Animation
* Liquids seem to have a very weak 'skin' surface effect which is caused by the bulk molecules attracting the surface molecules disproportionately.
# The natural rapid and random movement of the particles means that liquids ‘spread’ or diffuse. Diffusion is much slower in liquids compared to gases because there is less space for the particles to move in and more ‘blocking’ collisions happen.
Gases
Inter Molecular Foeces
# Particles of gases are "more rarefied" than either liquids or solids. This means that the forces of attraction that hold them together are very
# Gases have no surface, and no fixed shape or volume, and because of lack of particle attraction, they always spread out and fill any container (so gas volume = container volume).
# A gas has no fixed shape or volume, but always spreads out to fill any container.
Arrangement Of Particles
# The particles are widely spaced and scattered at random throughout the container so there is no order in the system.
# The Particles occupy any available spaces
# Gases have a very low density (‘light’) because the particles are so spaced out in the container (density = mass / volume).
* Density order: solid > liquid >>> gases
# Gases are readily compressed because of the ‘empty’ space between the particles.
* Ease of compression order: gases >>> liquids > solids (almost impossible to compress a solid)
Motion
# The particles move rapidly in all directions, frequently colliding with each other and the side of the container.
# With increase in temperature, the particles move faster as they gain kinetic energy.
# Gases flow freely because there are no effective forces of attraction between the gaseous particles - molecules.
* Ease of flow order: gases > liquids >>> solids (no real flow in solid unless you powder it!)
* Because of this gases and liquids are described as fluids.
# Particles Move at high speeds randomly so they have no fixed shape or volume, but always spreads out to fill any container.
EVIDENCE IN SUPPORT OF RANDOM MOTION OF GAS MOLECULES
The striking evidence of the molecular agitation of matter comes from such physical phenomena as diffusion, evaporation, Brownian motion, etc.
(a) Brownian Motion
It is named after the English Botanist Robert Brown. He discovered this motion in 1827. He observed that pollen suspended in water shows a continuous disorderly motion when viewed under a powerful microscope. [Fig. 22.4] The irregular motion of suspended particles is called Brownian motion. The proper explanation of Brownian motion was possible only after the development of kinetic theory.
When we look at a glass of still water on the table, it seems incredible that millions of molecules are moving about restlessly inside it.Robert Brown, however, discovered evidence of this movement over 200 years ago. He was observing through a microscope some very tiny particles of pollen in suspension in water. To his surprise he saw that some of the smaller particles were moving about continuously in a haphazard way. It is due to the bombardment of tiny particles by the liquid molecules all around it. The direction of the resultant force is constantly changing. Hence the motion of the particles is completely random. Thus 'Brownian motion' is defined as the random or zig zag motion of the suspended molecules.
Experiment :
Brownian motion can be demonstrated simply by releasing some smoke particles from burning cord into a small glass container and putting a cover plate to seal the smoke and air into the cell.
To investigate liquid molecular movement place some water with graphite particles suspended in it in the cell.
Now adjust the microscope slightly until you can see very bright specks. The particles of graphite (or smoke) scatter (reflect) the light shining on them and so appear as bright points of light darting about in a random or erratic motion. Note that the graphite (or smoke) particles are much larger than the water (or air) molecules. The particles can be seen by the light they scatter but the molecules themselves are too small to be seen.
The irregular movement of the visible particles of graphite (or smoke) is explained as being due to an uneven bombardment of the particles by the invisible molecules of water (or air). It is due to Brownian motion.
We can conclude that lighter the particles faster the motion and denser the particles slower the motion
(b) Evaporation
The phenomenon of evaporation is associated with the random motion of liquid molecules. At any given temperature, all the molecules do not move with the same velocity. Some move much faster than others. If the fast moving molecules possess energy greater than the energy with which they are held, they will escape from the surface of the liquid. This phenomenon is called evaporation. The escape of fast moving molecules lowers the mean kinetic energy of the remaining liquid molecules. This causes a decrease in the temperature of the liquid. That is why evaporation causes cooling.
(c) Diffusion
It is the process by which the molecules of one kind penetrate and intermix with the irolecules of another kind.
If a bottle of ammonia is opened in a room, the odour spreads to all parts of the room. This is because the molecules of ammonia escape from the bottle and diffuse into the air molecules.
If a jar full of hydrogen is held mouth to mouth over a jar full of carbon dioxide, hydrogen diffuses into carbon dioxide. [Fig. 22.5] This is inspite of the fact that hydrogen is lighter than carbon dioxide.
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