Kinetic theory has got more applications in daily life, in most of the tools and apparatus we use in our activities. This articles will focus on the major application.
The Rates of Diffusion and Kinetic Theory
Diffusion is the process by which the molecules of one
substance mix with those of another as a result of their motion. The substances
move freely from a region of high concentration to a region of low
concentration at their own pace. The rate of diffusion depends on the
temperature and the density of the substances involved as proved in experiment
1.1 and 1.2 below.
Experiment 1.1: Diffusion in gases
Aim: to demonstrate the diffusion in hydrogen
Requirements: hydrogen gas, porous jar, glass tube, coloured water, clamp and stand,
Hydrogen gas passes quite easily through earthenware
which is porous. Arrange the apparatus as shown in the figure below. Introduce
hydrogen into a vessel surrounding porous jar. What do you observe?
Diffusion of hydrogen |
Observation:
The bubbles of the gas start to escape through the
water: The hydrogen will diffuse through
the porous jar and increases the gas pressure in the jar and this will cause
some of the gas to bubble out. Air molecules will diffuse out of the vessel at
the same time as the hydrogen molecules diffuse in but because the air
molecules are large and heavier they diffuse out at a much slower rate.
Conclusion: The rate of diffusion in gases depends on its
density.
Air is a
uniform mixture of various gases because of diffusion. However, diffusion does
not only take place in gases, it even occurs in liquids as well. If convection
currents and stirring are avoided, it can be demonstrated that diffusion in
liquid is quite slow using the following setup.
Experiment 1.2: Diffusion in liquids
Aim: To show
diffusion in water
Materials: beaker, water,
dropper, copper sulphate (CuSO4)
Procedure:
Fill a gas jar
with water. Place a thistle funnel into it so that it extends to the bottom of
the jar and carefully pour a solution of copper sulphate until the bottom of
the jar is filled to a height about one centimetre. Let the jar stand
undisturbed for a few days. What do you
observe?
Diffusion in a liquid |
Observation: You will observe an upward diffusion of the blue copper sulphate against gravity.
Conclusion: The rate of
diffusion in liquid depends on the temperature.
The diffusion
in fluids can be explained using the kinetic theory model.
·
When diffusion takes place, molecules move from one
substance to another to fill up the empty spaces.
·
The spaces between the molecules are such that
molecules are able to pass through them.
·
The continuous movement of the molecules ensures a
thorough mixing of the substances as they collide.
·
When temperature is increased, the kinetic energy
increases through the increased speeds of molecules and consequently, the rate
of diffusion increases.
BROWNIAN MOTION
In 1827 Robert Brown was studying pollen grain
suspended in water using a microscope, he was very surprised to observe that
they were in a state of continuous random motion. He could not account for this
movement, but later scientists discovered that the movement of the pollen grain
was due to the collision of the water molecules against the pollen grains.
If you observe smoke particles in air, you will notice
that they experience similar movement. As a result of such observations,
scientists concluded that the molecules of liquids and gases are in a state of
constant, random motion, an idea known as the “kinetic theory of matter”
Experiment 1.3:
Brownian motion
Aim: To demonstrate Brownian motion in air/ a smoke cell
Materials: glass (smoke) cell, optical microscope, converging
lens, a lamp/ torch lamp
Procedure:
Set up apparatus as shown in the figure below; a small
glass cell in which smoke has been trapped is view through a microscope. A
converging lens or glass rod is used to focus the light from a lamp into the
glass cell.
Observation of Brownian motion |
Observation:
When the light strikes the smoke particles it is
scattered and the smoke particles are observed as bright specs of light. They
are also seen to be moving about in a zig-zag manner.
Conclusion:
This zigzag movement is due to the collisions of the
smoke particles with invisible air molecules that are also moving about
randomly in the smoke cell. The zigzag pattern of movement is illustrated in
figure below.
The zigzag movement of the smoke particles |
EVAPORATION
Evaporation is the change of a liquid into a gas at
the surface. It occurs at any temperature but occurs more rapidly at higher
temperatures because heat imparts more kinetic energy to the molecules and they
escape from the surface faster.
Increased gas pressure on the surface of the liquid
reduces the rate of evaporation because more collisions occur between the
evaporating liquid molecules and the gas molecules, resulting in some of the
evaporated liquid molecules bouncing back into liquid.
Some factors increase the rate of evaporation namely:
-
Increase in temperature: If we observe the wet clothes, we will notice that on
a warm day, they dry faster than on a cold day. This is because more particles
of the water have enough energy to escape.
-
Increase in surface area: Water in a puddle on the road will dry more quickly
than water in a cup. This is because more water molecules in the puddle are
close to the surface and escape in bigger number than in the cup.
-
Air blowing across the surface: When a day is windy, the clothes dry faster than when
it is quiet or you can dry your wet hand by getting them closer to the fan.
This is because the moving air carries escaping water molecules away from the
liquid, preventing them from bouncing back into the liquid as result of air
pressure over the liquid.
-
Reduction of humidity: when air contains more water vapour, it is said to be
very humid. Wet clothes will take longer to dry in very humid air because
molecules in the water vapour return to the liquid almost at the same rate as
water molecules escape from it.
During the evaporation process, the molecules that
escape from the liquid are the most energetic ones. The average kinetic energy
of those that remain is therefore reduced, and the temperature reduces. This is
cooling by evaporation.
COOLING EFFECT OF EVAPORATION
Evaporation results in a general cooling of the
liquid. This cooling is brought about by the lowering of the overall kinetic
energy of the liquid as the more energetic molecules escape from the surface.
If energy is supplied to the liquid molecules (for
example by heating), the rate of evaporation increases. If this energy comes
from the surroundings, then the surroundings lose energy and experience a drop
in temperature. The cooling effect of evaporation can be demonstrated by the
following experiment:
Experiment 1.4: Cooling effect of evaporation.
Aim: To demonstrate the cooling effect of evaporation.
Requirements: a small beaker, ether, wooden block, water, glass tube
Procedure:
Place a small beaker, about one third full of ether,
on a flat piece of wood which has a layer of water on it. Place a glass tube
obliquely into the beaker to allow air in the beaker.
Cooling by evaporation |
Observation:
Air is bubbled through the ether and effectively
increases the surface area of the liquid and allows a lot more evaporation to
take place. As the evaporation continues, the water between the beaker and
wooden block will gradually start to freeze and the beaker will stick to the
wooden block. Water droplets will also form on the outside of the beaker.
Explanation:
The energy required for this increased rate of
evaporation is drawn from the beaker, the water and the wooden block
Water droplets form on the outside of the beaker due
to condensation of water vapour from the surrounding air coming into contact
with the beaker.
The cooling effect of evaporation is utilised in
refrigerators. A refrigerator basically transfers heat from the inside cabinet
to the outside environment. The principle involves a liquid that is constantly evaporating
and thereby drawing energy (or heat) from the inside of the cabinet. The
transfer of the heat from the inside to the outside is quite rapid and the
inside becomes very cool. Once outside the cabinet the heat is transferred to
the surrounding environment through the thin tube containing the vapour and the
vapour is condensed back into liquid. A pumping mechanism ensures the movement
of the liquid vapour round a closed system of conducting pipes thereby keeping
the same liquid flowing continually.
Illustration of cooling process in domestic refrigerator |
The figure above represents a typical refrigerator
system. The electric compressor motor forces a gas at high pressure through a
heat exchanger (condenser) on the rear outside wall of the refrigerator, where
QH is given off, and the gas cools to become liquid. The liquid
passes from a high-pressure region, via a valve, to low-pressure tubes on the
inside walls of the refrigerator; the liquid evaporates at this lower pressure
and thus absorbs heat (QL) from the inside of the refrigerator. The
fluid returns to the compressor, where the cycle begins again
Other Cooling effects of Evaporation
Another every day application of cooling by
evaporation is well observed in sweating:
When animals sweat, fine drops of moisture are released either through the skin
or through pores on the tongue. The passage of air over these drops results in
evaporation which causes a cooling of the animal’s body in a similar way as
described in the experiment above. Dogs, for example, remove excess heat from
their bodies by panting.
No comments:
Post a Comment