Simple Kinetic Molecular Model of Matter - HSphysics

Simple Kinetic Molecular Model of
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Learning Objectives
• compare the properties of solids, liquids and gases.
• infer from Brownian motion experiment the evidence for the movement of
• describe qualitatively the molecular structure of solids, liquids and gases, relating
their properties to the forces and distances between molecules and to the motion of
the molecules.
• describe the relationship between the motion of molecules and temperature.
• explain the pressure of a gas in terms of the motion of the molecules.
• recall and explain the following relationships using the kinetic model (stating of
the corresponding gas laws is not required):
• a change in pressure of a fixed mass of gas at constant volume is caused by a
change in temperature of the gas.
• a change in volume of a fixed mass of gas at constant pressure is caused by a
change in temperature of the gas.
• a change in pressure of a fixed mass of gas at constant temperature is caused
by a change in volume of the gas.
• use the relationships stated above to solve problems (qualitative treatment would
The kinetic molecular model of matter
describes matter as being made up of
molecules in continuous , random motion.
What is Brownian Motion?
Brownian motion is the random motion of a molecule, or
other very light object. It is observed by reflections of light
when the object is big enough to reflect the light, as a speck
of dust in a light beam, either in air or in water.
We will see random movement of specks of light with the
use of a microscope.The speck of dust in air is being struck
at random by molecules of air, and keeps changing direction
because of that.
Modern equipment to observe Brownian motion
Brownian motion
We will see random, erratic and haphazard
movement of the specks of light. The
molecules bombard the air particles or dust at
high speeds.
Heat, temperature and the
motion of molecules are all
related. Temperature is a
measure of the average kinetic
energy of the molecules in a
material. Heat is the energy
transferred between materials
that have different temperatures.
Increasing the temperature
increases the translational
motion of molecules.
When heat is added to a substance, the molecules and
atoms vibrate faster. As atoms vibrate faster, the space
between atoms increases. The motion and spacing of
the particles determines the state of matter of the
substance. The end result of increased molecular
motion is that the object expands and takes up more
Mass of the object remains the same, however. Solids,
liquids and gases all expand when heat is added. When
heat leaves all substances, the molecules vibrate
slower. The atoms can get closer which results in the
matter contracting. Again, the mass is not changed.
Pressure in Gases
From the kinetic theory of gases, a gas is composed of a large number of
molecules that are very small relative to the distance between molecules.
The molecules of a gas are in constant, random motion and frequently
collide with each other and with the walls of any container.
The molecules possess the physical properties of
mass, momentum, and energy. The momentum
of a single molecule is the product of its mass
and velocity, while the kinetic energy is one half
the mass times the square of the velocity. As the
gas molecules collide with the walls of a
container, as shown on the right, the molecules
impart momentum to the walls, producing a
force perpendicular to the wall. The sum of the
forces of all the molecules striking the wall
divided by the area of the wall is defined to be
the pressure.
The Pressure Law
The pressure law states that for a
constant volume of gas in a sealed
container the temperature of the
gas is directly proportional to its
pressure. This can be easily
understood by visualising the
particles of gas in the container
moving with a greater energy
when the temperature is increased.
This means that they have more
collisions with each other and the
sides of the container and hence
the pressure is increased.
The graph below shows the pressure of a fixed mass of gas at
constant volume is directly proportional to the absolute temperature
in Kelvin.
p/T = constant or (p1/T1) = (p2/T2)
Where T1 and T2 are the absolute temperatures, before and after the
change respectively.
Boyle's Law
Boyle's Law states that for a given mass of gas, at
a constant temperature, the value of pressure
multiplied by the volume is a constant. As a
mathematical equation, Boyle's law is:
Where P is the pressure (Pa), V the volume (m3)
of a gas, and k1 (measured in joules) is the
constant from this equation—it is not the same as
the constants from other equations.
Graph of Volume against pressure
Boyle's Law formula
P1V1=P2V2 = P3V3
Charles’s Law
An inflatable pool float may seem quite firm as it sits on
a deck in the hot sun. However, minutes after you toss
to float into the cold pool, the same float may seem
under-inflated. You may suspect that the float has
developed a slow leak, but that may not be the most
likely explanation for the apparent loss of air
pressure. It may be that Charles's law is responsible.
Charles's law, discovered by Jacques
Charles, states that the volume of a
quantity of gas, held at constant
pressure, varies directly with the Kelvin
Gases expand as they are heated and they contract when
they are cooled. In other words, as the temperature of a
sample of gas at constant pressure increases, the volume
increases. As the temperature goes down, the volume
decreases as well. The mathematical expression for
Charles's law is shown below:
V1/T1 = V2/T2
Remember that Charles's law calculations must be done
in the Kelvin scale.
Example 1
The volume of a fixed mass of gas in a cylinder is decreased at a
constant temperature.
Why does the pressure exerted by the molecules of the gas increase?
As the volume decreases, the gas molecules strike the
cylinder walls more often.
Example 2
The figure on the right shows some air
trapped by a layer of mercury. When the
beaker of water is heated, explain what
happens to the layer of mercury.
As the temperature increases, the kinetic energy of the air molecules
increases. These molecules will strike the mercury and the walls of
the test tube with greater frequency. The pressure of the trapped air
increases and pushes up the layer of mercury.