GenChem1-Gas and Atmospheric Pressure.ppt

GAS AND ATMOSPHERIC PRESSURE

1. Kinetic Theory Of Kinetic Theory Of
GasesGases
•Gases or atmosphere consist of molecules
which far apart and in random motion at high
speed.
GAS PRESSUREGAS PRESSURE

3. Atmospheric pressure
1.The earth is surrounded by a thick layer of
atmosphere.
2.Atmospheric pressure is the pressure exerted pressure exerted
by the atmospheric on the surface as well as all by the atmospheric on the surface as well as all
objects on Earth.objects on Earth.
Activity

4. Characteristics of atmospheric or gas Characteristics of atmospheric or gas
pressure.pressure.
1. Unit : 1 atmosphere= 1 x 10
5
Pa.
= 1 x 10
5
Nm
-2
= 1000 milibar
= 760 mm Hg
= 10 m water
1 milibar= 100 Pa
2.Atmospheric pressure act in all direction.
3.It not affected by surface area of the object.

5. Continued..
4. The atmospheric pressure depend on altitude. Hence as
the altitude increases, the atmospheric decrease.
5. The atmospheric pressure can support a column of
mercury at 760mm in high or column of water at 10m
high.

6. The gas pressure inside a container depends on;
–Density of molecules of gases
–Temperature
–Volume of the container

6. MEASURING INSTRUMENTSMEASURING INSTRUMENTS
1. The height of the mercury column, h = 76 cm, represents the
atmospheric pressure.
2. The atmospheric pressure, P
P = h ρ g
= 0.76 m x 1.36 x 10
4
kg m
-3
x 10 N kg
-1

= 101 300 Pa
Mercury BarometerMercury Barometer

7. Aneroid barometer
1. Consist of a flexible metal can be which most of the air inside
the can has been taken out of it.
2. When the surrounding pressure is higher, the top of the can is
squeezed down slightly causing the pointer to move along
the curved scale measuring the corresponding pressure.
3. The spring prevents the flexible metal can from collapsing
completely by the atmospheric pressure.

8. ManometerManometer
1.Consist of a U-Tube that is filled with a liquid like water, oil
or mercury
2.When both ends of the tube is exposed to atmospheric
pressure, both level are the same.
3.When one end is connected to a gas supply, the difference
in level, h gives the pressure of the gas, P
where
P = Atmospheric pressure + h ρ g

9. Bourdon GaugeBourdon Gauge
1. The movement of a gas into a hollow copper tube
causes it to straighten slightly.
2. A system of lever and gear will enable a pointer to move
along the round scale indicating the pressure of a gas
measured.

10. APPLICATION OF GAS / APPLICATION OF GAS /
ATMOSPHERIC PRESSUREATMOSPHERIC PRESSURE
• Straw
• Siphon
• Rubber sucker
• Lifting pump
• Vacuum cleaner

11. Drinking Straw
•When we suck
through a straw, the
air pressure in the
straw is lowered.
Then the pressure of
the atmosphere
acting on the surface
of the drink in the
glass pushes the
water up the straw

12. Rubber Sucker
•When the sucker is
pressed into place, most
of the air behind it is
squeezed out. The sucker
is held in position by the
pressure of atmosphere
on the outside surface of
the rubber. If the seal
between the sucker and
the surface is airtight, the
sucker will stick
permanently.

13. Syringe
•Pulling up the piston
reduces the
atmospheric pressure
inside the cylinder.
•The atmosphere on
the liquid surface then
pushes the liquid up
into the syringe.
•If we then hold the
plunger in place and
lift the syringe out of
liquid, none will fall
out. This is again due
to atmospheric
pressure.

14. Vacuum Cleaner
•Produces only a
partial vacuum.
•The fan inside the
cylinder blows air out
of the vents. – less air
inside, air pressure
drops.
•The atmospheric
pressure outside then
pushes air up the
cleaner hose, carrying
dust and dirt with it.

15. Example & Example &
exercise;exercise;
Example 1 :
The figure below shows a simple barometer used in the laboratory to
measure atmospheric pressure. The length of the glass tube is 100 cm. The
height of a vacuum column is 10 cm and the height of the mercury column is
76 cm.
a) What is a value of the atmospheric pressure in cm Hg ?
b) What is the pressure at point X in cm Hg ?
c) If the density of a mercury is 1.36 x 10
4
kg m
-3
and the g = 10 m s
-2
.
Calculate the pressure at X in Pascal.

16. Answer;
Atmospheric pressure = 76 cm Hg.
Pressure at point X = ( 76 + 14 ) cm Hg
= 90 cm Hg.
Pressure at point X in Pascal
P = h ρ g
= 0.90 m x 1.36 x 10
5
kg m
-3
x 10 m s
-2.
= 1.224 x 10
6
Pa.

17. Example 2 :
A mercury manometer with one end attached to a gas supply measures
a difference in the level of mercury of 32 cm as shown in figure below.

Calculate the pressure of a gas supply in
a) cm Hg
b) pascal
( Atmospheric pressure = 76 cm Hg, g = 10 m s
-2
and
density of mercury = 1.36 x 10
4
kg m
-3
)

18. Solution :Solution :
a) P gas = Atmospheric pressure + Pressure the column of
mercury.
= ( 76 + 32 ) cm Hg
= 108 cm Hg.
b) P gas = 1.08 m x 1.36 x 10
5
kg m
-3
x 10 m s
-2
.
= 1.47 x 10
6
Pa.

19. ExerciseExercise
;;
The diagram shows a manometer which is connected to a gas supply.
The atmospheric pressure is 76 cm Hg
What is the pressure at point A?
(a)Find the pressure of the gas supply,
(b)The tap is opened and the mercury is allowed to flow out until the
level B falls to the division 60 cm of the ruler.
(c)State the new position of level A.
1)

20. Figure shows an apparatus used in the laboratory to measure atmospheric
pressure. The length of the glass tube is 100 cm.
The height of the vacuum column is 8 cm and the height of the mercury
column is 76 cm.
(a) Name the apparatus as shown in the figure
(b)(i) What is the value of the atmospheric pressure in cm Hg?
(ii) What is the pressure at point X in cm Hg?
(iii) If the density of the mercury is 1.36 × 10
4
kg m
-3
and the acceleration
due to gravity is 9.8 ms
-2
.Calculate the atmospheric pressure in Pascal.
2.)

21. (c) What will happen to the height of mercury column from the surface of
the mercury in the container if
(i) the tube is raised through a height of 12 cm?
(ii) the tube is lowered through a depth of 6 cm?
(iii) the glass tube is inclined about 2o from the vertical line?
(iv) give reason for you answer in (c) (iii)
(d) The apparatus above can be used to measure altitude
(i) Explain why this can be done
(ii) Explain why the apparatus above is not suitable to
measure altitude.