The document explains the Kinetic Molecular Theory (KMT), which describes the behavior of gases in terms of molecular energy states, motions, and interactions. It outlines the differences between ideal gases, which conform to KMT postulates, and real gases, which exhibit molecular volume and attractive forces, leading to deviations from ideal gas behavior. Understanding KMT enhances comprehension of gas laws and general chemistry principles.

1. Copyright Sautter 2015

2. 2
THE KINETIC MOLECULAR
THEORY
UNDERSTANDING SUBSTANCES
ON A MOLECULAR LEVEL

3. 3
WHAT IS THE DIFFERENT BETWEEN SOLIDS,
LIQUIDS AND GASES ON A MOLECULAR LEVEL?
•THE ENERGY STATE OF THE MOLECULES
•SOLIDS CONTAIN THE LEAST ENERGY
•LIQUIDS ARE AT AN INTERMEDIATE
ENERGY STATE
•GASES ARE AT THE HIGHEST ENERGY
STATE

4. 4
WHAT DOES ENERGY STATE MEAN AT
A MOLECULAR LEVEL ?
•THE ENERGY CONTENT OF A MOLECULE RELATES
TO THE TOTAL ENERGY OF ITS:
•(1) MOTION THROUGH SPACE (TRANSLATIONAL
ENERGY)
•(2) SPINNING (ROTATIONAL ENERGY)
•(3) “SPRINGINESS” OF BONDED ATOMS
(VIBRATIONAL ENERGY)
•(4) MOTION OF THE ELECTRONS IT CONTAINS
(ELECTRONIC ENERGY)
•AT A TEMPERATURE CALLED ABSOLUTE ZERO (-273
DEGREES CELSIUS) NO ENERGY IS PRESENT IN THE
SYSTEM (EXCEPT ELECTRONIC)

5. 5
MOLECULAR MOTIONS
•MOTION OF GAS MOLECULES
(TRANSLATION, ROTATION & VIBRATION)
•MOTION OF LIQIUD MOLECULES
(ROTATION & VIBRATION)
•MOTION OF SOLID MOLECULES
(VIBRATION)

6. 6
RANDOM MOTION
OF GAS MOLECULES
MOLECULES MOVE
MOST RAPIDLY
IN THE GAS PHASE

7. 7
THE KINETIC MOLECULAR THEORY
PRIMARY DESCRIBES GASES
•THIS THEORY CONTAINS FIVE BASIC
STATEMENTS ABOUT GASES
MOLECULES AND HELPS US TO
BETTER UNDERSTAND THE BEHAVIOR
OF GASES

8. 8
POSTULATES OF THE KMT
(POSTULATES MEANS IDEAS OF PROPOSALS)
•(1) THE MOLECULES OF A GAS ARE IN CONSTANT,
RANDOM, STRAIGHT LINE MOTION. AT ABSOLUTE
ZERO (-273 DEGREES C) MOLECULAR MOTION
CEASES. (RANDOM MEANS WITHOUT PATTERN,
LIKE THE PING PONG BALLS IN THE LOTTO
MACHINE)
•(2) TEMPERATURE MEASURES THE AVERAGE
KINETIC ENERGY OF THE MOLECULES IN THE
SYSTEM. (KINETIC ENERGY IS THE ENERGY OF
MOTION. THE HIGHER THE TEMPERATURE THE
FASTER THE AVERAGE MOLECULE IS MOVING
HOWEVER, ALL THE MOLECULES ARE NOT MOVING
AT THE SAME SPEED)

9. 9
MORE ABOUT TEMPERATURE AND KINETIC ENERGY
•THE KINETIC ENERGY OF AN OBJECT IS RELATED TO ITS
MASS AND ITS SPEED. LARGE, FAST MOLECULES HAVE
MORE ENERGY THAN SMALL, SLOW ONES!
•HOWEVER, LARGE, SLOW MOLECULES CAN HAVE THE
SAME ENERGY AS SMALL, FAST ONES. WHAT THE LARGE
ONES LACK IN SPEED THEY MAKE UP FOR WITH MASS AND
WHAT THE SMALL MOLECULES LACK IN MASS THEY
MAKE UP FOR WITH SPEED. IN BOTH MASS AND SPEED
COUNT KINETIC ENERGY!!
•IN ANY GAS AT A PARTICULAR TEMPERATURE THERE IS
A DISTRIBUTION OF KINETIC ENERGIES. SOME
MOLECULES HAVE LOW KINETIC ENERGY, SOME HAVE
HIGH KINETIC ENERGY, MOST HAVE AN AVERAGE
AMOUNT ! (THIS IS LIKE THE SPEED OF CARS ON A
HIGHWAY, SOME TRAVEL FAST, SOME TRAVEL SLOWLY,
MOST TRAVEL AT AN AVERAGE SPEED NEAR THE SPEED
LIMIT !)

10. 10
KINETIC ENERGY DISTRIBUTION CURVE
TEMPERATURE = T1
TEMPERATURE = T2
KINETIC ENERGY
N
U
M
B
E
R
O
F
M
O
L
E
S
TEMP 2 > TEMP 1
AT HIGHER
TEMPERATURES,
MOLECULES HAVE
HIGHER ENERGIES
ON AVERAGE

11. 11
POSTULATES OF THE KMT (CONT’D)
•(3) THE MOLECULES OF A GAS ARE SO TINY COMPARED TO
THE SPACE THEY OCCUPY THAT THEIR VOLUME CAN BE
CONSIDERED TO BE ZERO.(IN REALITY IT IS IMPOSSIBLE FOR
ANY OBJECT TO HAVE ZERO VOLUME !)
•(4) THE MOLECULES OF A GAS NEVER ATTRACT
EACHOTHER. (IN ACTUALITY, ALL MOLECULES ATTRACT
EACHOTHER BUT IN GASES ONLY SLIGHTLY)
•(5) WHEN MOLECULES COLLIDE WITH EACHOTHER OR THE
WALLS OF A CONTAINER THE COLLISIONS ARE PERFECTLY
ELASTIC. THIS MEANS THAT ENERGY IS EVER LOST FROM
THE SYSTEM DUE THE MOLECULES HITTING EACHOTHER OR
THE CONTAINER WALLS. ENERGY OF MOTION IS MERELY
TRANSFERRED BETWEEN MOLECULES

12. 12
IDEAL GASES AND REAL GASES
•WHAT DOES THE WORD “IDEAL” MEANS ?
•IDEAL – “PERFECT, OF HIGHEST
STANDARD”
•IDEAL GASES THEN ARE THOSE THAT
CONFORM PERFECTLY TO THE POSTULATES
OF THE KINETIC MOLECULAR THEORY. NO
GAS IS IDEAL. AN IDEAL GAS IS A
THEORETICAL GAS WHICH REAL DOESN’T
EXIST !

13. 13
IDEAL GASES AND REAL GASES
(CONT’D)
•IF AN IDEAL GAS DOES NOT EXIST,
WHY DO WE TALK ABOUT IT ?
•MANY GASES UNDER THE RIGHT
CONDITIONS ACT SIMILARLY TO AN
IDEAL GAS SO IT GIVES US A GOOD
IDEA OF “REAL GAS” BEHAVIOR.

14. 14
IDEAL GASES AND REAL GASES
(CONT’D)
•A REAL GAS IS ONE THAT DOES NOT
CONFORM EXACTLY TO THE KINETIC
MOLECULAR THEORY AND DOES NOT
FOLLOW THE GAS LAWS EXACTLY
(THE GAS LAWS WILL BE DISCUSSED
LATER)

15. 15
IDEAL GASES AND REAL GASES
(CONT’D)
•IN WHAT WAYS DO REAL GASES DIFFER
FROM AN IDEAL GAS ?
•(1) IN REAL GASES THE MOLECULES DO
HAVE A VOLUME. IN AN IDEAL GAS THE
MOLECULES DON’T HAVE VOLUME
•(2) IN REAL GASES, THE MOLECULES
ATTRACT EACHOTHER. IN IDEAL GASES
THE MOLECULES HAVE ZERO ATTRACTION
FOR EACHOTHER.

16. 16
IDEAL GASES AND REAL GASES
(CONT’D)
•AS A RESULT OF MOLECULAR VOLUME AND
ATTRACTION, WHAT WILL REAL GASES DO THAT
IDEAL GASES WILL NOT ?
•(1) REAL GASES WILL NOT GIVE ALL THE
VALUES OF PRESSURE, TEMPERATURE AND
VOLUME THAT ARE PREDICTED BY THE GAS
LAWS
•(2) REAL GASES WHEN COOLED AND
PRESSURIZED WILL CONDENSE TO LIQUIDS AND
SOLIDS. AN IDEAL GAS WOULD NEVER
CONDENSE BUT WOULD REMAIN A GAS TO
ABSOLUTE ZERO AT WHICH POINT IT WOULD
SHRINK TO ZERO VOLUME. (AN IMPOSSIBLE
RESULT!)

17. 17
No Gas is Ideal
real gases liquefy before
absolute zero
Volume
T
E
M
P
E
R
A
T
U
R
E
- 273 0C
Absolute Zero
An ideal gas would
shrink to zero volume
at absolute zero

18. 18
WHAT IS PRESSURE AND WHAT CAUSES IT ?
•PRESSURE IS THE PUSH OF GAS
MOLECULES ON THE WALLS OF A
CONTAINER AS THEY BOUNCE OFF
THE SURFACE. IT IS DEFINE AS FORCE
DIVIDED BY AREA.
•IF MOLECULES HIT HARDER AND/ OR
MORE OFTEN, THEY PUSH HARDER
AND PRESSURE INCREASES.

19. 19
WHAT MAKES MOLECULES HIT THE WALLS OF A
CONTAINER HARDER AND / OR MORE OFTEN ?
•(1) INCREASING MOLECULAR SPEED (AS
TEMPERATURE INCREASES, THE AVERAGE KINETIC
ENERGY INCREASES AND MOLECULES MOVE
FASTER)
•(2) SHORTER DISTANCES BETWEEN COLLISIONS
WITH THE WALLS OF THE CONTAINER (IF THE
VOLUME OF THE CONTAINER IS REDUCED, THE
MOLECULES MOVE A SHORTER DISTANCE AND HIT
THE WALL MORE FREQUENTLY)
•(3) MORE MOLECULES IN THE CONTAINER (MORE
MOLECULES ALLOW MOLECULES TO STRIKE THE
CONTAINER WALLS MORE OFTEN)

20. 20
GRAHAM’S LAW OF DIFFUSION
(ANOTHER APPLICATION OF THE KMT)
WHY DO MOLECULES OF SOME GASES MOVE
FASTER THAN OTHERS AT THE SAME
TEMPERATURE ?
•GRAHAM’S LAW – AT ANY SPECIFIC TEMPERATURE, THE
SPEED OF A MOLECULE IS INVERSELY PROPORTIONAL TO ITS
MASS (AS MASS INCREASES, MOLECULAR SPEED
DECREASES)
•LIGHTER MOLECULES LIKE HELIUM MOVE MUCH FASTER
THAN HEAVIER ONES LIKE CARBON DIOXIDE AT THE SAME
TEMPERATURE.WHY ?
•BECAUSE AT THE SAME TEMPERATURE EACH GAS HAS THE
SAME AVERAGE KINETIC ENERGY, WHAT HELIUM LACKS IN
MASS IT MUST MAKE UP IN SPEED. THE MORE MASSIVE
CARBON DIOXIDE CAN MOVE MORE SLOWLY AND STILL
HAVE THE SAME KINETIC ENERGY AS THE HELIUM.

21. 21
SUMMARY OF THE KINETIC MOLECULAR
THEORY (KMT) AND ITS CONSEQUENCES
•TYPE OF ENERGY
(1) TRANSLATIONAL
(2) ROTATIONAL
(3) VIBRATIONAL
(4) ELECTRONIC
•FIVE POSTULATES OF KMT
(1) CONSTANT, RANDOM MOTION
(2) TEMPERATURE MEASURES AVERAGE KE
(3) MOLECULES HAVE NO VOLUME
(4) MOLECULES NEVER ATTRACT EACHOTHER
(5) MOLECULAR COLLISIONS ARE ELASTIC

22. 22
SUMMARY OF THE KINETIC MOLECULAR
THEORY (KMT) AND ITS CONSEQUENCES
(CONT’D)
•PRESSURE = FORCE / AREA
•AS TEMPERATURE INCREASES, PRESSURE INCREASES
•AS VOLUME DECREASES, PRESSURE INCREASES
•AS THE NUMBER OF MOLES INCREASES, PRESSURE
INCREASES
•AS TEMPERATURE INCREASES, AVERAGE MOLECULAR
SPEED INCREASES
•AS MOLECULAR MASS INCREASES AT A GIVEN
TEMPERATURE, THE AVERAGE MOLECULAR SPEED
DECREASES

23. 23
SUMMARY OF THE KINETIC MOLECULAR
THEORY (KMT) AND ITS CONSEQUENCES
(CONT’D)
•AN IDEAL GAS CONFORMS EXACTLY TO
KMT AND THE GAS LAWS
•REAL GASES ARE NOT DESCRIBED
EXACTLY BY THE KMT AND THE GAS LAWS
BECAUSE THEY HAVE MOLECULAR
VOLUME AND ATTRACTIVE FORCES
•REAL GASES WILL FORM LIQUIDS AND
SOLIDS UPON COOLING. AN IDEAL GAS
NEVER WOULD CONDENSE.

24. 24
UNDERSTANDING THE KINETIC
MOLECULAR THEORY WILL GREATLY
INCREASE YOUR UNDERSTANDING OF THE
GAS LAWS AND CHEMISTRY IN GENERAL !!