1. Gases
Chapter 14

2. Properties of Gases as demonstrated by experiments
Gases have mass
Gases exert pressure
Gases can expand to take up large volumes
Gases can be compressed to fit in small volumes

3. Kinetic Molecular Theory
The gas consists of very small particles, all with non-zero mass.
These molecules are in constant, random motion. The rapidly moving particles constantly collide with the walls of the container.
The collisions of gas particles with the walls of the container holding them are perfectly elastic.
The interactions among molecules are negligible. They exert no forces on one another except during collisions.

4. Boyle’s Law
Boyle’s law states that at constant temperature, the absolute pressure and the volume of a gas are inversely proportional. Meaning, as pressure increases, volume decreases.
For Example:

5. Symbolic Representation of Boyle’s Law
As pressure increases, volume decreases.
P * V = constant
So, P1V1 = P2V2
Pressure
(kPa)
Volume (L)

6. Measuring pressure Many ways to measure pressure of a gas.
Atmospheres (atm) force per unit area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere.
mm Hg (mmHg) pressure of 1 mm of mercury in a column of mercury.
torr equal to 1 mmHg
pascals or kilopascals

7. Measuring pressure in Chemistry class
Pascals (Pa) or kilopascals (kPa)
1 Pa = the amount of force 1 kg exerts on 1 m2 for every 1 s2.
1 Pa = 1 x 10-6 atm
1 kPa = the amount of force 1000 kg exerts on 1 m2 for every 1 s2.
1 kPa = 1 x 10-3 atm

8. Boyle’s Law Problem
A balloon contains 20.0 L of helium gas at 103kPa. The balloon rises to an altitude where the pressure is only 15.0 kPa. What happens to the volume of the balloon?
P1V1 = P2V2
P1= 103 kPa
V1 = 20.0 L
P2 = 15.0 kPa
V2 = ?
(103kPa)(20.0L) = (15.0kPA)V2

9. Charles’ Law
When pressure is held constant, the volume and temperature of a gas are directly proportional. Meaning as temperature increases, volume increases.
For example,

10. Symbolic Representation of Charles’ Law
As temperature increases, volume increases.
So, V/T = constant
Volume (L)
Temperature (K)

11. Measuring Temperature
Fahrenheit
(based on the freezing and melting point of water)
Celsius
(based on the freezing and melting point of water but in base 10)
Kelvin
(based on the idea that at 0 Kelvin or absolute zero, atoms are NOT in motion)

12. Converting between Celsius and Kelvin
Kelvin, K = oC + 273
So, 32oC = = 305 K
100oC = = 373 K

13. Charles’ Law Practice
A balloon has a volume of 1.00 L. The temperature of the room where the balloon is located is 273 K. If the room is heated to 300 K, what happens to the volume of the balloon?
Make a qualitative assessment of what is going to happen to the volume of the balloon after an increase in temperature.

14. Gay-Lussac’s Law
If the volume is constant, as the temperature of the gas increases, the pressure increases. Temperature and pressure are directly proportional.
For example,

15. Symbolic Representation of Gay-Lussac’s Law
As temperature increases, pressure increases.
So, P/T = constant
Pressure (kPa)
Temperature (K)

16. Gay-Lussac’s Practice
A sealed cylinder of gas contains nitrogen gas at 1,000 kPa and a temperature of 293K. When the cylinder is left in the sun, the temperature of the gas increases to 323K. What happens to the pressure in the cylinder?
Step #1: Qualitative Assessment of the Problem
Step #2: Quantitative Assessment of the Problem

17. Combined Gas Law
Assume the number of gas molecules remains constant, temperature, pressure and volume are related.

18. Combined Gas Law Example
A sample of gas has a volume of 283 mL at 25oC and atm pressure. What is the volume at 100oC and 1.00 atm?
V2 = 177 mL

19. Avogadro’s Law
If the temperature and pressure are constant, as the number of particles of the gas increases, the volume increases. Number of particles and volume are directly proportional.

20. Symbolic Representation of Avogadro’s Law
As number of particles increases, volume increases.
So, V/n = constant
Volume
(L)
Number of Particles

21. Ideal Gas Law
An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces.
Think of a gas as a collection of perfectly hard spheres which collide but which otherwise do not interact with each other. Like Billiards!!

22. More about Ideal Gas Law
An ideal gas can be characterized by three variables: absolute pressure (P), volume (V), and absolute temperature (T).
The relationship between them may be deduced from kinetic theory.

23. Exceptions to the Ideal Gas Law
at low temperatures (close to 0 K) the gas molecules have less kinetic energy (move around less) so they do attract each other.
at high pressures (like many, many kPa) the gas molecules are forced closer together so that the volume of the gas molecules becomes significant compared to the volume the gas occupies.

24. Symbolic Representation Ideal Gas Law
PV = nRT
n = number of moles
R = universal gas constant = J/mol*K
One mole of an ideal gas at STP (Standard Temperature and Pressure) occupies 22.4 L.

25. Ideal Gas Law Example
What volume is needed to store moles of helium gas at kPa and 400 K?
P = kPa n = mol T = 400K V = ? L R = J/mol*K
PV = nRT
(202.6)(V) = (0.050)(8.314)(400)
(202.6)(V) =
V=166.28/202.6
V = 0.821

26. Another Example
What pressure will be exerted by g hydrogen gas in a 7.5 L cylinder at 20oC?
What do we know?
What are we trying to find?
V= 7.5 L
T= 20 Celsius
R= J/mol*K
n= g
Pressure!
Do we need to change any units?
Celsius  Kelvin
20 C = 293
Mass:
Grams  moles
mass = g
molar mass of H2 = 2 x =2.016 g/mol
NOW SOLVE!