1. Unit 9: Gases

2. Pressure
Gas pressure: the result of collisions of billions of rapidly moving gas particles with other objects
Atmospheric pressure results from the collisions of atoms and molecules in air with objects
SI unit of pressure: pascal (Pa)
Other units include
millimeters of mercury: mmHg
atmospheres: atm
torr

3. Pressure conversions
1 atm = 760 mmHg =760 torr = kPa ** Remember- STP is defined as a temp of 0°C (273 K) and kPa (1 atm)

4. Volume Volume is a measure of how much space an object takes up
SI unit for volume = Liter
Remember: 1 L = 1000 mL

5. Temperature
Temperature is a measure of the average kinetic energy of the particles in a substance
SI unit for temp: Kelvin
Remember: K = °C

6. 1. Boyle’s Law: Pressure and Volume
Under constant temperature, as the pressure of a gas increases, the volume decreases (and vice versa)
P1V1 =P2V2

7. 2. Charles’ Law: Temperature and Volume
Under constant pressure, as the temperature of a gas increases, the volume increases (and vice versa)
TEMPS MUST BE IN KELVINS!

8. 3. Gay-Lussac’s Law: Pressure and Temperature
Under constant volume, as the temperature of a gas increases, the pressure increases (and vice versa)
TEMPS MUST BE IN KELVINS

9. The Combined Gas Law

10. Warm-Up
If I initially have a gas at a pressure of 1.1 atm, a volume of 23 liters, and a temp. of -73°C, and then raise the pressure to 990 mmHg and increase to standard temp, what is the new volume of the gas?
27 L

11. Kinetic Molecular Theory of Gases (KMT)
**When we use any of the 3 gas laws or the combined gas law, we assumed that these gases are ideal. Ideal gases obey in these 4 ways: 1) Gases consist of individual particles that move in random straight lines 2) Gas particles have no attraction for one another

12. Kinetic Molecular Theory of Gases (KMT)
3) Gas particles are separated by such large distances that the particles themselves have negligible volume 4) When gas particles collide, the collisions are perfectly elastic (particles may transfer energy, but there is no net loss of energy)

13. When do gases behave most like ideal gases?
Under conditions of low pressure and high temperature
Lighter (smaller) gases behave more like ideal gases than heavier ones

14. Avogadro’s Principle
One mole of any gas at STP occupies a volume of 22.4L

15. Avogadro’s Principle, cont.
Equal volumes of gases at the same temperature and pressure contain an equal number of particles

16. mol  L and L  mol Conversions
Convert 0.42 mol H2 (g) to liters
Convert 64.3 L O2 (g) to moles

17. Partial Pressure
Partial pressure: the contribution each gas in a mixture makes to the total pressure

18. Dalton’s Law of Partial Pressure
Ptotal = P1 + P2 + P3 + …
- At constant V and T, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the component gases

19. Graham’s Law
Diffusion: the tendency of molecules to move from higher to lower concentration
Effusion: when a gas escapes through a tiny hole in its container
Graham’s Law: Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass

20. Real vs. Ideal gases
Particles of a real gas ARE attracted to one another (and can condense)
Real gas particles DO have volume
When gas particles collide, the collisions are NOT perfectly elastic (there is some loss of energy)

21. What is vapor pressure?
Vapor pressure: a measure of the force exerted by a gas above a liquid in a closed container
The higher the vapor pressure, the WEAKER the intermolecular forces (IMFs)

22. Dynamic Equilibrium
A constant vapor pressure is reached when rate of condensation = rate of evaporation
Known as dynamic equilibrium

23. Vapor Pressure and Temperature Change
An increase in temp. of a contained liquid increases the vapor pressure
Why? Particles have more kinetic energy, allowing them to escape the attractive forces of the liquid phase

24. What is boiling point?
Boiling Point: the temperature at which the vapor pressure of a liquid equals the external or “outside” pressure
Normal boiling point: b.p. of a liquid at standard pressure (101.3 kPa)

25. Boiling Point and Pressure Changes
When external pressure is:
Less than 1 atm, the b.p. of the liquid is lower than its normal b.p.
Equal to 1 atm, the b.p. of a liquid is called the normal boiling point.
Greater than 1 atm, the b.p. of the liquid is greater than its normal b.p.