Chapter 4B: Gas Laws p. 66-73 Questions p. 69-73 (47-71)

Gas Laws There are a few laws that all gases must follow based on their Pressure, Volume, and Temperature. Do you have to memorize them? NO! Table T has one and you can modify it for the other two. Boyle’s Law Charles’ Law Combined Gas Law

Boyle’s Law P1V1 = P2V2 So, you need Pressure and Volume Volume is expressed as mL or L Pressure is expressed in atm or kPa At constant temperature, demonstrates the relationship between pressure and volume Indirect relationship Pressure increases, volume decreases Pressure decreases, volume increases

Boyle’s Law

Example 1: A gas has a volume of 50mL at a pressure of 1atm. If the volume is increased to 100mL, and the temperature is constant, what is the new pressure?

Example 2: A gas occupies 25mL at 2atm. What will be the volume of this gas if the pressure becomes 4atm and the temperature remains constant?

Charles’ Law V1/T1 = V2/T2 So, you need Volume and Temperature Volume is expressed in mL or L Temperature MUST be expressed in KELVIN! So, first, you have to convert temperature to the Kelvin scale- it avoids using a negative number. Table T At constant pressure, demonstrates the relationship between volume and temperature Direct relationship Temperature increases, volume increases Temperature decreases, volume decreases

Charles’ Law

Example 3: A gas has a volume of 100. mL at a temperature of 200. K. Calculate the new volume if the temperature was increased to 400. K and the pressure remains constant.

Example 4: At -73oC, the volume of a gas is 100mL. The temperature is raised to 27oC, and the pressure remains constant. What is the new volume?

Combined Gas Law (P1V1)/T1 = (P2V2)/T2 This law combines both Boyle’s Law and Charles’ Law into one law. This is the formula that is given to you in your Reference Tables Table T Fill in what is given to you and leave the rest alone. Just ignore it! Only use Kelvin temperatures

Example 5: A gas has a volume of 15.0L at a temperature of 300.K and a pressure of 4.00atm. What will be the new volume when the temperature is changed to 400K and the pressure is changed to 2.00atm?

Example 6: The volume of a sample of a gas at 273oC is 200.0L. If the volume is decreased to 100.0L at constant pressure, what will be the new temperature of the gas?

STP Whenever a problem says that the gas is at STP, (according to Table A of your Reference Tables) that means: Standard Pressure 101.3 kPa 1 atm Standard Temperature 273 K 0oC Which one you use will depend upon the units that are given.

Kinetic Molecular Theory of Gases Gas laws describe HOW gases behave, but not WHY they behave the way that they do The Kinetic Molecular Theory (KMT) is a theory that is used to explain the behavior of gases Also called Ideal Gas Laws

Kinetic Molecular Theory (aka Ideal Gas Laws) Gases contain particles that are in constant, random, straight-line motion. Gas particles collide with each other and with the walls of the container. These collisions may result in a transfer of energy among the particles, but there is no net loss of energy as the result of these collisions. The total energy remains constant. Gas particles are separated by relatively great distances (for their size). Because of this, the volume occupied by the particles themselves is negligible and need not be accounted for. In other words, it is mostly empty space. Gas particles do not attract each other.

Ideal verses Real Gases KMT explains the behavior of gases by using a model gas called an “ideal” gas A gas is said to be “ideal” if it behaves exactly as predicted When gas laws are used to solve problems involving “real” gases, the answers obtained often do not exactly match the results obtained in the lab. Its theoretical Ideal gases do not exactly match the behavior of real gases This is because 2 of the assumptions made by KMT are not exactly correct.

Ideal Gas Law Discrepancies- #3 Gas particles do not occupy volume. Although gas particles themselves occupy a small volume of space under normal conditions, as pressure increases the volume occupied by the particles can no longer be ignored. At high pressures, the increased concentration of particles leads to more frequent collisions and far greater chances of combining

Ideal Gas Law Discrepancies- #4 Gas particles do not attract one another. In most cases, the attractive forces between gas particles are so small that they can be disregarded. However, in some extreme cases, these small forces become important. ie- Water molecules in the atmosphere attract each other when the temperature becomes cold enough. The water molecules combine to form snow or rain. Real gas particles have weak intermolecular forces of attraction. Anything that has mass has an attraction!

What is “ideal?” A gas is said to be “ideal” if it behaves EXACTLY as predicted No gas is truly “ideal” Hydrogen (H) and Helium (He) are nearly ideal because they are so small MEMORIZE THIS! Gases are most ideal at low pressures and at high temperatures

Ideal Gases A real gas behaves most like an ideal gas at a LOW pressure and a HIGH temperature. Why? Low Pressure: gas particles are TOO FAR apart for weak intermolecular forces to form High Temperature: gas particles are moving TOO FAST to have any intermolecular forces of attraction

Avogadro’s Number (RECAP) Collective noun ie- Dozen=12, Gross=144, etc. Equal volumes of all gases under the same conditions of temperature and pressure have equal numbers of molecules One mole (mol) has 6.02 x 1023 particles/mol. 12L of Nitrogen gas at STP would contain the same number of molecules as 12L of Oxygen at STP One mole of gas occupies a volume of 22.4L at STP MEMORIZE THIS!!! It is NOT given to you in your Reference Tables!

Example 7:

Example 8: At STP, equal volumes of N2(g) and CO2(g) contain equal numbers of Protons Electrons Atoms Molecules

Example 9: How many molecules of H2(g) are in 44.8L of the gas?

Open your workbook to p. 69 (47-66) F f