Chemistry Chapter 12: Gas Laws

Study Notes

Gas laws describe how gases behave
The Law of Combining Volumes states that when gases react, the volumes of reactants and products (measured at equal temperatures and pressures) are in whole number ratios.
Joseph Louis Gay-Lussac formulated this law
2 volumes of hydrogen gas react with 1 volume of oxygen gas to produce 2 volumes of water vapor.

The Law of Multiple Proportions states that the masses of elements combining can be expressed in small, whole-number ratios
John Dalton formulated this law

Amedeo Avogadro (1776-1856) contributed to molecular theory (Avogadro's Law)
This law combines Gay-Lussac's and Dalton's ideas
The volume of a gas is related to the amount of gas present, calculated from the mass.
Equal volumes of gases under the same temperature and pressure conditions contain equal numbers of molecules.
Avogadro's law provides a mathematical relationship between the volume of a gas (V) and the number of moles of gas present (n).
V ∝ n, or n = kV, or V₁/n₁ = V₂/n₂
where n = number of moles, V = volume and k = a constant

One mole of any gas at the same temperature and pressure occupies the same volume
Molar volume is the space occupied by one mole of a gas, expressed as L/mol

A 1.30 L container, with a mass of 7.43 g, has nitrogen gas added, until the pressure is 98.0 kPa at 22.0 °C. The total mass is 6.18 g. Calculate the molar volume of nitrogen gas at STP. (Solution provided)
- Given values include initial pressure (P₁), initial volume (V₁), initial temperature (T₁), final pressure (P₂), and the mass added (m)
- Find the volume of 1 mole of nitrogen at STP

At standard temperature and pressure (STP) gases behave like ideal gases.
At high pressure and/or low temperatures, gas molecules no longer behave like ideal gases
- Gas particles have their own volume
- Gas particles attract each other
- Collisions are not perfectly elastic (there is some energy loss)
This is because gas molecules have their own volume and also that there is an attraction between them
In these conditions molecules will have smaller volumes, be closer together and allow for gas particle attraction

### The Ideal Gas Law

- PV = nRT
- R is the universal gas constant (8.314 kPa·L/mol·K)

Guidelines for using ideal gas law:
- Convert temperature to Kelvin
- Convert masses to moles
- Convert volumes to liters
- Convert pressures to kilopascals (kPa)

Use Gay-Lussac's law of combining volumes to solve for stoichiometry problems involving gas-phase reactions when gases react with other gases
- Balancing chemical equations is important
Example: Calculate the volume of ammonia gas produced from 12.0 L of nitrogen gas, if there is excess hydrogen.
- Balanced chemical equation
- Ratio between nitrogen and ammonia (1 mol N₂ to 2 mol NH₃)

To solve stoichiometry problems when gases are involved, convert volumes into moles first
Compare the moles
Convert back to volumes