Physical Properties of Gases

1. Physical Properties of Gases Ideal Gas law General Gas Law Stoichiometry of Gases Dalton’s Law of Partial Pressures

2. The Simple Gas Laws Review - the simple gas laws allowed us to compare the relationship of two different variables while keeping the other two constant. • Boyle’s law: the relationship between pressure an volume (constant: temperature and quantity of gas) • Charles’ law: the relationship between absolute temperature and volume (constant: pressure and quantity of gas) • Gay-Lussac’s law: the relationship between absolute temperature and pressure (constant: volume and quantity of gas) • Avogadro’s law: the relationship between volume and quantity of gas (constant: pressure and temperature) .

3. The Simple Gas Laws Remember that the concept of an ideal gas is only hypothetical; these laws assume that all gases will behave in a specific way and we can predict their behavior if we only manipulate two variables at a time. However, real gases will only behave like ideal gases at conditions close to STP and SATP.

4. The Simple Gas Laws - Compared Hypothesis 1 states that “the particles of a gas are infinitely small and the size of a particle is negligible to the volume of the container that holds the gas.” If pressure is exerted on the container the volume occupied by the gas decreases (Boyle’s law) and the particles become increasingly closer together. In the case of a real gas, since the particles are now closer together, their size in relation to the volume in no longer negligible.

5. The Simple Gas Laws - Compared Hypothesis 2 states that “the particles of a gas are in constant motion and move in a straight line in all directions.” In the case of a real gas, collisions occur between particles and energy is transferred or lost to the surrounding environment. The result of this is that the pressure of a real gas is less than the pressure of an ideal gas (whose collisions are perfectly elastic).

6. The Simple Gas Laws - Compared Hypothesis 3 states that “the particles of a gas do not exert any force of attraction or repulsion on each other.” In the case of a real gas, as temperature decreases and particle movement slows, there are weak bonds that begin to form between gas particles as they become a liquid.

7. The Ideal Gas Law Each of the gas laws were designed to compare a sample of the same gas under different conditions. Separately, they were each proportional to a constant: The four simple gas laws can be combined to form the ideal gas law, which shows the relationship between all four variables (absolute temperature, pressure, volume and quantity of a gas: Where R is the gas constant, taking into account the proportionality constants of the simple gas laws and derived from the conditions at STP. In calculations, ensure that the units used respect the units of the gas constant.

8. The General Gas Law • The simple gas laws allow us to determine missing quantities from one set of conditions to another, given that two of the four conditions (temperature, pressure, volume and quantity of gas) remain constant. • The ideal gas law allows us to find out a missing variable if we know the other conditions that the given gas in exposed to.

9. The General Gas Law • The four variables (temperature, pressure, volume and quantity of gas) can be combined into a general gas law that will allow us to compare the initial conditions of a gas to the modified conditions. This law is a combination of two sets of conditions using the ideal gas law: • From this equation we can determine each of the simple gas laws by eliminating the conditions that remain constant.

10. Stoichiometry of Gases • Remember: At the beginning of the year, there was a lot of emphasis placed on stoichiometry. Now we will be putting that knowledge to use in combination with our knowledge of the gas laws! • Stoichiometry allows you to predict the quantity of reactants and products in a chemical reaction based on the molar ratios in the balanced chemical equation. • See notes from the beginning of the year for more details

11. Stoichiometry of Gases • One of the fundamental laws of chemistry is: When gases undergo a chemical reaction they are always present in whole-number ratios; temperature and pressure must remain constant. • The molar volumes at STP (22.4 L/mol) and SATP (24.8 L/mol) can simplify calculations. • Crash Course – Fundamental Laws of Chemistry http://www.youtube.com/watch?v=QiiyvzZBKT8

12. Dalton’s Law • Dalton’s law of partial pressures states that, at a given temperature, the total pressure of a mixture gases will equal the sum of the pressures of each of the individual gases (partial pressures).

13. Dalton’s Law • How is this explained using the kinetic theory of gases? The movements that gases undergo cause them to exert pressure on the walls of their container. The pressure that each individual gas in a mixture exerts is independent of any of the other gases in the mixture; according to Hypothesis 3, gas particles do not exert any force of attraction or repulsion on each other. This means that the pressure of each individual gas (partial pressure) contributes to the overall pressure of the mixture.

14. Dalton’s Law • The individual pressures that a gas exerts in a mixture must be calculated, they cannot be determined with a manometer. A manometer can only determine the total pressure of a system. The partial pressure of a gas is proportional to the composition of the mixture. • The partial pressure of a gas (if the total pressure is known):