Topic 5 Gases

Topic 5 Overview 5.1 Describing Gases - Phases of matter - Units of pressure 5.2 Ideal and Non-Ideal (Real) Gases - Ideal gas behaviour - Non-ideal gas behaviour 5.3 Mixtures and Stoichiometry - Describing mixtures - Mole conversions

5.1 Describing Gases The three most common states of matter are – Gases (subscript ) – Liquids (subscript ) – Solids (subscript ) Gases are the ________ ordered (most disordered = highest entropy) Gas state is the _________ energy state of matter (by the fact that the particles are in constant random motion)

5.1 Describing Gases Some elements have a standard gas state: H 2(g) N 2(g) O 2(g) F 2(g) Cl 2(g) He (g) Ne (g) Ar (g) Kr (g) Xe (g) Rn (g)

5.1 Describing Gases Some compounds also have a standard gas state: CO (g) CO 2(g) NO (g) N 2 O (g) NO 2(g) SO 2(g) SO 3(g) and smaller hydrocarbons Note: You should remember which elements have gaseous standard state (and which diatomic molecules).

5.1 Describing Gases Gases have measurable and relatable properties: (some do depend on experimental conditions) Pressure (p) Volume (V) Both p and V can be affected by the amount of gas (number of moles) and the temperature of the system (T).

5.1 Describing Gases Pressure (p) – Measures collisions of gas molecules to inside of container – SI units : – Other units often used: Volume (V) – Defined by container

5.2 Ideal and Non-Ideal (Real) Gases (Textbook Chapter 2.2, 2.7) Ideal Gas behaviour – When (α = “is proportionate to”, changes affect both in the same manner) (1/α = “is inversely proportionate to”, changes affect both in opposite manners)

5.2 Ideal and Non-Ideal (Real) Gases Combination of these behaviour properties forms the Ideal Gas Equation R = gas constant (value and units depend on units of pressure used in system) = OR =

5.2 Ideal and Non-Ideal (Real) Gases Example:

5.2 Ideal and Non-Ideal (Real) Gases Changing conditions can be easily observed by noting initial and final conditions: R will always be constant At least one variable (p, V, n, and/or T) must remain constant (is often number of moles)

5.2 Ideal and Non-Ideal (Real) Gases Example:

5.2 Ideal and Non-Ideal (Real) Gases “Ideal” behaviour relations allowed by the Kinetic-Molecular Theory of Gases 1.Gas particles are __________________ compared to the distance between the particles. 2.Collisions between gas particles ___________ (no loss of kinetic energy; assumed to have no intermolecular forces).

5.2 Ideal and Non-Ideal (Real) Gases Real Gas behaviour Kinetic-Molecular Theory assumptions have some flaws: 1. The size of the particles is not entirely negligible (they do take up some space…) 2.Intermolecular forces are not entirely negligible (intermolecular forces have some attraction between the gas particles)

5.2 Ideal and Non-Ideal (Real) Gases The reason the ideal gas equation holds is because the two flaws cancel each other... …at least at relatively low pressure. Below _________, ideal gas equation works well. Above __________…not so much.

5.2 Ideal and Non-Ideal (Real) Gases Use the Real Gas Equation to correct failures of ideal gas equation. Corrections applied using van der Waal’s constants:

5.2 Ideal and Non-Ideal (Real) Gases van der Waal’s constant b: Size correction Applies to volume variable Value of b different for each gas Units = Ideal  real

5.2 Ideal and Non-Ideal (Real) Gases van der Waal’s constant a: Intermolecular forces correction Applies to pressure variable Value of a different for each gas Units = Ideal  real

5.2 Ideal and Non-Ideal (Real) Gases Change Ideal Gas Equation pV = nRT to Real Gas Equation

5.2 Ideal and Non-Ideal (Real) Gases Example:

5.3 Mixtures and Stoichiometry (Textbook Chapter ) Many gases are mixtures of two or more compounds. Examples: Air

5.3 Mixtures and Stoichiometry These mixtures can be described in two ways: By ________________ – Each compound in the container has its individual pressure – Written as: p A (Partial pressure of compound A) By __________________ – Sum of the partial pressures of all the compounds in the container – Written as: p or p total

5.3 Mixtures and Stoichiometry Partial and total pressures can be related by mole fraction of the gases: Mole fraction of A = X A = (n A /n total ) (no units since X is mol/mol) p A /p total = X A Sum of mole fractions of all species of gases in mixture = 1.00

5.2 Ideal and Non-Ideal (Real) Gases Example:

5.3 Mixtures and Stoichiometry Scale of mixes: Typical units are ppm and ppb 1ppm = 1 molecule out of every 10 6 molecules 1ppb = 1 molecule out of every 10 9 molecules You have already seen a similar unit with relative spread (in ppt).

5.3 Mixtures and Stoichiometry Something to add to equations of chemical reactions: phases Subscripts to the compounds in the reaction: – Ex: CO (g) = carbon monoxide __________ – Ex: H 2 O (l) = __________ water – Ex: Fe 2 O 3(s) = __________ iron oxide – Also: NaCl (aq) = sodium chloride in an ___________

5.3 Mixtures and Stoichiometry The state of the compound will tell you which phase conversion to consider (when finding the number of moles): n = m/M n = c/V n = pV/RT Can combine to convert between types or units (ex: mass of liquid when only volume was known)

Topic 5 Practice Problems Tutorial practice problems List of textbook questions Bonus assignment(s)