chem20 - properties of gases

gas

- fluids that have no shape or volume of their own, but are able to take on the shape and volume of the container in which they are confined

- EX. oxygen, hydrogen, helium, carbon dioxide

compressable

gases are __________________ (can be compacted very tight)

expand

gases _____________ as temperature is increased

low resistance to flow

gases have much ______ _______________ ___ _______ (i.e. low viscosity)

-> means a material or system allows movement or passage (like electricity, water, or other fluids) with minimal obstruction or opposition.

lower densities

gases have much ____________ ________________ than liquids or solids (the measurement of how tightly a material is packed together)

miscible

gases mix evenly and completely (_______________)

- they can mix in all proportions to form a homogeneous mixture, due to the high kinetic energy and large distances between gas particles.

kinetic molecular theory

a scientific model for explaining the behavior of gases; remember that KINETIC means MOVEMENT!

- based on an "ideal gas," although in reality no gas is ever ideal, the theory is accurate enough to describe real gases

KMT has 3 points

1) gas molecules are in CONSTANT RANDOM MOTION

2) molecules of an ideal gas are POINT MASSES (has no volume/takes up no space)

3) gas molecules interact w/ one another and with the walls on a container only through ELASTIC COLLISIONS

gas molecules are in CONSTANT RANDOM MOTION

the molecules travel in straight lines until they collide with other gas particles or the walls of the container

gases are point masses

- a point mass is a mass that takes up no space (ie. it has no volume)

- while this is not actually possible, the gas molecules approximate point masses when the volume of the container is much larger than the volume of the gas itself

gas molecules interact through ELASTIC COLLISIONS

- gas molecules interact through ELASTIC COLLISIONS between one another and with the walls of a container

- the molecules do not exert attractive or repulsive forces on one another

- unlike ideal gas molecules, real gas molecules interact with each other through attractive forces, but these interactions are so small as to be negligible.

pressure

force per unit area

- kPa, atm, mmHg, torr, bar

pressure of gases

related to the number of forces of collisions of gas particles with each other and with the walls of the container

boyle's law

A principle that describes the relationship between the pressure and volume of a gas at constant temperature

- decreasing volume brings gas molecules closer together, increasing the number and force of collisions between gas molecules and walls of container --> increasing pressure

- thus, there is an INVERSE RELATIONSHIP between the P and V.

"as the pressure on a gas increases, the volume of the gas decreases proportionally, provided that the temperature and chemical amount (#moles) of the gas remains constant."

charles' law

the law that states that for a fixed amount of gas at a constant pressure, the volume of the gas increases as the temperature of the gas increases and the volume of the gas decreases as the temperature of the gas decreases

• “as the temperature of a gas increases, the volume increases proportionally, provided that the pressure and chemical amount of substance remains constant.”

Charles’ law equation

V represents volume in litres (L)

T represents Kelvin temperature

K represents a constant of proportionality

Then V/T=K

V1/T1 = V2/T2

or

V1T2=V2T1

Boyle’s law equation

V represent volume in litres (L)

P represent pressure (as long as you are consistent it does not matter which units you use for pressure)

k represent a constant of proportionality

Then, PV = k

P1V1 = P2V2

1 = initial 2 = final

combined gas law

• combination of BOYLE'S LAW and CHARLES’ LAW

• “for any fixed chemical amount of gas, the product of the pressure and volume of a gas sample is proportional to its absolute temperature in Kelvin.”

• when using this law, the chemical amount of gas remains constant

combined gas law equation

• V represents volume in litres (L)

• P represents pressure (as long as you are consistent, it does not matter which unit you use for pressure)

• T represent temperature in Kelvin (K)

P1V1/T1 = P2V2/T2

law of combining volumes

• “at the same temperature and pressure, gas reactants and products in a reaction are in SIMPLE RATIOS of WHOLE NUMBERS”

• the relationship between VOLUME ratios and COEFFICIENT ratios

law of combining volumes example

N2(g) + 3H2(g) —> 2NH3(g)

coefficient ratio: 1N2 : 3H2 : 2NH3

mole ratio: 1mol N2 : 3mol. H2 : 2mol NH3

volume ratio: 1L N2 : 3L H2 : 2L NH3

molar volume of gases

• molar volume is the volume that one mole of gas occupies at a specified temperature and pressure

• molar volume is the SAME for all gases at the same temperature and pressure

• SATP: molar volume of a gas is 24.8L/mol

• STP: molar volume of a gas is 22.4L/mol

molar volume of gases formula

• n = moles

• v = volume (L)

• V = molar volume (L/mol)

n = v/V

• as we only know molar volume when gases are at STP or SATP, it is only in these conditions that we can use this formula

avogadro’s law

“equal volumes of gases at the same temperature and pressure contain equal numbers of molecules”

ideal gas law

combining boyle’s law, charles’ law, and avogadro’s theory provides us with

• gas particles have negligible volume, no intermolecular forces, and experience perfectly elastic collisions

• the pressure (P) and volume (V) of a gas are directly proportional to the number of moles (n) and the absolute temperature (T), with the proportionality constant being the ideal gas constant (R). 

PV=nRT

ideal gas law equation

• V = volume in litres (L)

• P = pressure in kilopascals (kPa)

• T = temperature in kelvin (K)

• R = universal gas constant (8.31kPaL/Kmol)

PV = nRT

low temperature effects

• on real gases…

• as temperature decreases, molecules of a gas are going to slow down

• due to the reduced speed, the molecules are unable to overcome the attractive forces between them (IMF)

• attractive forces will cause the molecules to condense into a liquid

  • molecules with stronger attractive forces will CONDENSE at a HIGHER TEMPERATURE (ie. higher boiling point)

• gases condense at high pressures or low temperatures

high pressure effects

• on real gases…

• under atmospheric pressure, gas molecules are far enough apart that interactions between them are not frequent enough to cause gases to behave non-ideally

• as pressure increases, the molecules are pushed closer together, increasing the frequency of their interactions, exposing them to attractive forces

• the molecules will then be pulled away from the walls of the container, essentially reducing their volume (ie. the space they are taking up inside the container)