Ch. 7 - Gases

pressure definition

force per unit area

pressure characteristics

• pressure is directly proportional to force

• pressure is inversely proportional to area

• collision of gas molecules is pressure

barometer

an instrument for measuring the pressure of a gas, especially the atmosphere

standard temp and pressure (STP)

• STP used to be defined as 0C and 1 atm pressure

• IUPAC redefined STP to be 0C and 100kPa pressure

• 100 kPa = 0.98692 atm = 1 bar

4 properties of gases

1. volume in L

2. amount in mol

3. temperature in K

4. pressure depends on given 

boyle’s law proportionality

pressure is inversely proportional to volume

charles law proportionality

volume is directly proportional to absolute temperature

avagadro’s law proportionality

volume is directly proportional to number of moles

dalton’s law of partial pressure

the total pressure of the mixture is equal to the sum of the individual pressures of the components of a gaseous mixture

dalton’s law of partial pressure formula

p_total = p1 + p2 + … + pn

• SF depend on d.p

mole fraction (X_i)

the number of moles of a component divided by the total moles in the mixture 

mole fraction (X_i) formula

X_i = ni/ntotal

molar mass (MM)

m/n

density of gas (g/L)

d = mass/volume

d = MM x (P/RT)

Gay-Lussac’s law of combining volumes

the volume ratio of the gases in a chemical reaction is equal to the mole ratio in the balanced equation if all gases are at the same temperature and pressure

• purpose: can do chemical switcharoo stoich with gases

kinetic molecular theory

theory that explains the gas laws (and other phenomena) in terms of the motions and characteristics of the molecules of a gas 

random motion

motion of molecules in random direction

• gas particles travel in straight line in any direction until they hit something

perfectly elastic collision

a collision that occurs with no loss of kinetic energy

• gas particles bounce off one another without any loss of energy

5 postulates of kinetic molecular theory 

1. gases are composes of small molecules that are in constant, random motion.

2. the volume that is taken up by the molecules themselves is insignificant compared with the overall volume occupied by the gas

3. forces between the molecules are negligible, except when the molecules collude with one another

4. molecular collisions are perfectly elastic; that is, no energy us lost when molecules collide

5. the average kinetic energy of the gas molecules is directly proportional to the absolute temperature of the gas

diffusion

the spreading out of gas particles by random motion and collusion to occupy an entire volume 

effusion

the movement of gas particles through a tiny opening without collisions

• a helium balloon slowly losing gas over time

graham’s law of effusion

rates of diffusion and effusion of gases are inversely proportional to the square roots of their molecular masses

usually gas B is the heavier gas