Unit 5 Gases

properties of gas

1. they may be compressed (volume decreases)

2. they will expand (their volume) to uniformly fill their containers

3. they all have low densities

4. gases homogeneously mix (in a fixed volume)

5. they exert uniform pressure on containter

homogenous

all compounds are in the same “phase” and are all dissolved together

heterogenous mixture

compounds in more than one phase

gases explained by the ideal gas model

gas atoms/molecules behave as if they are independent particles, there are no forces between them, because the molecules are so far apart they don’t interact much with each other

pressure of gases vs liquids

they exert uniform pressure on any point of the walls of a container, whereas liquids exert variable pressure, usually depending on the depth of the liquid

pressure measured with what and how

a barometer and air pushes down on a dish filled with mercury and pushes some of the mercury up a tube

atmospheric pressure

the same way that the weight of the atmosphere weighs down on the barometer, air also weighs down on you

definition of pressure

Pressure(P)=Force(F)/area(A)=F/A=ma/a

si units of pressure

pascals (Pa=N/m²)

atm to torr to Pa

1 atm =14.696 PSI 760 torr = 101,325 Pa

Boyles law

describes inverse relationship between pressure and volume at constant temperature

P1V1=P2V2

Charles law

demonstrates that volume and temperature are directly proportional to each other at constant pressure

V1/T1=V2/T2

gay Lussac’s law

demonstrates that pressure and temperature are directly proportional to each other at a fixed volume

P1/T1=P2/T2

how was 0K (absolute zero) determined

experiments like this: a plot of T vs. V at constant pressure show that at 0K the volume in a constant P vessel goes to zero at 0K regardless of P

combined gas law

for a constant amount of gas volume is directly proportional to temp and inversely proportional to pressure

(P1V1)/T1=(P2V2)/T2

standard temp and pressure

stp 273.15K and 1atm

law of combining volumes

discovered by gay-lussac that at constant T and P, gases react (products form) in small whole number V ratios. these volume ratios correspond to the molar ratios

avagadros law

states that the volume of gas is directly proportional to the number of moles of the gas at constant temp and pressure. In other words more gas molecules will take up more volume at constant temp and pressure

k=V/n or V1/n1=V2/n2

ideal gas law

PV=nRT and temp must be in K

what does n=

n=m/mm

density of water

1.00 g/mL (=g/cm³)

density of gases

usually reported in g/L (=g/dm³)

gas density equation

D=MM(P/RT)

3 features of gas density

1. at constant temp: Density is proportionally related to Pressure (P inc so does D)

   1. because P compresses gas into smaller V without changing its mass (m)

2. at constant P: D is inversely related to Temp (T inc and D dec)

   1. because T expands gas into larger V without changing its mass (m)

3. At constant temp and pressure: density is directly proportional to MM (MM in and D inc)

   1. because equal moles of gas occupy equal volumes at constant T and P (so m and D increase with MM)

effects of gas density

increasing gas density will increase the frequency that molecules collide (with the container walls and each other). at high densities molecules will travel less far between collisions

molar volume

volume per 1 mole

MV=V/n=RT/P

any MV at stp =22.4 L/mol

daltons law of pressures

Ptotal=P1+P2+P3+…

Mole fractions

Mole fraction A =Xa=na/ntotal

Pa/Ptotal=na/ntotal=Xa

partial pressure equation from mole fraction

Pa=XaPtotal

kinetic molecular theory

the state of matter that a substance is in is related directly to the speed of molecular motion

5 assumptions of the ideal gas model

1. a gas consists of atoms or molecules that move randomly about with high velocities

2. the actual volume that gas atoms or molecules occupy is negligible compared to the space they take up

3. gas atoms/molecules behave as if they are independent molecules (no IMFs

4. gases consist of atoms/molecules freely (and constantly) moving in straight lines any collisions involving gas molecules are elastic

   1. the average kinetic energy of gas atoms or molecules is proportional to the temp (K)

what are the conditions of being an ideal gas

1. IMFs must be small-to-negligible compared to their kinetic energy

   1. the molecules must have negligible volume compared other the volume of the container

ideal gass effects: number of molecules

if # of moles in a fixed volume is doubled …

doubling number will double the number of collisions aka pressure will double

ideal gas effects: volume

when the volume of container is halved there will also be double number of collisions so P doubles

ideal gas effects on temp

doubling temp will increase the number of collisions and force of collisions to P doubles

average kinetic energy per mole equation

EKE (mole)=3/2 (RT)

equation of speed of a molecule

V=Sq root ((3RT)/MM) T is in K and MM is in Kg

Van der Waals equation

(Preal+(n²a)/V²)x(V-nb)=nRT