AP Chem Unit 4(Gas Laws)

What are common characteristics of Gases?

• Expand to fit all containers

• Compressible

• Extremely low dense

• Volumes are proportional so can be used as mol values when comparing amounts

What is pressure, and its equation?

Amount of force applied to an area, equation is P=F/A

Atmospheric Pressure

The weight of air per unit of area

What is gas pressure caused by

Collisions of mcs with the side of container & other mcs

Standard pressure

Normal atmospheric pressure at sea level

Units of pressure & values, conversions to 1 atm

• 1.00 atm

• 760 Torr(760 mmHg)

• 101,325 Kpa

mmHg

The difference in the heights measured in mm(h) of two connected Hg columns

Barometer

Used to measure atmospheric pressure

How does air pressure effect levels of Hg supportred?

• Higher pressure results in higher Hg levels supported(warm and dry days)

• Lower air pressure results in lower levels of Hg supported(stormy, unstable air)

Manometer

Used to measure difference between atmospheric pressure and that of a gas in a vessel

Calculations for manometer

• If more gas in the tube - p_gas=p_atmosphere + ph

• if more mercury - p_gas = p_atmosphere = ph

Boyle’s law

The volume of a fixed quantity of gas at a constant temp is inversely proportional to pressure

• P1V1=P2V2

What does a graph of V and P look like, why

A curve, as they are inversely proportional

What does a graph of -V=K(1/p) look like?

A slope

Charles law

The volume of a fixed amount of gas at constant pressure is directly proportional to its absolute temp

• V1/T1 = V2/T2

• Plot would be a slope

Avogadro’s Law

• Vol of a gas at constant temp and pressure is directly proportional to the number of mols of gas

• V=Kn

Ideal-gas equation

The constant of proportionality is known as R, gas constant PV=nRT

What are requirements for STP?

• temp=0 C (273 K)

• 101325 Pa, 101.325 KPa

• 1.00 atm

• 760 mmHg

• 760 Torr

Molar Volume

All gases at STP occupy around 22.42 L/1 mol

How many mcs does 1 mole of gas equal? and how many mcs of gas at STP occupy 22.4 L

6.022×10²³ for both

Avogadro’s principal

Allows you to interrelate mass, volume, mols, pressure, and temp for any sample of gas

Dalton’s Law

The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone

• P_total = P₁ +P₂ +P₃ + …

What is the effect of collecting a gas over water

There is water vapor mixed with the gas

How to find partial pressure

Subtract the vapor pressure of H₂O(depends on temp only) from P_total

Mol fraction

(gas mol/total mol) x P_total

• can be in mols/liters

When mole fraction is combined with dalton’s law, what can you find

Pressure exerted by individual gases in a mixture

How does temperature affect density in gases of the same composition

Warmer gases are less dense than cooler gases

How does gas density and absolute temp relate in proportionality

gas density is inversely proportional to absolute temp

• IE Gases under high pressure are more dense than those under low pressure

Kinetic Molecular theory

• Combined volumes of the gas is negligible relative to the total volume where gas is contained

• Gases consist of large numbers of mcs that are in continuous, random motion

• Attractive and repulsive forces between gas molecules are negligible

• The average kinetic energy of the molecules is proportional to absolute temp

• Energy can be transferred between molecules during collisions, but the average KE of the molecules does not change as long as temp doesn’t change

Diffusion

The spread of one substance of throughout a space or throughout a 2nd substance

Effusion

The escape of gas molecules through a tiny hole into an evacuated space

Real gases

Only behave ideally under low pressure(less than or equal to 1 atm) and a high temp

Why is it an issue that real gases have volume

• vol of container is not availale to gas

• v-nb, adjustment to euquation

What forces do real gases exert on each other

attractive forces

• nb represents volume of molecules

• These attractive forces would decrease the pressure that would be expected for an ideal gas

What does (n/v)² represent in van der waals

concentration and number of possible collisions

What do gases need to be ideal

• A larger volume(lower pressure) = more ideal behavior( Vol of molecules becomes insignificant)

• Higher temp= more ideal behavior(faster so less attraction)

  • Therefore the most ideal conditions are high temp, low p, low mols

• Real gases would be when there is low temp, high pressure, and higher amount of mols