Gases

Kinetic Theory of Gases

A gas consists of small particles that

move rapidly in straight lines. have essentially no attractive

(or repulsive) forces. are very far apart.have very small volumes

compared to the volume of the container they occupy.

have kinetic energies that increase with an increase in temperature.

Properties That Describe a Gas

Gases are described in terms of four properties: pressure (P), volume(V), temperature(T), and amount(n).

Gas Pressure

Gas pressureis described as a force acting on a specific area.

Pressure (P) = Force AreaHas units of atm, mm Hg, torr, lb/in.2 and

kilopascals(kPa).

1 atm = 760 mm Hg (exact)1 atm = 760 torr

1 atm = 14.7 lb/in.2

1 atm = 101.325 kPa

1atm = 1013 millbars

Closed End Manometer Reading

Closed End ManometerDirect read instrumentHeight(h) is the mmHg pressure of the gas in the bulb (ALWAYS).

Open End Manometer Reading

Open End ManometerAtmosphere vs. gasHigher pressure is downDistance between Hg levelsis the difference between thethe pressure of the two sides

“There is a Battle going on”

Learning Check

A. What is 475 mm Hg expressed in atm?

1) 475 atm 2) 0.625 atm

3) 3.61 x 105 atm

475 mm Hg x 1 atm = 0.625 atm 760 mm Hg

Learning Check

B. The pressure in a tire is 2.00 atm. What is this pressure in mm Hg?

1) 2.00 mm Hg 2) 1520 mm Hg 3) 22 300 mm Hg

2.00 atm x 760 mm Hg = 1520 mm Hg 1 atm

Atmospheric Pressure

Atmospheric pressureis the pressure exerted by a column of air from the top of the atmosphere to the surface of the Earth.

Altitude and Atmospheric Pressure

Atmospheric pressure

is about 1 ATM (atmosphere) at sea level.

depends on the altitude and the weather.

is lower at high altitudes where the density of air is less.

is high for sunny and low for cloudy and rainy conditions

Barometer & Barometric pressure

A barometer

measures the pressure exerted by the gases in the atmosphere.

indicates atmospheric pressure as the height in mm of the mercury column.

Dalton’s Law

Pt = px + py + pz….

Pressure total is the sum of all of the individual gas pressures(partial pressure) in a container

The pressure of one gas does not affect the pressure of other gases.

Could be more than 3 gases.

Vapor Pressure of Water

Vapor pressure of water is the pressure of the gas molecules above the liquid surface at a given temperature.

Higher temperature means more molecules leave the surface and HIGHER pressure.

What is the Nitrogen pressure when caught over water at 22°C with a total pressure of .5Atm.

At 22°C, Vapor pressure (H2O) = 19.8mmHgPt = .5 Atm = 380 mmHg (Use DA)Using Dalton’s formula Pt = pN2 + pH20 380mmHg = pN2 + 19.8mmHgpN2 = 360.2mmHg

Pressure and Volume(Boyle’s Law)

Boyle’s Law

Boyle’s Law states that

the pressure of a gas is INDIRECTLY related to its volume when T and n are constant.

if volume decreases, the pressure increases

P1*V1=P2*V2

Learning Check

If a sample of helium gas has a volume of 120 mLand a pressure of 850 mm Hg, what is the newvolume if the pressure is changed to 425 mm Hg ?

1) 60 mL 2) 120 mL 3) 240 mL

Known: P1 = 850 mm Hg P2 = 425 mm Hg V1 = 120 mL V2 = ??

P1V1=P2V2

V2 = V1 x P1 = 120 mL x 850 mm Hg = 240 mL P2 425 mm Hg

Pressure ratioincreases volume

P or V changes if H2O is present

A canister contains Hydrogen gas that was generated over water. It is currently at 20L, 25˚C and 150mmHg. Find the ρH2, ρH2O & PT after the piston moves changing the volume to 5.5L

PT = ρH2 + ρH2O ρH2 =150mmHg-23.7mmHgρH2 =126.3mmHgP1*V1=P2*V2

= 459.3mmHgPT = 459.3 + 23.7 = 483mmHg

Multiple Cylinder Problem

Scenario:Bulb A: Ne, 3L, 3AtmBulb B: He, 7L, 2000mmHgBulb C: Ar, 10L, manometeris 200mm Bulb side down w/Barometric Pressure = 750mmHgValves between currently closed.

Find the , PT, and manometer read after the valves are opened.

3Atm = 2280mmHgP1*V1= P2*V2 (will be used for each gas)ρNe=2280mmHg* 3L=X* 20L=342mmHgρHe=2000mmHg* 7L=X* 20L=700mmHgρAr=950mmHg * 10L=X* 20L=475mmHgPT= ρNe + ρHe + ρAr = 1517 mmHg

Current manometer:There is a battle going on between 1517 and 750. 1517 wins the battle.

1517>750 bulb side down by 767 mmHg

Temperature and Volume (Charles’ Law)

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Charles’ Law

In Charles’ Law,the Kelvin

temperature scale MUST be used in Gas Las relationships

K = °C + 273P and n are constant.Direct relationship:

when the temperature of a gas increases, its volume increases.

V1/T1=V2/T2

Calculations: Charles’ Law

A balloon has a volume of 785 mL at 21°C. If the temperature drops to 0°C, what is the new volume of the balloon (P constant)?

Be sure to use the Kelvin (K) temperature ingas calculations.

1. Set up data table:Conditions 1 Conditions 2V1 = 785 mL V2 = ?T1 = 21°C = 294 K T2 = 0°C = 273 K

Calculations: Charles’ Law

2. Solve Charles’ law for V2:

V1 = V2

T1 T2

V2 = V1 x T2

T1

V2 = 785 mL x 273 K = 729 mL 294 K

Mole Fraction Formula

If 71 grams of Chlorine gas and 2 g of Hydrogen are found in a gas bulb with a pressure of 820 mmHg. Find each gas.

71 grams of Cl2 is 1 mole of Chlorine2 grams of H2 is 1 mole of HydrogenTotal moles = 2 moles TotalCl2 = (1m Cl2/2mTotal)(820mmHg) =410mmHgH2 = (1m H2 /2mTotal)(820mmHg) =410mmHg

Ideal Gas Law Calculations

Molar Volume of all gases(STP)=22.4L/mIf temperature varies from 0˚C or if the

pressure varies from 1 Atm then Molar Volume changes. This will find mass or volume when only one is given.

To find NEW MOLAR VOLUME use:

Then use Dimensional analysis to solve

Sample New Molar Volume

If 100 g of H2 are stored at 20Atm and 50˚C find the volume of the gas.

Use New Molar Volume Formula first

V2 = 1.33 liters/mole(New Molar Volume)

Then Dimensional Analysis = 66.5 liters H2

Molar Volume of Hydrogen Gas Lab

Mass of Mg Ribbon: 1.1g/meterLength of Mg Ribbon: 2cmBarometric Pressure: 30.27inchesTemperature: 23˚CVolume of wet H2 measured in tall cylinder

as shown below: 22.5ml Causes pressure equilization