Chapter 10- Gases

Chapter 10- GasesWhat are the characteristics of gases?Variable shapeVariable volume

The atmosphere is composed of gases. The two major components are nitrogen(78%) and oxygen(21%).

Kinetic Molecular TheoryA theory that explains the behavior of gases at the molecular levelBased on the Ideal Gasa gas for which the product of the pressure and volume is proportional to the absolute temperatureIf a gas is moving it has energyKinetic EnergyKE=1/2mv2

Four Variables describe a Gas

1. TemperatureBased on absolute temperatureKelvin

How do we convert to Kelvin?

K=°C + 273

Cont.2. VolumeV= l•w•hCommon units are L, ml, or cm3

3. Pressure- force exerted over an areaP=Force/areaForce is measured in Newtons(N)Area is measured m2 or other similar units

Pressure cont.How is pressure measured?Barometer- device used to measure atmospheric pressure

What are the units of Pressure?A. Millimeters of mercury(mm Hg)

1mm of Hg= 1 torrAtmospheric pressure at sea level at 0° C is 760 mm

Pressure cont.B. Atmosphere of pressure(atm)

1 atm= 760 mm Hg

C. Pascal(Pa) – the pressure exerted by a force of one newton acting on an area of one square meter.1atm= 1.01325 x 105 Pa or 101.325 kPa

Standard Temperatureand Pressure

To compare volumes of gases, it is necessary to know the temperature and pressure at which the volumes are measured.The standard conditions are called Standard Temperature and Pressure(STP)STP= 0°C and 1.0000atm

4. Number of moles(n)The volume of one mole of gas at STP is called Molar Volume.

The Gas Laws1. Charles’s Law-the volume of a gas at constant pressure is directly proportional to the temperature.

V1/T1=V2/T2

Example of Charles’s LawA sample of gas occupies 24 m3 at 100. K. What volume would the gas occupy at 400.K?What do you know?V1=24 m3

T1=100. K

T2= 400. K

V2= ?

Boyle’s LawThe volume of a gas at constant temperature is inversely proportional to the pressure.

P1V1=P2V2

Example of Boyle’s LawThe gas in a balloon has a volume of 4 L at 100 kPa. The balloon is released into the atmosphere, and the gas in it expands to a volume of 8 L. What is the pressure on the balloon at the new volume?What do you know?V1= 4 L P1= 100 kPa

V2= 8 L P2 = ?

Gay-Lussac’s LawThe pressure of a fixed mass of gas at constant volume varies directly with the Kelvin temperature.P1/T1=P2/T2

The gas in a container is at a pressure of 3.00 atm at 25°C. Directions on the container warn the user not to keep it in a place where the temp. exceeds 52°C. What would the gas pressure in the container be at 52°C?

Dalton’s Law of Partial Pressures

For a mixture of gases in a container the total pressure exerted is the sum of the pressures that each gas would exert if it were alone.

PTotal = P1 + P2 + P3 + …

ExampleHydrogen gas is collected over water at a total pressure of 95.0 kPa. The volume of hydrogen collected is 28 ml at 25° C. If the pressure of water vapor at 25° C is 3.17 kPa, what is the partial pressure of hydrogen gas?

Graham’s LawDiffusion- movement of particles from regions

of high density to regions of low densityExample: smelling food throughout the houseEffusion- passage of a gas under pressure

through a tiny openingExample: a helium balloon is buoyant at first

but later hangs toward the ground because helium has escaped through holes in the balloon

Con.Graham’s Law- the ratio of the rates of effusion(diffusion) of two gases is equal to the square root of the inverse of their molar masses or densitiesRate1/Rate2= M2/M1

ExampleAn oxygen molecule travels at about 480 m/s at room temperature. How fast would a molecule of sulfur trioxide travel at the same temperature?

Combined Gas LawExpresses the relationship between pressure, volume, and temperature of a fixed amount of gas.

P1V1 = P2V2

T1 T2

Example of Combined Gas Law

A helium balloon with a volume of 410. mL is cooled from 27° C to -27° C. The pressure on the gas is reduced from 110. kPa to 25 kPa. What is the volume of the gas at the lower temperature and pressure?What do you know?V1= 410. mL V2= ? P1= 110. kPa P2= 25. kPa

T1= 27°C= 300 K T2= -27°= 246 K

Ideal Gas LawIs the mathematical relationship among pressure, volume, temperature, and the number of moles of a gas.

PV= nRTP = pressureV=Volumen= number of molesT= temperatureR=8.314 L•kPa mol•K

Example of Ideal Gas LawA sample of carbon dioxide with a mass of .250 g was placed in a 350 ml container at 400. K. What is the pressure exerted by the gas?What do you know?V= 350 mlT= 400. KM= .250 g of carbon dioxideP= ?

Ideal gas and Stoichiometry

How many liters of hydrogen gas will be produced at 280. K and 96.0 kPa if 40.0 g of sodium react with excess hydrochloric acid according to the following equation?

2Na(s) + 2HCl(aq) 2NaCl(aq) + H2(g)

What do we know?P= 96.0 kPaR= 8.314 L•kPa/mol•KT= 280. KN= calculate with molar mass and mole ratioV=?