GasesThe kinetic-molecular theory of gases:

• A gas consists of particles, atoms, or molecules that move randomly and rapidly.

• The size of gas particles is small compared to the space between the particles.

• Gas particles exert no attractive forces on each other.• The kinetic energy of gas particles increases with increasing temperature.• Gas particles collide with each other, rebound and travel in new directions.

Gas Pressure• When gas particles collide with the container, they exert pressure (F/A) on the

sides. The pressure exerted by the gas incr. with incr. in temp.• Gases in the atmosphere exert a pressure called the atmospheric pressure, which

decrease with increase in altitude.

• Measured by a barometer

• Millimeters of Mercury ( mm Hg) used in a barometer (1 atm. = 760 mm Hg, at

sea level). One millimeter of mercury is also called one torr.

1 atm = 760 mm Hg = 760 torr= 1.013 x 105 Pa

= 101.3 KpaNB: STP (Standard Temperature and Pressure) refer a pressure of 1 atm. and temperature of 273 K (0oC).

GAS LAWSFour variables define the behaviour of gases: pressure (P), volume (V), temperature (T) and moles (n).

1. BOYLE’s LAW: relates volume and pressure of a gas.

• The volume occupied by a fixed amount of gas is inversely proportional to the pressure, at constant temperature.

• P1 α 1/V1

• The product of the two quantities is a constant

• P x V = K

• P1V1 = K, P2V2 = K

• At constant temp. P1V1 = P2V2

• 2. CHARLES’ LAW : (relates volume and temperature of a gas)

• The volume occupied by a fixed amount of gas is directly proportional to its

temperature at constant pressure.

• V1 α T1, V1/T1 = k

• Similarly

• V2 α T2, V2/T2 = k

• V1/T1 = V2/T2 (at constant pressure)

• 3. GAY-LUSSAC’S LAW: (relates temperature and pressure of a gas)

• The temperature of a fixed amount of gas is directly proportional to the pressure

exerted by a gas, at constant volume.

• P1 α T1

• P1/T1 = k and (P2/T2= = k)

• P1/T1 = P2/T2, ( at constant volume)

• 4. COMBINED GAS LAW.

P1V1/T1 = P2V2/T2

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Gas LawsBoyle’s Law

Example

Example

If a 4.0-L container of helium gas has a pressure of 10.0 atm, what pressure doesthe gas exert if the volume is increasedto 6.0 L?

Step [1]

Step [1]

Identify the known quantities and the desired quantity.

P1 = 10.0 atmV1 = 4.0 L V2 = 6.0 L

known quantities

P2 = ?

desired quantity

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Gas LawsBoyle’s Law

Step [2]

Step [2]

Write the equation and rearrange it to isolatethe desired quantity on one side.

P1V1 = P2V2

Solve for P2 by dividingboth sides by V2.

P1V1

V2

= P2

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Gas LawsBoyle’s Law

Step [3]

Step [3] Solve the problem.

P1V1

V2

P2 = =(10.0 atm)(4.0 L)

(6.0 L)

Liters cancel.

= 6.7 atm

Answer

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Avogadro’s LawHow Volume and Moles are Related

Avogadro’s law: When the pressure and temperature are held constant, the volume of a gas is proportionalto the number of moles present.

• If one quantity increases, the other increases as well.

Volume

Number of moles= constant

V

n= k

• Dividing the volume by the number of moles is a constant, k.

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Avogadro’s Law• If the number of moles of gas in a cylinder is increased, the volume of the cylinder will increase as well.

V1

n1

=V2

n2

• This behavior can be explained by the equation:

initial conditions new conditions

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Avogadro’s Law

• Often, amounts of gas are compared at a set of standard conditions of temperature and pressure, abbreviated as STP.

• STP conditions are:

1 atm (760 mm Hg) for pressure

• At STP, 1 mole of any gas has a volume of 22.4 L.

• 22.4 L is called the standard molar volume.

273 K (0 oC) for temperature

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The Ideal Gas Law

All four properties of gases (i.e., P, V, n, and T) can be combined into a single equation called the ideal gas law.

PV = nRT

• R is the universal gas constant:

For atm: R = 0.0821 L • atmmol • K

For mm Hg: R = 62.4 L • mm Hgmol • K

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Dalton’s Law and Partial Pressures

Dalton’s law: The total pressure (Ptotal) of a gas mixture is the sum of the partial pressures of its component gases.

• For a mixture of three gases A, B, and C:

Ptotal = PA + PB + PC

partial pressures of A, B, and C

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Dalton’s Law and Partial Pressures

Sample Problem 6.9

Sample Problem 6.9

A sample of exhaled air contains four gases withthe following partial pressures: N2 (563 mm Hg),O2 (118 mm Hg), CO2 (30. mm Hg), and H2O (50. mm Hg). What is the total pressure of the sample?

Ptotal = PN2 + PO2 + PCO2

+ PH2O

Ptotal = 563 + 118 + 30. + 50.

Ptotal = 761 mm Hg

Answer

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The Ozone Layer

• Ozone (O3) is a gas formed in the upper atmosphere by the reaction of oxygen molecules (O2) with oxygen atoms (O).

• Ozone acts as a shield that protects the earth by absorbing destructive ultraviolet radiation.

• Chlorofluorocarbons (CFCs), once widely used as refrigerants and aerosol propellants, destroy ozone in the upper atmosphere.

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Carbon Dioxide and Global Warming

• CO2 is a greenhouse gas because it absorbs thermal energy that normally radiates from the earth’s surface. •Higher levels of CO2 may contribute to an increase in the average temperature of the earth’s atmosphere, termed global warming.