Chapter 10

Gases

GASESGASES

John Dalton

Characteristics, Pressure, Laws, and Ideal-Gas Equation

Sections 10.1-10.4

Objectives

• Compare distinguishing characteristics of gases with those of liquids and solids

• Study gas pressure and the units that express it

• Examine volume, pressure, and temperature and their relationship to gases.

• Use the ideal-gas equation to make calculations.

Characteristics of Gases

• Earth’s Atmosphere:– N2 (78%)

– O2 (21%)

– Other gases, ex: Argon (0.9%)

• Diatomic gases- halogens

• Monatomic gases- nobles

• Vapors

Gases vs. Solids & Liquids

• Gases, unlike solids and liquids,:– Expand to fill their containers’ volumes– Are highly compressible– Form homogeneous mixtures with each other

Pressure

P = F

A

• Gases exert pressure on any surface they contact

Atmospheric Pressure

• Gases experience gravitational acceleration BUT they have tiny masses

• So gravity operates on atmosphere as a whole to press down on Earth

– Atmospheric pressure

Magnitude of Atmospheric Pressure

F = ma

Force = mass x acceleration

a= 9.8 m/s2

SI Units

• Force: kg-m/s2, the newton (N)

• Pressure: N/m2, the pascal (Pa)

Barometer

• Early 17th century

• Evangelista Torricelli, student of Galileo

• Proved that atmosphere had weight

Height of Hg, h, measures atmospheric

pressure

Standard Atmospheric Pressure

• Pressure at sea level

• Supports a column of Hg 760mm high

– 1.01325 x 105 Pa

• Defines non-SI units of pressure

– Atmospheres (atm)

– Millimeters of mercury (mm Hg)

• a.k.a. torr (for Torricelli)

Pressure Conversions

1 atm = 760 mm Hg = 760 torr = 1.01325 x 105 Pa

Gas Laws

Pressure and Volume

(Boyle’s Law)

Temperature and Volume (Charles’ Law)

Quantity and Volume(Avogadro’s Law)

Boyle's Law

British chemist Robert Boyle (1627-1691)The pressure of a gas is inversely related to

the volume when T does not changePV product remains constant

P x V = k (constant) when T remains constant

Boyle’s Law and the Breathing

P and V Changes

P1= 8atm

P2= 4atm

V1= 2LV2 = 4L

Pressure/Volume Changes

P1V1 = P2V2

P1V1= 8.0 atm x 2.0 L = 16 atm L

P2V2= 4.0 atm x 4.0 L = 16 atm L

Charles’ Law

V = 125 mL V = 250 mL

T = 273 K T = 546 K

Observe the V and T of the balloons. How does volume change with temperature?

Charles’ Law

• French scientist, Jacques Charles (1746-1823)

• At constant pressure, the volume of a gas is

directly related to its absolute (K) temperature

V= constant x T

V1 = V2

T1 T2

Learning Check

Use Charles’ Law to complete the statements:

1. If final T is higher than initial T, final V

is (greater, or less) than the initial V.

2. If final V is less than initial V, final T is

(higher, or lower) than the initial T.

Solution

V1 = V2

T1 T2

1. If final T is higher than initial T, final V

is (greater) than the initial V.

2. If final V is less than initial V, final T is (lower) than the initial T.

Avogadro’s Law

• Joseph Louis Gay-Lussac (1778-1823)– At a given P and T, the V of gases react with one

another in a ratio of small whole numbers

• Amedeo Avogadro (1776-1856)– Hypothesis: Equal volumes of gases at the same T

and P contain equal numbers of moecules– Law: V of a gas maintained at constant T and P is

directly proportional to the number of moles of gas.

V= constant x n

Ideal Gas Law

The four variables in the gas laws all deal with proportionality:

Boyle’s: V 1/P (constant n, T)Charles’: V T (constant n, P)

Avogadro’s: V n (constant P, T)

They combine into a general gas law:V nT P

Ideal Gas Equation

If the proportionality constant is R, than:

PV = nRT

R = ideal gas constant

Units for Ideal-Gas Equation

• T must be in K• n is expressed in moles• P is usually atm• V is typically L

*

*

*

STP

• Standard Temperature and Pressure

• 0 ºC

• 1 atm

• V of 1 mole of ideal gas at STP = 22.41 L

Temperature Conversions

ºF

ºC

K

-459 32 212

-273 0 100

0 273 373

32FC 95 K = ºC + 273