Gases for Students

8/4/2019 Gases for Students

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The Gaseous State of Matter


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General Properties

The least dense and most mobile state of

matter

Molecules move at very high velocities;

thus have high kinetic energy.

Mixtures are uniformly distributed within

the container they are confined


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Behavior of Gases

Any form of matter exists under suitable

conditions of temperature, pressure and

volume.

The effect of these three factors is quite

obvious in gases.


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Behavior of Gases

Under the same sets of conditions,

behavior of gases can be explained by the

Kinetic Molecular Theory (KMT).


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Kinetic Molecular Theory

Studies begun in the 17th century by Robert

Boyle


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All gases are made up of submicroscopic

(tiny), particles called molecules.

These molecules are very small in relation

to the distances between them.


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Gas molecules have no attraction for one

another.

These molecules are in constant random

motion causing them to collide with one

another and the walls of the container.


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The collisions of molecules are perfectly

elastic. No energy is lost by the collision of

gas molecules with another or with the

walls of the container.

The average KE of molecules varies

directly with the Kelvin temperature.


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The type of gas behaving according to the

kinetic molecular theory (KMT) is called

an ideal or perfect gas.


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No ideal gases exist, but under certain

conditions of P and T, real gases approach

ideal behavior, or at least show only small

deviations from it.


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Under extreme conditions, such as very

high P and low T, real gases deviate

greatly from ideal behavior.


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Note

All gases have the same KE at the same

temperature, thus; lighter molecules have

greater velocities than the heavier ones.


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Diffusion

The ability of two or more gases to mix

spontaneously until they form a uniform

mixture.

Because of the molecular motion of gases.


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Effusion

The process by which gas molecules pass

through a very small orifice from a

container at higher pressure to one at

lower pressure.


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Grahams Law of Effusion

Thomas Graham was able to observed the

rate of effusion was dependent on the

density of a gas.


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Grahams Law of Effusion

r

r

MW

MW

1

2

2

1 1


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Pressure

Pressure is defined as a force that is

exerted per unit of area.

This is obtained by dividing the force by

the area on which the force acts.


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Pressure

The atmosphere exerts pressure on the

earth known as atmospheric pressure.

The atmospheric pressure is

approximately 1 atmosphere, but varies

with elevation or altitude.


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Conversion of Pressure

1 atm = 760 mm Hg 1 Torr = 1 mm Hg

= 76 cm Hg 1 Pa = 1 N/m

= 14.7 lbs/in2

= 760 Torr

= 101,325 Pa= 101.325 KPa


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Temperature

Temperature is a measure of the degree of

hotness or coldness of a body.

To measure, temperature, thermometers

are used.

Three temperature scales are currently in

use.


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Temperature


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Temperature

Molecules of matter when heated move

faster but when cooled, the particles move

slower and slower until, theoretically all

motion stops.

The lowest possible hypothetical T at

which V is zero is -273.15C (all molecules

will stop moving.)


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Equations for Conversion

K = C + 273.15

100

FF - 32 or

5

9 FF - 32

CC

CC

180

F F100

+ 32 or F 9 F

5+ 32

180

CC

CC


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Volume

Volume maybe described as the space

that a specific substance occupies.

Several units of volume maybe used such

as liter (L), milliliters (mL), cubic

centimeter (cm), cubic meter (m), etc.


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Properties of Gases

Gases assume the volume and shape oftheir containers.

Gases have low densities.

Gases are able to exert pressure.

Gases diffuse rapidly.

Gases expand when heated.

Gases are the most compressible of thestates of matter.


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Boyles Law

The volume of a fixed amount of gas is

inversely proportional to its pressure when

its temperature is kept constant.

The Pressure of the gas Varies Inversely

with Volume at Constant Temperature.

Robert Boyle


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In Equation

V1

P


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Boyles Law

The heart of Boyles Law is the experimental factthat the product of pressure and volume isconstant.

This can be expressed in the following form,which is easier to use in calculations. Thesubscripts 1 and 2 rearranging,

P1V1 = P2V2


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Charles Law

The volume of a given amount of gas heldat constant pressure is directlyproportional to the absolute (Kelvin)

temperature. The Absolute Temperature (Kelvin) of the

gas varies proportionally with volume atconstant pressure.

Jacques Charles


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In Equation

V T

VT = VT 1

1

2

2


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Gay Lussacs Law

The pressure of a fixed amount of gas is

directly relationship proportional to the

absolute temperature (Kelvin) only when

its volume is held constant.

French scientists Jacques Charles and

Joseph Gay Lussac,


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In Equation

P T

P

T =

P

T 1

1

2

2


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STP Condition

Under STP,

pressure is fixed at 1 atm

temperature at 0 0C or 273.15 K

volume of 1 mole of a gas is constant at 22.4 L

called the standard molar volume.


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Combined Gas Law

The volume of a fixed amount of a gas is

directly proportional to the ratio of its

absolute temperature and its pressure.


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In Equation

VT

P

P V

T=

P V

T1 1

1

2 2

2


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Avogadros Principle

The volume of a gas is directly

proportional to the number of moles of the

gas present at constant pressure and

temperature

V n


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Summary of Gas Laws

Boyle's Law: V 1/P (constant n & T)

Charles' Law: V T (constant n & P)

Avogadro's Law: V n (constant P & T)

V nTP


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Ideal Gas Equation

PV = nRT

V = R

nT

P


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Universal Gas Constant, R

R = 0.08205 atm-Lmol-K

= 62.36mmHg-L

mol-K

= 62,358.97torr - mL

mol-K

82.05 atm-mLmol-K


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Gas Stoichiometry

Stoichometry problems involving moles

and masses were discussed in the previous

chapter.

There is also a relationship between

volumes and amounts of reactants and

products in a balanced chemical equation.

The flow chart suggests the basic tools for

solving problems involving gases.


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Mass-Volume Relationship

The balanced equation for the metabolicbreakdown of glucose (C6H12O6) is similaras that for the combustion of glucose in

air. This equation maybe written as:C6H12O6 (s) + 6O2(g) 6CO2 (g) + 6H2O(l)

Calculate the volume of CO2 produced at40C and 760 torr when 6.50 g of glucose isused up in the reaction.


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Volume-Volume Relationship

Ammonia gas, NH3, may be prepared bythe Haber Process. In this process for thesynthesis of ammonia,

N2(g) + 3H2(g) 2NH3(g)

how many liters of N2 are needed to reactcompletely with 50.0 L of H2, if thevolumes of both gases are measured atSTP?


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Air Pollution

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