2asc0302 w2- gas - liquid mzd 240912 edit.ppt

7/27/2019 2ASC0302 W2- Gas - Liquid MZD 240912 edit.ppt

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1

Gases: Learning Outcomes

Define Daltons law.

Calculate partial pressure of a gas and its

composition based on Daltons law

Explain phenomena of diffusion and effusion.

Use Grahams law to detemine molecular mass of a

gas

Explain Boyles, Charles and Gay Lussacs laws

for ideal gases using the kinetic-molecular theory.

Explain the limitation of ideality at high pressuresand low temp.


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We now need to consider mixtures of gases

One useful way to describe a composition of a mixtureis in terms of its mole fract ions

The mole fraction is the ratio of the number of moles of

a given component to the total moles of all

componentsFor a mixture ofA , B, substances, the mole fraction

of substance i(Xi)is

Daltons Law of Partial Pressures

T

AA

i

ZBA

AA

n

nX

innnn

nX

ofmoles,...


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3

Daltons law of partial pressures states:The total partial pressure of a mixture of gases isthe sum of their individual partial pressures


...1

........

BA

BTotalATotal

BATotal

XX

XPXPPPP

Partial pressure, Px

Px= PTotal( nx/ nTotal)


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Example: A balloon contains 0.1 moles of oxygen and

0.4 moles of nitrogen. If the balloon is at standard

temperature and pressure, what is the partial pressureof the nitrogen?

nTotal= noxygen+ nnitrogenn

Total

= 0.1 mol + 0.4 molnTotal= 0.5 mol

Ptotal = 1 atm

Pnitrogen = PTotal ( nnitrogen / nTotal )

Pnitrogen = 1 atm ( 0.4 mol / 0.5 mol )

Pnitrogen = 0.8 atm


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Partial pressures can be used to calculate molefractions.

This is possible because the number ofmolesof eachgas is directly proportional to its partial pressure

Using the ideal gas equation foreach gas

For a given mixture of gases, the volume andtemperature is the same for all gases

RT

VPn AA



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Gases are often collected over water in the

laboratory

These (collected) gases are saturated with water

The space above any liquid contains some of theliquids vapour

The pressure this vapor exerts is called the

vapour pressure



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As the gas bubbles through the water, water vapor

gets into the gas so the total pressure inside the bottle

includes the partial pressure of the water vapour.


Gases are often collected over water in the laboratory


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The total pressure is the pressure of the gas plus

the vapor pressure of water

aporwatertotalgas

aporwatergastotal

PPP

PPP

v

v or



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Vapor pressure of water at various temperatures.

760.0100

55.3240

31.8230

24.025

17.5420

4.5790

(torr)PressureVaporC)(eTemperaturo



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Diffusion is the spontaneous intermingling of themolecules of one gas with another

Effusion is the movement of gas molecules

through a tiny hole into a vacuum

Diffusion and Effusion of Gases

The rates of both diffusion and effusion depend on

the speed of the gas molecules

The faster the molecules, the faster diffusion andeffusion occur


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13(a) Diffusion (b) Effusion



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He found that the effusion rate of a gas was inversely

proportional to the square root of the density (d)

This is known as Grahams law

A

B

A

B

M

M

d

d

B

A

TP

)(rateeffusion

)(rateeffusion

)and(constantd

1rateeffusion


Thomas Graham studied the effusion of gases

(From Ideal Gas Law: d= PM/RT)

where Miis the molar mass of species i


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Diffusion of gases

Gaseous NH3 & HCl vaporize from cotton plugs at

ends of tube, diffuse & meet to form a white ionicsolid NH4Cl which serves as a time marker for the

relative diffusion rates of the gases.

Rate of diffusion for NH3 = Distance traveled by NH3/Time

required for ring formation

Rate of diffusion for HCl = Distance traveled by HCl/Time

required for ring formation

Ratio of rate of diffusion = Rate of diffusion for NH3/ Rate

of diffusion for HCl


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This apparatus consists of aglass tube sealed at one end

with plaster that has holes

large enough to allow a gas to

enter or leave the tube.When the tube is filled with H2

gas, the level of water in the

tube slowly rises because the

H2 molecules inside the tube

escape through the holes inthe plaster more rapidly than

the molecules in air can enter

the tube.

Diffusion of gases


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If the gas escapes from the tube faster than the

air enters the tube, the amount of water in the

tube will increase. If air enters the tube faster

than the gas escapes, water will be displaced

from the tube.

By studying the rate at which the water level inthis apparatus changed, Graham was able to

obtain data on the rate at which different gases

mixed with air.

Graham found that the rates at which gasesdiffuse is inversely proportional to the square

root of their densities.

Diffusion of gases


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Graham's law of effusion can be

demonstrated using this apparatus.

A thick-walled filter flask is

evacuated with a vacuum pump.

A syringe is filled with 25 mL of gas

and the time required for the gas toescape through the syringe needle

into the evacuated filter flask is

measured with a stop watch.

The experimental data in the tablebelow were obtained by using a

special needle with a very small

(0.015 cm) hole through which the

gas could escape.

Effusion of gases


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The rateat which the gases

effuse is therefore inversely

proportional to the square

root of the molar mass.

Effusion of Gases

Compound

Time(s)

MolarMass

H2 5.1 2.02

He 7.2 4.00

NH3 14.2 17.0

air 18.2 29.0

O2 19.2 32.0

CO2 22.5 44.0

SO2 27.4 64.1

The time required for 25-

mL samples of different

gases to escape through

a 0.015 cm hole into a

vacuum


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Boyle's Law:

Gases can be compressed because most of the

volume of a gas is empty space. If we compress a gas

without changing its temperature, the average kinetic

energy of the gas particles stays the same. There is no

change in the speed with which the particles move, butthe container is smaller. Thus, the particles travel from

one end of the container to the other in a shorter

period of time. This means that they hit the walls more

often. Any increase in the frequency of collisions withthe walls must lead to an increase in the pressure of

the gas. Thus, the pressure of a gas becomes larger as

the volume of the gas becomes smaller.

24

fixed)n&(TV

1P


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AC002-Gas Liquid Solid 1 25

Compressing a gas increases its pressure. A molecularview of what happens when a gas is squeezed into a

smaller volume. The number of collisions with a given

area of the walls increases which causes the pressure

to rise.

A molecular description of Boyles Law


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Charles' Law:

The average kinetic energy of the particles in a gas

is proportional to the temperature of the gas.

Because the mass of these particles is constant, the

particles must move faster as the gas becomeswarmer. If they move faster, the particles will exert a

greater force on the container each time they hit the

walls, which leads to an increase in the pressure of

the gas. If the walls of the container are flexible, it

will expand until the pressure of the gas once morebalances the pressure of the atmosphere. The

volume of the gas therefore becomes larger as the

temperature of the gas increases.

26

fixed]nand[PTV


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The last postulate of the kinetic molecular theory

states that the average kinetic energy of a gas

particle depends only on the temperature of the gas.

Thus, the average kinetic energy of the gas particles

increases as the gas becomes warmer. Because themass of these particles is constant, their kinetic

energy can only increase if the average velocity of

the particles increases. The faster these particles are

moving when they hit the wall, the greater the forcethey exert on the wall. Since the force per collision

becomes larger as the temperature increases, the

pressure of the gas must increase as well.

27

Gay-Lussacs Law: fixed]nand[VTP


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Avogadro's Hypothesis: As the number of gas particles increases, the frequency

of collisions with the walls of the container mustincrease. This, in turn, leads to an increase in the

pressure of the gas. Flexible containers, such as a

balloon, will expand until the pressure of the gas inside

the balloon once again balances the pressure of the gasoutside. Thus, the volume of the gas is proportional to

the number of gas particles.

28

)andconstant(at PTnV


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A plot ofPV/Tversus Pfor an ideal gas is a straight

line. The same plot for oxygen is not a straight line.

Deviation from Ideal Gas Law

J. D. van der Waals corrected the ideal gas equation

in a simple, but useful way.


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He did this by modifying the measured pressureand volume of a real gas so it fits the ideal gas

equation

The constants aand bare called the van der

Waals constants

valuegasidealtomeasuredreduces:

valuegasidealtoupmeasuredbrings:2

2

2

2

Vnb

PV

an

nRTnbVV

anP

measured

measured

measured

measured

Deviation from Ideal Gas Law


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0.030495.464OHWater,

0.037074.170NHAmmonia,

0.026610.02444HHydrogen,0.017090.2107NeNeon,

0.023700.03421HeHelium,

molL

molatmLSubstance

2

3

2

122 ba

Real Gases: Deviations from Ideality


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Liquids

Define viscosity and surface tension.

Explain viscosity, surface tension, capillary

action and wetting based on adhesive and

cohesive forces.

Learning outcomes:


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Properties of Liquid

38

Liquid retain their volume when placedinto a container, but conform to the

shape of the container.

They are fluid and are able to flow.

Liquid s are nearly incompressible.

Molecules tightly packed but with littleorder.

They are able to move past each other

with little difficulty.

Intermolecular attractive forces arerelatively strong.


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ViscosityProperties of Liquids

Viscosity is defined as the resistance in the flow ofa liquid.

It describes the internal friction of a moving fluid.

A liquid with high viscosity is thick and flows

slowly.A liquid with low viscosity is thin and flows quickly.

Liquids such as honey and ethylene glycol have

high viscosity while ethanol and water have low

resistance to flow.

Motor oils are more viscous than gasoline, for

example, and the maple syrup used on pancakes is

more viscous than the vegetable oils used in salad

dressings.


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Viscosity

40

Properties of Liquids

Viscosity depends on

intermolecular attractions

and molecular shape

Viscosity is measured by

determining the rate at

which a liquid flows through

a small-diameter glass tube.

viscometer


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Viscosity of Water at Several Temperatures

Temperature(0C) Viscosity (N*s/m2)*

20

40

60

80

1.00x10-3

0.65x10-3

0.47x10-3

0.35x10-3

*The units of viscosity are newton-seconds per

square meter.


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Properties of LiquidsSurface Tension

There is a force of attraction

between molecules in liquids.

Below the surface of the liquid,

the force of cohesion (literally,"sticking together") between

molecules is the same in all

directions.

There is also a force ofadhesion (literally, "sticking")

between a liquid and the walls

of the container.


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Surface Tension

The surface tension of a liquidresults from an imbalance of

intermolecular attractive

forces, the cohesive forces

between molecules:

A molecule in the bulk liquid

experiences cohesive forces

with other molecules in all

directions.

A molecule at the surface of aliquid experiences only net

inward cohesive forces.


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Molecules at the surface have higher potential

energy than those in the bulk of the liquid

The surface tension of a liquid is proportional to

the energy needed to expand its surface area

In general, liquids with strong intermolecular

attractions have large surface tensions

Surface Tension


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Surface Tension and Forces Between Particles

Substance Formula

Surface Tension

(J/m2) at 200C Major Force(s)

diethyl ether

ethanol

butanol

water

mercury

dipole-dipole;

dispersion

H bonding

H bonding;

dispersion

H bonding

metallic bonding

1.7x10-2

2.3x10-2

2.5x10-2

7.3x10-2

48x10-2

CH3CH2OCH2CH3

CH3CH2OH

CH3CH2CH2CH2OH

H2O

Hg

Surface Tension


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Capillary action is the ability of aliquid to rise (or fall) in a glass

tube immersed in the liquid.

This occurs due to the adhesive

forces exceed cohesive forces.

The liquid creeps up the inside ofthe tube (as a result of adhesive

forces between the liquid and the

inner walls of the tube) until the

adhesive and cohesive forces ofthe liquid are balanced by the

weight of the liquid.

The smaller the diameter of the

tube, the higher the liquid rises.

Capillary Action




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Wetting is the spreading of a liquid across a surface to

form a thin filmWhen the force of adhesion is more than half as large

as the force of cohesion between the liquid molecules,

the liquid is said to "wet" the solid.

The force of adhesion between water and wax is verysmall compared to the force of cohesion between water

molecules. As a result, rain doesn't adhere to wax. It

tends to form beads, or drops, with the smallest

possible surface area, thereby maximizing the force of

cohesion between the water molecules.

The same thing happens when mercury is spilled on

glass or poured into a narrow glass tube.

Properties of LiquidWetting


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Surfactants

48

Surfactants are compounds that lower the surface

tension of a liquid and lowering of the interfacialtension between two liquids, or between a liquid and

a solid.

Surfactants are usually organic compound thatAre amphiphilic, meaning they contain both

hydrophobic groups (theirtai ls) and hydrophilic

groups (theirheads).

Therefore, a surfactant molecule contains both a

water insoluble (or oil soluble component) and a

water soluble component.

2asc0302 w2- gas - liquid mzd 240912 edit.ppt

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