kinetic theory & thermal properties notes igcse avg

8/18/2019 Kinetic Theory & Thermal Properties Notes IGCSE AVG

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Matter: Kinetic Molecular Model, andThermal Properties

In these notes: bullet points are from the Cambridge IGCSE syllabus. Text in italics issupplementary (extended).

1. Solids, Liquids and Gases: Properties and Particles

• State the distinguishingproperties of solids, liquidsand gases.

• Descrie qualitati!el" themolecular structure ofsolids, liquids and gases.

• Relate the properties ofsolids, liquids and gases to

the forces and distancesbetween molecules and tothe motion of the molecules.

State DistinguishingPropert"

Explanation in terms of particles

Solid(Atomsmole!ules held"ery !lose together bystrong attra!tions.

They stay in theirpositions# but "ibrate $mo"e repeatedlyba!%&ards andfor&ards.)

'ixed shape. (Cantbe poured.)

The particles are strongly attractedand can’t change their positions.

Cannot be

!ompressed has axed "olume.

The particles are very close together

(and if you try to push them closer,they repel each other).

Liquid(Atomsmole!ules are"ery !lose together butthe attra!tions are&ea%er than in a solid.So they !an mo"e

around# !hangingpositions.*eighbouring parti!lesmay be temporarilyattra!ted# but thenbrea% apart again.)

Can be poured# or+o&.

The particles are not stronglyattracted, so they can move past eachother.

'ills the bottom of its!ontainer.

Gravity pulls the liquid down, and the particles can’t escape from each other

because they are attracted. But theattractions are not strong, so the

particles move around and the liquidows to !ll the bottom of thecontainer.

Cannot be!ompressed has axed "olume.

The particles are very close together(and if you try to push them closer,they repel each other).

Gas(Atomsmole!ules are

far apart# and there isno attra!tion bet&eenthem. They mo"e inrandom dire!tions# at

'ills its !ontainer(!hanges its shape

and "olume).

The particles are not attracted, so they do not stay together.

(This is beyond "G#$%, but in case you’re wondering& gas particles can'defy’ gravity because they have a lotof energy for their sie. i*e a fast+

, -r. A. George

#arning$ Students oftenthin% the parti!les aresigni!antly farther apart in aliuid than in a solid. This isnot true. If you dra& theparti!les in a liuid# youshould dra& them tou!hing.

The &orld /particles is used a lot belo&. It !anmean either atoms# or mole!ules (t&o or moreatoms bonded together). 'or example# a parti!leof iron is simply a single iron atom0 &hereas aparti!le of &ater is a mole!ule of 1

23 (t&o

hydrogen atoms and one oxygen atom# bondedtogether)# and a parti!le of su!rose is themole!ule C

,21

223

,,.


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high speed.) moving roc*et or a !rewor*, they areenergetic enough to overcomegravity.)

Can be !ompressed does not ha"e axed "olume.

The particles are far apart, so they canbe pushed closer together.

4o& density. ot many particles per unit volume, sohas a low mass per unit volume.

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%. &eat and Temperature

• 'nterpret the temperature of a gas in terms of the motion of itsmolecules.

'irstly# heat and temperature are not the same thing (although they are related).

&eat: a type of energy. 1eat !an be transferred from one ob5e!t or pla!e to another#and &e measure amounts of heat in 5oules. 6hen some heat enters an ob5e!t# itstemperature usually rises (but not al&ays $ see later).

Temperature: the temperature of an ob5e!t is related to the a"erage %ineti! energyof its parti!les. The faster its parti!les are mo"ing (either "ibrating or mo"ingaround)# the higher its temperature. (7eyond IGCSE: the temperature of an ob5e!t isdirectly proportional to the a"erage %ineti! energy of its parti!les.) 8sually if you gi"ean ob5e!t some more heat# that extra energy is shared bet&een the ob5e!ts parti!les.

The a"erage %ineti! energy (and the speed) of the parti!les in!reases. This means ithas a higher temperature than before.

(. Thermal )*pansion

• Descrie qualitati!el" the thermal e*pansion ofsolids, liquids and gases.

• 'dentif" and e*plain some of the e!er"da"applications and consequences of thermale*pansion.

• Explain in terms of motion and arrangement ofmolecules the relative order of magnitude of theexpansion of solids, liquids and gases.

Thermal expansion !an be explained by the %ineti! mole!ular model of matter.If the temperature of a substan!e rises# that means itsparti!les are mo"ing faster. This !auses !hanges &e !anmeasure and sometimes a!tually see:

Solid Liquid Gas The parti!les "ibrate fasterand farther. They pushea!h other farther apart#so the solid expands a

little.

The parti!les mo"earound faster. They hitea!h other &ith morefor!e# pushing ea!h other

farther apart. So theliuid expands a little.

The parti!les mo"e aroundfaster. They hit the insidesof their !ontainer harder andmore often. If the !ontainer

is able to be pushedout&ards by their in!reasedfor!e (e.g. a balloon# asyringe)# then the gas &illexpand. Gases expandmu!h more than solids andliuids.

-articles in a solid are very strongly attracted to their

-articles in a liquid arequite strongly attracted

There is no attractionbetween the particles to

9 -r. A. George

#arning$ Studentsoften &rite that &hen asubstan!e is heated# itsparti!les expand;. e. Thesubstan!e itself

expands# be!ause itsparti!les move fartherapart .


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neighbours this stopsthem moving much farther apart, so the e/pansion ina solid is usually small.

this stops the liquide/panding as much as agas, but allows it toe/pand more than asolid.

resist the e/pansion, sogases e/pand a lot whentheir temperature rises.

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Thermal expansion @ uses and ris%s:

+pplications

Thermometers: The liuid (mer!ury# or more !ommonly al!ohol) in a thermometerexpands &hen heated# and !an be used to sho& the temperature:

7imetalli! strips:-ierent metals expand by dierent amounts &hen heated. If strips of t&o dierentmetals are &eldedstu!% together# their dierent amounts of expansion !auses themto bend. This phenomenon !an be used for temperature measurement# or for !uttingo a !ir!uit &hen it gets too hot.

A bimetalli! strip thermometer (left). This strip is bent into a spiral. 6hen it expands#the spiral starts to open out# and the pointer mo"es round.

Dealing ith -iss

6hen large stru!tures li%e bridges expand in hot &eather# this !ould !ause seriousdanger and damage: the bridge !ould bend or !ra!%. The pi!tures belo& sho&/expansion 5oints in bridges: they gi"e room for the bridge to expand safely.

The same applies to rail&ay tra!%s. 3n the left is a tra!% &hi!hhas got hot# expanded and bu!%led (noti!e the derailed train inthe ba!%ground). It is important to in!lude small gaps in the

rails so this doesnt happen.

B -r. A. George


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/. )!aporation

• Descrie e!aporation in terms of the escape of more energetic moleculesfrom the surface of a liquid.

• -elate e!aporation to the consequent cooling.

• Demonstrate understanding of how temperature, surface area and airow over a surface inuence evaporation.

E"aporation !an be explained by the %ineti! mole!ular model of matter (matter ismade of parti!les).E"aporation is a !hange of state of a liuid into a gas at a temperature belo& theboiling point of the liuid. It happens at the liuid surfa!e# &here some of the faster@mo"ing parti!les es!ape. (In a liuid# there is an a"erage parti!le speed# but someparti!les &ill be faster than this and some slo&er.) This lo&ers the a"erage %ineti!energy of the parti!les in the liuid $ so the liuids temperature drops.

0actors which increase the rate of evaporation&

-r. A. George


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0. Melting and oiling

• Descrie melting and oiling in terms of energ" input ithout a changein temperature.

• Distinguish between boiling and evaporation.

• Descrie condensation and solidi2cation.

• State the meaning of melting point and oiling point.

• Use the terms latent heat of vaporisation and latent heat of fusion and

give a molecular interpretation of latent heat .

Changes of State

/States of matter are the physi!al states in &hi!h matter !an exist: solid# liuidand gas. e. 7ut &hen &ater gets !lose tofree>ing# its parti!les a!tually get farther apart. Thats be!ause in solid &ater (i!e)# the parti!les ttogether in a parti!ular arrangement &hi!h is

relati"ely spa!ed out (see diagrams). That means i!eis less dense than &ater# &hi!h has some important!onseuen!es for our planet

Boiling and %vaporation

Evaporation Boiling"nvolves liquid turning into gas. "nvolves liquid turning into gas.1appens at any temperature (aslong as the substance is aliquid).

2nly happens at one particular temperature(the boiling point).

2nly happens at the surface ofthe liquid.

1appens throughout the liquid. Bubblesappear and rise to the surface. Thesebubbles are !lled with the gas version ofthe liquid. (%.g. when water boils, the

H -r. A. George

S3L'D'4')S54-))6


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bubbles are full of water in the gas state.)3oes not need any heat to enter the liquid.

1eat has to enter the liquid from outside toma*e boiling happen.

$low. 0ast.

-r. A. George


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atent 1eat

4hen you boil a liquid, its temperature doesn’t go above the boiling point untilit has !nished boiling. 0or e/ample, while you are boiling water, thetemperature of the water stays at 5667# until all the water has boiled to watervapour.

This may seem strange& you are putting heat energy into the liquid, yet its temperature does not rise. 4here is the energy going8

The heat energy is used to brea* the intermolecular attractionsbetween the molecules of the liquid after the liquid has turned to gas, thisenergy is stored in the gas as potential energy. (The molecules nowhave the potential to come bac* together again to form a liquid. "f they do, the stored chemical energy will turn bac* into heat energy.)

The same thing happens when a solid melts. 0or e/ample, if you heat

ice so that it melts, the temperature of the ice9water mi/ture will stay at 67# until all the ice has melted.

This diagram shows the energy and temperature changes when a substance melts orboils. (%* means *inetic energy of the particles % p means potential energy of the

particles.)

Below is a heating curve for water a heating curve is a graph showing thetemperature of a substance plotted against the amount of energy it hasabsorbed. :ou may also see a cooling curve, which shows the temperaturewhen a substance cools down.

J -r. A. George


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7. eha!iour of Gases

• Descrie qualitati!el" the pressure of a gas in terms of the motion of its

molecules.• Descrie qualitati!el" the e8ect of a change of temperature on the

pressure of a gas at constant !olume.

• Descrie qualitati!el" the e8ect of a change of temperature on the

!olume of a gas at constant pressure.

• Relate the change in volume of a gas to change in pressure applied tothe gas at constant temperature and use the equation p ! constant atconstant temperature.

6hy do gases exert apressureK

6hat happens to the pressure&hen you in!rease the

temperature (but %eep the "olumethe same $ say# by trapping thegas inside a strong box)K

Cool gas 1ot gas:parti!les hitthe &allsharder# andmore often

6hat happens to the"olume &hen you

in!rease thetemperature (but %eepthe pressure the same $say# by %eeping thesame &eight on top ofthe gas)K

4o&er temp. 1ighertemp.:parti!lesmo"e faster#so hit the&alls harderand moreoften. If thegas !an# it&ill expand.

"f you *eep the temperature of a gas the same but change its pressure, the volumewill change. 2r if you change the volume, the pressure will change&

,D -r. A. George


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,, -r. A. George


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"n a gas at constant temperature, the pressure (p) is inversel" proportional to thevolume (;)& that means if the pressure increases the volume decreases, and if the

pressure decreases the volume increases. The relationship loo*s li*e this&

kinetic theory & thermal properties notes igcse avg

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