Chapter 3, Solid & Liquid

8/2/2019 Chapter 3, Solid & Liquid

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The Kinetic Theory for the Liquid

StateContinual heating: solid liquid (melting point)vapour/gas(boiling point)

The energy content of the particles increases in the

order: solid < liquid < gas. In the liquid state, molecules move in rapid &

random manner & have vibrational, rotational,translational motions

Molecules of liquid are held together so close & verylittle space between them.

Liquids are more difficult compress than gases.


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Molecules in liquid are held together by 1 or moreattractive forces.

The molecules of gas can move past one another,

so a liquid can flow, can be poured and assumesthe shape of its container.

1 mol of substance: volumesolid < volume liquid the rate of condensation Hence, the vapour pressure will increase with time.

When the rate of vaporisation = the rate of condensation &a dynamic equilibrium is established.

The concentration of the vapour is max. & the max. vapourpressure is exerted.

This max. vapour pressure is called the saturated vapourpressure (SVP) of the liquid at the said temperature.

The Saturated vapour pressure increases with temperature.


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The shaded areaunder the curveshows the fraction ofparticles that have

enough kinetic energy(Evaporisation) tovaporise.


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If the temperature of liquid is increased, moreparticles will possess energy higher thanEvaporisation & the rate of evaporation will increase.


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Boiling Point & Vapour Pressure

The vapour pressure of a liquidincreases with temperature(does not depend on theamount of the liquid present).

If a liquid is heated in an opencontainer, a point is reachedwhen the vapour pressure ofthe liquid = the atmosphericpressure is called as boilingpoint).

This process is called boiling.


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Example: water boils at 100oC at 101.3kPa (1 atm). If the external pressure isreduced, the boiling point is decreased.

The boiling point of a liquid at 1 atm (101.3kPa) is called the normal boiling point.


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Volatility & Vapour Pressure

When 2 liquids are compared(same temperature), the liquidwhich evaporates morereadily is described as being

more volatile. Eg. Volatility of water 95.5oC


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The Allotropes ofCarbon

Diamond Fullerene

Graphite


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Diamond Graphite

Face-centered cubic structure A layered lattice structure in thehexagonal crystalline system

Each C atom is joined to 4 othercarbon atoms by covalent bonds inatetrahedralarrangement

Each C atom is covalentlybonded to 3 other carbon atoms inthe same plane, forming layers ofhexagonal C atom rings.

E h b t i 3 E h b t i 2


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Each carbon atom is sp3hybridised; bond angle is 109.5o

Each carbon atom is sp2

hybridised; bond angle 120o

All the 4 valence electrons of the Catoms involved in the formation ofcovalent bonds making diamond anon-conductor of electricity.

The unhybridised p electrons forma delocalised electrons cloud enablegraphite to conduct electricity &giving it shiny appearance.

The 3-D network of covalent bondsbinds all the atoms into a giantmolecular structure. This reflected inthe great hardness & high meltingpoint (3823 K).

The weak van der Waals forcesbetween the layers enable them toslide over each other.

It is used as a lubricant. Strongcovalent bonds within each layerresulted in high melting point (3925K).

Density = 3.5 g cm

-3

Density = 2.3 g cm-3

Uses: Diamond-tipped tools areused for cutting & engraving;diamond has a high refractive indexmaking it a prized jewel.

Uses: Lubricants & as inertelectrodes in electrochemical &electrolysis cells


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Fullerenes

Fullerene (full name:

Buckminsterfullerene) with amolecular formula of C60, isthe third allotrope of carbon,discovered in 1985.

The C60 molecule has 32faces; 12 pentagonal faces &20 hexagonal faces.

Each C atom undergoes sp2hybridisation.

Fullerenes are used to makesuperconductors,semiconductors, lubricants,micro-ballbearings innanomachines & catalyst.

The Phase Diagram of a Pure


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The Phase Diagram of a PureSubstance

Most matters can exist in 3 physical phases ~ solid,liquid & gas, depend on temperature & pressure.

E g. At high T & low P, substance usually exist as gas.

Sometimes, an equilibrium exists between 2 phases ~liquid-vapour, solid-vapour & solid-liquid.

All this information can be summarised on a graphknown as a phase diagram or vapour pressure diagram.

The phase diagram enables us to predict the physicalstate of a substance under different temperature &pressure.


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The Phase Diagram of Water

The graph is divided into 3 areas. Each area representsa single phase (or physical state).

Each line indicates the conditions of temperature &pressure when 2 phases exist in dynamic equilibrium.


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The Curve TB

Is called the vapour pressure curve.

It shows the boiling point of water at differentpressure.

The normal b.p. of water is 100 oC at 1.0 atm &liquid & vapour exist together in dynamic

equilibrium.


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The Curve AT

Shows the saturated vapour pressure of ice (solid

water) at different temperature.Sublimation of the water molecule from the surface

of ice.

Known as sublimation curve.

Ice water vapour


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The Line CT

Is called the melting temperature curve.

Shows the melting point of ice (or the freezing pointof water) at different pressures.

The line is unusual because it slopes to the left.

This shows that as pressure increases, the melting

point of ice decreases


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The Curve OD This is a metastable condition, because water usually

does not exist as a liquid below its freezing point. This phenomenon is called supercooling.

Supercooling ~ the cooling of a liquid to below its freezingpoint without change from liquid to solid state.


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The Point O

Known as the triple point.

It marks the conditions of temperature & pressure(0.006 atm & 0.01 oC) when ice, liquid & water vapourare in dynamic equilibrium.

The Point B

Critical point for water which occurs at 218 atm (criticalpressure) & 374 oC (critical temperature).

Once the critical point is reached, there are no longer 2separate phases, because the densities of the gas &liquid are equal.

The liquid & the vapour become indistinguishable.


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The properties of water

Water has high specific heat.

Density of water solid (ice) is less than itsliquid form.

Because, volume of ice is greater thanwater liquid.


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The Unusual Behaviour of Water During the process of freezing, the volume of most

substances will decrease when the liquid changed intosolid.

The behaviour of water is unusual.

Ice crystals have an open structure due to hydrogen

bonding. When water freezes to form ice, its volume increases

slightly.

Conversely, when ice melts to form water, its volume

decreases slightly. Liquid phase is denser than solid phase.


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Effect of Pressure on Melting Point If volume of solid > liquid (same mass), the melting

point decreases with increases in pressure.Explaination:An increase in pressure will push the moleculescloser. Density liquid water > solid ice. Thus,increasing the pressure favours the formation ofliquid water.[For substance with the same mass, the smallerthe volume, the higher the density. An increase inpressure favours the higher density physical

state.]Since the latent heat of fusion is absorbed during

melting, the temperature of the system will fall &the melting temperature curve slopes to the left.

The effect of pressure on melting point can be


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The effect of pressure on melting point can bepredicted by using Le Chateliers principle.

When the pressure on a system in equilibrium is

increased, the equilibrium will shift in the directionto lower the pressure (to smaller volume).

Consider the equilibrium between water & ice:

Water Ice

When the ice melts, the volume of the system willdecrease & the pressure of system will be lowered.

The new equilibrium is established at a higherpressure & a lower temperature.

Volume increases

Volume decreases

The Phase Diagram of Carbon


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The Phase Diagram of Carbon

Dioxide

Basically, it same as the phasediagram of water. But, there are2 differences:

The triple point O is above the

atmospheric pressure (5.1 atm,-57 oC). Below triple point, solidCO2 will sublime to vapour.

The line OC slopes to the right

(typical behaviour of mostsubstances). It implies that anincrease in pressure willincrease the meltingpoint ofCO2.

Th ff f l i i d d h


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The effect of pressure on melting point curve depend on therelative densities between solid & liquid.

The density of solid CO2 > liquid CO2 because CO2

molecules are held together closely made the volumesmaller.

Solid carbon dioxide Liquid carbon dioxide

An increase in pressure will push the molecules closertogether. So, an increase in pressure favours the formationof solid CO2.

When the liquid becomes solid, the latent heat of fusion willbe liberated. The temperature of the equilibrium system will

increase. So, when the external pressure is increased, the melting

point will also increase.

Volume increases

Volume decreases

U f D I i I d t


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Uses of Dry Ice in IndustryUses Explanation

Refrigerant Very cold & does not leavemessy liquid when sublimes

Cloud seeding Spread into cloud to

accelerate rain fallFilms / stageperformance

Produce special effects ofcloud / mist / fog.

Carbonated drink Carbonate salt produced isused to make drinks.

Fire extinguisher CO2 is compressed & usedas agent.

Chapter 3, Solid & Liquid

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