Chapter 5
CHAPTER 5: STATES OF MATTER
Solids X-ray diffraction Giant molecular structures Ionic, metallic and simple molecular lattices Liquids Gases Vapour Pressure Avogadro's, oyle's and !"arles's La#s and ideal gas equation $inetic %"eory of gases &eal gases !ritical temperature and pressure and liquefaction of gases 5.1. WHAT WE SEE
()()( We do not see individual atoms or ions, #e do not see *onds, and #e rarely see idividual molecules( +e do see collections of atoms, of ions, and of molecules( sually t"ey are solids, sometimes t"ey are liquids, and on rare occasions #e see gases( +"et"er #e see t"em or not, it is t"e collective properties of atoms, ions, and molecules #"ic" #e eperience( ut #"at is t"e arrangement of t"e atoms, ions, or molecules in solids, liquids and gases. If #e /ne#, it mig"t increase our understanding of t"e collective properties #e eperience and o*serve( ()(0( The main difference *et#een solids, liquids, and gases is some#"at predicta*le( In solids t"e constituent particles tend to *e close toget"er and relatively una*le to move( In liquids t"ey are furt"er apart and moving quite freely( In gases t"ey are even furt"er apart and *u11ing a*out all over t"e place 2not a tec"nical term3( 5.2. SO!"S
(0()( So#ids are a good place to start *ecause #e eperience t"em most over#"elmingly( %o eamine t"e arrangement of t"e constituent particles in solids #e do, "o#ever, need some fairly po#erful met"ods( %"ese include X-ray diffraction, to a lesser etent electron diffraction and sometimes ot"er met"ods suc" as 45& 2nuclear magnetic resonance3 and 6S& 2electron spin resonance3 can give useful information(
(0(0( $%ra& diffraction uses X-rays, #"ic" "ave a similar #avelengt" to t"e si1e of atoms, ions etc( +"en t"e X-rays travel past atoms, ions and so on, t"ey are diffracted, or *ent, 7ust as visi*le lig"t rays are diffracted #"en t"ey pass t"e edge of a solid o*7ect( X-rays "itting an ionic crystal can, *y #ay of eample, *e used to find t"e arrangement of ions in t"e crystal( %"e X-rays start off #it" t"eir #aves in p"ase 2see 8IG( ()3( 9o#ever, rays diffracted *y individual ions in a crystal may or may not *e in p"ase #it" rays diffracted *y ot"er ions( +"en t"ey are, t"ey reinforce eac" ot"er( +"en t"ey are not, t"ey tend to cancel eac" ot"er out( %"e condition for reinforcement is s"o#n in 8IG( ()( %"e nett result is t"at particular arrangements of ions produce particular patterns of reinforced X-rays( %"e X-ray patterns can *e detected on, for eample, p"otograp"ic plates( nderstanding t"e patterns allo#s t"e arrangement of ions in t"e crystal to *e deduced(
%"e same tec"nique can *e used to find t"e arrangement of ions in metals and of atoms in molecular crystals( (0(:( 'iant mo#ec(#ar str(ct(res ; 6lements suc" as car*on and silicon may eist as solid giant molecular structures, as may compounds suc" as silicon dioide, proteins and plastics, to name *ut a fe#( %"ere are many different arrangements of atoms in suc" structures and it is difficult to tal/ a*out common patterns( %"e *est #e can do is loo/ at an eample(
i) Car*on is a frequently discussed eample( %"e t#o main structures are diamond and grap"ite( It is "ard to *elieve t"at one element could eist in t#o suc" vastly different forms( %"e differences are reflections of t"e different *onding and different arrangements of atoms in t"e t#o structures(
In diamond t"e atoms are sp: "y*ridised( 6ac" car*on is *onded tetra"edrally to four ot"er car*on atoms *y strong covalent *onds( 28IG( (0(3 In grap"ite t"e atoms are sp0 "y*ridised( 6ac" car*on is *onded triagonally to t"ree ot"er car*on atoms *y strong covalent *onds #it" multiple c"aracter( 28ig( (0(3
%"e properties do overlap( <*viously t"e strong covalent *onds are difficult to *rea/ in *ot" structures and *ot" "ave etremely "ig" melting points 2and *oiling points3( %"e strengt" of t"e *onds also eplains #"y diamond is one of t"e "ardest su*stances t"ere is( 5oreover, it is difficult to distort t"e *onds at all, #it"out *rea/ing t"em; diamond is *rittle as #ell as "ard( ut grap"ite is soft( %"is is a little surprising at first sig"t( %"e covalent *onds are slig"tly "arder to *rea/ t"an t"ose in diamond, *ecause t"ey "ave multiple c"aracter( 9o#ever, t"e car*on atoms are arranged in covalently *onded layers 28IG( (0(3 and t"e *onds *et#een one layer and anot"er are #ea/ Van =er +aals *onds( It is t"ese #ea/ *onds #"ic" are *ro/en first(
%"oug" it is not quite t"is simple( Grap"ite is soft only #"en it contains air molecules as an impurity( %"e air molecules occur *et#een t"e layers and act as >rollers> allo#ing t"e layers to easily slide over eac" ot"er and ma/ing grap"ite soft and slippery( It is even used as a lu*ricant( Imagine using diamond dust as a lu*ricant? it is actually used as an a*rasive( 4ote t"at in "ig" vacuums, t"e air molecules are removed from grap"ite, causing t"e layers to stic/( %"e rat"er unusual *onding in grap"ite accounts for anot"er property #"ic" it certainly does not s"are #it" diamond( %"e *onding electrons in diamond are strongly confined to t"e covalent *onds( =iamond does not conduct electricity( 9o#ever, t"e p-or*ital electrons in grap"ite are mo*ile in an electric field( %"us grap"ite does conduct electricity( %"e same p-electron cloud reflects lig"t and accounts for grap"ite's metallic lustre( %"e p-*onding or*ital is very similar to a metallic *onding or*ital in t"ese respects( (0(@( !onic+ meta##ic and sim,#e mo#ec(#ar #attices ; %"is may seem li/e a diverse *unc" of structures to consider in t"e same *reat"( +e already /no# from t"e previous c"apter t"at #e are dealing #it" a #ide variety of *ond types( It is 7ust *ecause #e are clear a*out t"ese distinctions t"at #e can ris/ t"is lumping toget"er( %"e point is t"is( +"et"er #e are dealing #it" oppositely c"arged ions attracted to eac" ot"er, or #it" positive ions attracted to a single *onding cloud, or even #it" simple molecules attracted *y relatively #ea/ van der +aals forces, t"e pac/ing particles #ill try to get close to eac" ot"er( %"ere are surprisingly fe# #ays of ac"ieving t"is( As a result, particles of #"atever type pac/ toget"er in a small num*er of commonly occurring arrangements( nli/e giant molecules, ionic, metallic and simple molecular lattices s"o# a fe# repeating patterns( %"ere are fourteen arrangements in metals, of #"ic" t"ree are s"o#n *elo#( i) -od&%centred c(*ic arrangements are found in caesium c"loride amongst ionic compounds and in iron, manganese and t"e al/ali metals amongst metals(
In t"e case of ionic compounds li/e caesium c"loride, t"e single anion s"o#n in 8IG( (:( can *e pictured as part of a simple cu*ic lattice of anions #it"in a simple cu*ic lattice of metal ions( %"e structure is said to s"o# ; co-ordination; i(e( eac" anion is surrounded *y cations, and eac" cation is surrounded *y anions( In contrast #it" caesium c"loride, sodium c"loride forms a simple cu*ic structure( 28IG(@(0(3; a simple cu*ic arrangement in caesium c"loride #ould not *e as tig"tly pac/ed *ecause t"e caesium ion is *igger t"an t"e sodium ion(
5etals #"ic" pac/ according to t"e *ody centred cu*ic arrangement use BC of t"e space( %"is is not one of t"e, so-called, close-pac/ed structures, *ut it is still a fairly efficient use of space( %"e co-ordination num*er is ( ii) Hea/ona# c#ose%,ac0in/ is found in nic/el, 1inc and magnesium amongst ot"er metals( It uses D@C of t"e space, and t"e co-ordination num*er is )0(
iii) C(*ic c#ose%,ac0in/ is found in aluminium, copper, lead and silver amongst t"e metals( It is also t"e arrangement ta/en up *y iodine molecules in solid iodine( Li/e "eagonal close-pac/ing, it uses D@C of t"e space, and t"e coordination num*er is )0(
<#ing to t"e nature of t"e unit cell in t"is arrangement, cu*ic close-pac/ed structures are also /no#n asface%centred c(*ic structures( 4ote from 8IGS( (( and (B( "o# t"e units are pac/ed 7ust as tig"tly in t"e t#o cases( %"e underlying difference is t"e pattern of repeating layers; a*a* or a*ca*c( %"ese produce different unit cells( i) !n ionic s(*stances t"e patterns are complicated *y t"e fact t"at more t"an one type of ion is present in t"e structure, and even in t"e case of *inary compounds t"ese may not *e present in a ratio of );)( %"e result is t"at ions in ionic crystals may often *e seen as a particular arrangement of one ion #it"in a particular arrangement of t"e ot"er( +e "ave already seen t"at t"e *ody centred cu*ic structure of caesium c"loride can *e seen as a simple cu*ic arrangement of caesium ions #it"in a simple cu*ic arrangement of c"loride ions 2or vice versa3( Anot"er eample is calcium fluoride( %"is may *e seen as a face centred cu*ic arrangement of calcium ions #it"in a simple cu*ic arrangement of fluoride ions(
6ven t"e apparently simple cu*ic structure of sodium c"loride can *e seen in a different #ay( It can *e seen as a face-centred cu*ic arrangement of sodium ions #it"in a face-centred cu*ic arrangement of c"loride ions(
(0(( "es,ite the simi#ar arran/ements of particles in ionic, metallic and simple molecular lattices, t"e properties are quite different( %"is is a function of t"e different *onding 2ta*le @()(3( i) !onic cr&sta#s are "ard and *rittle *ecause t"e *onding is strong and is difficult to distort #it"out *rea/ing( %"e strong *onding also leads to t"e "ig" melting and *oiling points(
%"ere are no mo*ile ions or electrons in ionic lattices so t"e compounds are not electrical conductors in t"e solid state( 9o#ever, #"en molten, t"e ions are
mo*ile and a*le to transport c"arge( 5olten ionic compounds are t"erefore good electrical conductors( Ionic compounds are solu*le in polar solvents *ecause several polar solvent molecules 2eg #ater3 #ill pull eac" ion a#ay from t"e lattice( It is t"e com*ined strengt" of attraction of several solvent molecules for eac" ion t"at may overcome t"e strong ionic *onding( Since suc" solutions contain mo*ile ions, t"ey conduct electricity( 9o#ever, t"e attraction of non-polar solvent molecules 2eg tetrac"lormet"ane3 for ions is so #ea/, t"at even large num*ers of t"em cannot pull ions a#ay from t"e lattice( Ionic compounds t"erefore tend to *e insolu*le in non-polar solvents(
ii) Meta##ic s(*stances tend to *e mallea*le and ductile, t"at is t"ey can *e "ammered into different s"apes, or stretc"ed into s"apes suc" as #ires( %"is is *ecause t"ere are no rigid, directed *onds and groups of ions can *e forced into ne# positions #it"in t"e cloud of *onding electrons(
4evert"eless, t"e *onding must not *e t"oug"t of as #ea/ overall( 5etals can "ave "ig" melting points and *oiling points, alt"oug" t"e >*est> metals in
c"emical terms 2"ig"ly electropositive elements3 "ave quite lo# melting points and *oiling points( In fact, electropositive metals do not eemplify many of t"e c"aracteristics #e associate #it" everyday metals li/e steel( 4ot only are ions #it"in t"e electron cloud movea*le, *ut also t"e electrons in t"e or*ital are mo*ile in an electric field( %"is ma/es metals good electrical conductors( %"e mo*ility of t"e *onding electrons also ma/es metals good conductors of "eat( +"ere t"e metal is "ot, vi*rating ions ecite electrons( %"e motion is t"en transmitted via t"e *onding or*ital electrons to ions in a cooler part of t"e lattice( iii) Sim,#e mo#ec(#ar cr&sta#s of su*stances suc" as iodine, napt"alene and ice tend to *e soft *ecause t"e #ea/ *onds *et#een one molecule and anot"er are fairly easily *ro/en( In iodine t"ese *onds are van der +aals *onds, #"ereas in ice t"ey are "ydrogen *onds(
5oreover, t"e su*stances "ave lo# melting points and *oiling points again *ecause t"e #ea/ *onds *et#een one molecule and anot"er *rea/ easily( In no state do straig"tfor#ard molecular su*stances contain mo*ile electron or ions, t"us t"ey do not conduct electricity( %"ey do not dissolve in polar solvents *ecause in t"is case t"e solvent molecules are too strongly attracted to eac" ot"er for t"em to attract non-polar molecules a#ay from t"e lattice( 9o#ever, in non-polar solvents *ot" t"e solute-solute attractions, and t"e solvent-solvent attractions are #ea/( Simple molecular solids may t"erefore dissolve in non-polar solvents #"en enoug" sufficiently strong *onds form *et#een solvent and solute molecules for t"e solute molecules to *e attracted a#ay from t"e lattice( %"e *alance is fine, and anot"er factor is often important( +"en a su*stance dissolves t"ere may *e an increase in freedom and a dissipation of energy #it"in t"e system( <*servations s"o# t"at naturally occurring processes al#ays involve an increase in overall dissipation of energy 2c"apter )03( Suc" c"anges in t"e dissipation of energy are even important in finely *alanced cases of ionic su*stances dissolving in polar solvents( Sometimes ionic su*stances dissolve in #ater even #"en t"e total attraction of #ater molecules for ions in t"e lattice is slig"tly #ea/er t"an t"e attraction of t"e ions for eac" ot"er( %"ey do so #"en t"ere is an overall increase in t"e dissipation of energy(
5. !34!"S
(:()( The *rief com,arison of so#ids+ #i(ids and /ases in section ()(0( #as misleading( %"e comparison may *e useful for solids, liquids and gases #ell
a#ay from t"eir melting points and *oiling points( 9o#ever, it #ould lead to #rong epectations a*out t"e microscopic c"anges #"ic" ta/e place at t"ese transition temperatures( Since melting 2or solidification3 and *oiling 2or condensation3 are t"e processes during #"ic" t"e /ey c"anges from one state to anot"er ta/e place, #e must understand t"e microscopic c"anges #"ic" occur during t"e processes( long time> and >far a#ay> are relative terms, *earing in mind t"e frequency of vi*rations 2around )F): 913 and t"e si1es of atoms and molecules( 6ac" time t"e particles move, t"ey move *y only a fraction of t"eir radius, *ut since t"ey do t"is around )F): times a second, t"eir average position can c"ange fairly rapidly on t"e type of time scale #e are used to( %"e /ey fact remains t"at liquids do not contain permanent *onds *et#een one particle and anot"er( 24ote #e are not tal/ing a*out *onds #it"in molecules of a particular liquid( %"ese remain permanent(3 %"is means t"at liquids are very su*7ect to eternal forces( nder t"e influence of gravity, or ot"er forces, t"ey
adopt t"e s"ape of t"eir container and flo# easily( 9o#ever, since t"eir particles are already close toget"er, t"ey are not easily compressed( Anot"er point s"ould *e made a*out liquids( Alt"oug" t"ey do not contain permanent *onds *et#een one particle and anot"er, at any given instant t"ere may *e a considera*le amount of structure #it" many particles *onded to ot"ers( %"e surface of a liquid is particularly li/ely to *e structured( In #ater at room temperature, for instance, t"ere can *e fairly etensive tracts of molecules "ydrogen *onded in ice-type lattices( In solutions, solute-solvent *onds can produce a fair degree of structure( %o a large etent a "ydrated ion *e"aves as single liquid particle( It also structures surrounding #ater molecules *eyond its immediate "ydration s"ell( 9o#ever, as t"e temperature of a liquid reac"es its *oiling point, t"e amount of structure decreases, so lac/ of structure s"ould not *e ta/en as a fundamental difference of gases( 8or suc" differences #e s"ould loo/ at t"e events #"ic" accompany t"e c"ange of state *et#een liquid and gas( 6vents #"ic" t"us define *asic properties of liquids as #ell as t"ose of gases(
