14 patterns in the chemical world - hkep · patterns in the chemical world properties of the oxides...

1414 Patterns in the Chemical World

Periodic Variation in Physical Properties of the Elements

PeriodicityPeriodicity is the recurrence of similar properties at regular intervals when elements are arranged by increasing atomic numbers.

Electrical conductivity• In general, metals are good

electrical conductors and non-metals are non-conductors of electricity.

• For metals, the electrical conductivity increases when the number of the delocalized electrons increases.

• For metalloids, the electrical conductivity increases when the temperature increases.

Density• The density of an element is

affected by:– Atomic mass– Atomic radius– Arrangement of the particles

Melting point• The amount of energy

required to melt a substance depends on:– The strength of the attractive

forces between the particles of the substance

– The arrangement of the particles in the substance

• The melting points of the elements depend on the nature of the bonding and structures.

Atomic radius• The atomic radius can be used

as a measure of the size of an atom, which can either be the covalent radius or the metallic radius.

• The atomic radius decreases across the Periodic Table.

• The atomic radius increases down a group.Bonding and structure

• Across a period, the type of element changes from metal through metalloid to non-metal.

• Across a period, the type of structure of the elements changes from giant metallic through giant covalent to simple molecular.

Patterns in the Chemical World

HK

EP


Properties of the Oxides of Period 3 Elements

Transition Metals

Nature of bonding and stoichiometric composition• Metals react with oxygen to

give metal oxides, which have giant ionic structures.

• Silicon reacts with oxygen to form silicon dioxide, which has a giant covalent structure.

• Non-metals react with oxygen to give non-metal oxides, which have simple molecular structures.

Introduction to transition metals• Transition metals are the

metals in the horizontal rows crossing the Periodic Table between group II and group III from the fourth period onwards.

• Transition metals have several common physical properties:– high electrical and thermal

conductivity– high melting point– high density– hard and rigid

Aqueous ions of transition metals• Transition metal ions are

usually coloured.

Formula Colour

Sc3+(aq) Colourless

Ti3+(aq) Purple

V3+(aq) Green

Cr3+(aq) Green

Mn2+(aq) Very pale pink

Fe2+(aq) Pale green

Fe3+(aq) Yellow-brown

Co2+(aq) Pink

Ni2+(aq) Green

Cu2+(aq) Blue

Zn2+(aq) Colourless

MnO4-(aq) Purple

Cr2O72-(aq) Orange

Acid-base nature• Generally, metal oxides are

basic.• Aluminium oxide is an

amphoteric oxide.• Non-metal oxides are usually

acidic, but some of them are neutral.

Catalytic properties of transition metals and their compounds• Transition metals and their

compounds are important catalysts in industry and biological systems.


Variable oxidation states of transition metals• Transition metals can exist in

more than one oxidation state in their compounds.

• Lower oxidation states are found in simple ions.

• Higher oxidation states of transition metals are formed by the formation of covalent bonding with highly electronegative atoms.

19

Rate of Reaction

• Activationenergy(Ea) is theminimumvalueofkineticenergywhichthe

collidingparticlesmusthavebeforetheyareabletoreact.

Figure 11.19

• Examplesofparticlescollidingwithandwithouttherightorientation:

2CO(g)+2NO(g) N2(g)+2CO2(g)

Figure 11.20

Factorsaffectingrateofreaction–concentration

• Increasingtheconcentrationsofthereactantsincreasestherateofreaction

involvingsolutionsandgasesinmostcases.

• Increasingtheconcentrationofareactantimpliesincreasingthenumber of

particles per unit volume.

• Astheconcentrationincreases,theparticlesaremorecrowded.Hence,the

frequency of collisionsbetweenthereactantparticlesincreases.

• Therefore,thefrequencyofsuccessfulcollisionsalsoincreases.

Figure 11.21

The further discussion of

activation energy is in the

elective part ‘ Industr ial

Chemistry’.

• Activation energy can

be considered as a

‘threshold value’.

• The particles with kinetic

energy lower than Ea

may still collide, but they

cannot trigger a reaction.

B

Frequency of collisions =

Number of collisions per

unit time

50

Chemistry: Key Notes & Exambuilder (Book 3)

Effectofchangingtheconditionsonchemicalequilibriumsystems

Equilibriummaybeaffectedbychangingcertainconditionsofachemicalequilibrium,includingconcentration,pressureandtemperature.

Effect of concentration changes on the chromate / dichromate

system

Figure 12.3

• Procedure:– Addaknownvolumeofpotassiumchromatesolutiontoatesttube.– Leaveitforawhileuntilthecolourofthesolutionstopschanging.– Adda fewdropsofdilute sulphuricacid to thepotassiumchromate

solution.Observethecolourchange.– After that, add a fewdrops of sodiumhydroxide solution to the

acidifiedpotassiumchromatesolution.Observethecolourchange.

• Resultsanddiscussion:

1. Whenthecolourofthepotassiumchromatesolutionstopschanging,thefollowingequilibriumsystemisestablished:

2CrO42-(aq)+2H+(aq)CCr2O7

2-(aq)+H2O(l) yellow orange

2. Whendilutesulphuricacidisadded,theconcentrationofH+ ions inthesolutionincreases.Thecolourofthesolutionchangesfromyellowtoorange.It indicatesthattheconcentrationofCrO4

2-(aq)decreasesandthatofCr2O7

2-(aq)increases.Theequilibriumpositionshiftstotheright.

3. Whensodiumhydroxidesolution isadded, theconcentrationofH+ionsinthesolutiondecreases.Thecolourofthesolutionchangesfromorange toyellow. It indicates that theconcentrationofCrO4

2-(aq)increasesandthatofCr2O7

2-(aq)decreases.Theequilibriumpositionshiftstotheleft.

4. Therefore, theconcentrationofH+ ionscanchangetheequilibriumposition.

C

The factors affecting the

position of the equilibrium

will be discussed in more

detail in Section 12.3.

74


Suggested Answers (STSE Connections)

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

(b) A lot of energy is needed to break the triple bonds within nitrogen molecules, hence the activation energy of any reaction involving nitrogen gas must be very high. Therefore, nitrogen gas is nonreactive.

(c) According to Le Chatelier’s Principle, when the pressure of the system increases, the equilibrium position shifts to the right in order to reduce the total number of gas molecules. Hence the product yield increases.

(a) WritethechemicalequationforthereactionintheHaberProcess.

(b) Explainwhynitrogengas isnonreactive in termsofbondbreakingand

activationenergy.

(c) According toLeChatelier’sPrinciple, explainwhy theHaberProcess is

performedathighpressure.

Haber Process

TheHaberProcess isan industrialnitrogen-fixingprocess. It turnsnitrogengas intoamoreusable form,ammonia,byreactingnitrogenwithhydrogen.Ammoniacanbeoxidizedtonitrateandnitrite,whicharethekeycomponentsofindustrialfertilizersandexplosives.

Nitrogenisaveryimportantelementinbothbiologyandindustry.Itisaconstituentelementofaminoacidsandthusofproteinsandnucleicacids(DNAandRNA).Ouratmosphereisrichinnitrogen(about78.1%).However,thenitrogeninaircannotbeuseddirectlybecausenitrogenisrelativelynonreactive.

Beforethe20thcentury,onlynitrogen-fixingbacteriaandarchaeacouldturnnitrogengastoamoreusableform.Intheearly20thcentury,theGermanchemistFritzHaberfoundthatammoniacouldbemadefromatmosphericnitrogenathighpressure.TheGermanchemistandengineerCarlBoschscaledupHaber’stabletopmachinetoindustrial-levelproduction.HaberandBoschwere laterawardedNobelPrizes in1918and1931respectively.Nowadays,aboutonethirdoftheworld’spopulationisfedbycorpsgrownbyfertilizersmadefromtheHaberProcess.

IntheHaberProcess,hydrogengasisproducedfromthemethaneinnaturalgas.Thehydrogengasisthenmixedwithnitrogengasandpassedthroughseveralbedsofironcatalystunder15–25MPaand300–550°C.Oneachpassonlyabout15%ofthegasesareconvertedtoammonia,butanyunreactedgasesarerecycled,sothateventually97%ofthegasesareconverted.

HKEP

76


ConceptCheck

Topics Common errors Correct concepts

Equilibriumconstant

Considerthefollowingequilibriumsystem:

Cr2O72-(aq)+H2O(l)C

2CrO42-(aq)+2H+(aq)

When we add water to the system, theequilibriumposition shifts to the right as[H2O(l)]increases.

The explanation is incorrect. [H2O(l)] isconstantatagiventemperature.

Addingwaterwilldiluteallspecies.

AsQc=[H+(aq)]2[CrO4

2-(aq)]2

[Cr2O72-(aq)]

,

thedecrease in concentrationof all speciesaffectsthenumeratormost,soQcdecreases.

Hence theequilibriumpositionshifts to therightinordertomakeQcrestoretoKc.

Equilibriumposition

Consider the following equilibrium systeminsideavesselwithconstantvolume:

N2O4(g)C2NO2(g)

Whenanoblegas (i.e.,Ne) isadded to thesystem,theequilibriumpositionshifts totheleftasthepressureincreases.

Thereisnoshiftintheequilibrium.

The concentrations of all species remainunchanged because the numbers of thereactant and product molecules, and thevolumeoftheflask,areunchanged.

HenceQc=Kcandtheequilibriumpositiondoesnotshift.

Catalyst

ConsidertheHaberProcess:

N2(g)+3H2(g)C2NH3(g)

AddingFe(s)(acatalyst)canincreasetheyieldofammonia.

AddingacatalystdoesNOTchangetheyieldoftheproduct.Itjustincreasestherateofthereaction.

Backward reaction逆向反應 Equilibrium system平衡體系

Chemical equilibrium化學平衡 Forward reaction正向反應

Closed system密閉體系 Haber Process哈柏法

Contact Process接觸法 Irreversible reaction不可逆反應

Dynamic equilibrium動態平衡 Reaction quotient反應商數

Equilibrium constant平衡常數 Reversible reaction可逆反應

176


achiral非手性的 chiral手性的

acidic hydrolysis加酸水解 cholesterol膽固醇

actual yield實際產率 cis-isomer順式異構體

addition reaction加成反應 condensation polymer縮合聚合物

alcohol醇 condensation polymerisation縮合聚合作用

aldehyde醛 dacron的確涼

aldohexose己醛糖 dehydration脫水作用

aldose醛糖 detergent清潔劑

alkaline hydrolysis加鹼水解 dipeptide二肽

alkane烷烴 disaccharide雙糖

alkene烯烴 distillation蒸餾

alkyl group烷基 electromagnetic radiation電磁波

amide酰胺 emulsifying乳化

amine胺 emulsion乳狀液

analgesic止痛劑 enantiomer對映異構體

anti-inflammatory消炎 enantiomerism對映異構

aryl group芳香基 ester酯

aspirin阿士匹靈 esterification酯化作用

biodegradable生物降解的 ether醚

carbohydrates碳水化合物 fractional distillation分餾

carbonyl group羰基 fructose果糖

carboxyl group羧基 functional group官能基

carboxylic acid羧酸 functional group isomerism官能基異構

catalysed hydration催化水合作用 gastric acid胃酸

catalytic hydrogenation催化氫化作用 geometrical isomerism幾何異構

cellulose纖維素 glucose葡萄糖

chain isomerism鏈異構 glycerol甘油

189

Chemistry of Carbon Compounds

Section A Multiple-choice QuestionsHint 1 Identifythefunctionalgroup(s)ofthesubstancesfirst.

Hint 2 Determinewhichpartofesterisfromalcoholandcarboxylicacidrespectively.

Hint 3 RecallthechemicalreactionsofLiAlH4andcarboncompounds.

Hint 4 Determinethelengthofthemainchain.

Hint 5 Determinethetypeofthereactionfirst.

Hint 6 HardwatercontainsCa2+(aq)andMg2+(aq).

Hint 7 Calculatethemolecularmassofthestartingreagentandtheproductrespectively.

Hint 8 Determinethefunctionalgroupofeachcompoundinthesyntheticroute.

Section B Short Questions and Structured / Essay-type Questions

Hint 9 Identifythefunctionalgroupandmolecularmassofeachofthesubstances.

Hint 10 “Givesgasbubbleswithsodiumcarbonatesolution”meansthesubstancehaveacidicproperties.

Hint 11 Acidifiedpotassiumpermanganateisanoxidizingagent.

Hint 12 Considertheirmolecularshapes.

Hint 13 Thesubstancewithmorenegativeenthalpychangeofformationismorestable.

Hint 14 Considertheavailabilityofforminghydrogenbondsofeachsubstance.

Hint 15 Potassiumcyanidedissolveswellinwater.

Hint 16 Dothereactionsgotocompletion?

Hint 17 WhatisthefunctionofsubstanceZ?

Hint 18 TheoxidationofethanebyanoxidizingagentgivesHOCH2CH2OH.

Hint 19 Soapydetergentandsoaplessdetergentcanbededucedfromtheionicheadofthedetergent.

Hint 20 Biodegradabledetergentshouldhaveonebranchedchaininhydrocarbontail.

Hint 21 Theproductshouldbeopticallyactive.

192


• Whatisperiodicity?

• WhatisthenatureofthebondingandstructuresoftheelementsfromGroupItoGroup0?

• Canyouexplaintheperiodicvariationsofthemeltingpointsandtheelectricalconductivitiesof theelementswithregardto thebondingandstructure?

PeriodicVariation inPhysicalPropertiesoftheElements

Periodicity• ThePeriodicTable is an arrangementof the elements in theorderof

increasingatomicnumbers.

• ElementsinthemodernPeriodicTablearedividedintofourcategories:

– Maingroupelements

– Transitionmetals

– Lanthanoids

– Actinoids

• Periodicityistherecurrenceofsimilarpropertiesatregular intervalswhen

elementsarearrangedbyincreasingatomicnumbers.

Periodicvariationsinthebondingandstructuresoftheelements• Thenatureofthebondingandstructuresoftheelementsacrossperiods2

and3inthePeriodicTablearesummarizedasfollows:

Period 2

Group I II III IV V VI VII 0

Element Li Be B C N O F Ne

Typeofelement

Metal Metalloid Non-metal

Natureofbonding

Metallic Covalent Amongmolecules:VanderWaals’forces

Withinmolecule:Covalent(ExceptNe)

Typeofstructure

Giantmetallic Giantcovalent Simplemolecular

Table 14.1

14.1

A

B

Ne exists as a monatomic

molecule.

200


14.1A. Determine whether each of the following statements is correct. Put a 3 in the appropriate box.

Correct Incorrect

(a) ElementsinthemodernPeriodicTablecanbedividedinto4categories.

(b) Onlymetalloidshavegiantcovalentnetworks.

(c) Theatomicradiusdecreasesacrossperiod2andperiod3.

(d) Themeltingpointsofallnon-metalsareverylow.

(e) Theelectrical conductivityof anelement increaseswith thenumberofdelocalizedelectrons.

B. Fill in the blanks:

• (1) is the recurrence of similar properties at regular intervals when elements are arranged by increasing atomic numbers.

• Magnesium has more delocalized (2) than sodium, and the atomic radius of magnesium is (3) than that of sodium. Therefore, the strength of the metallic bond of magnesium is greater than that of sodium. Hence, magnesium has a (4) melting point than sodium.

• All (5) can conduct electricity because they have mobile electrons.

(For answers, see the bottom of the page.)

Suggested Answer (Check Your Progress 14.1)

(a) Correct. They can be divided into main group elements, transition metals, lanthanoids and actinoids.

(b) Incorrect. Non-metals (i.e., carbon) can also have a giant covalent network.

(c) Correct.

(d) Incorrect. Carbon has a very high melting point.

(e) Correct.

(1) Periodicity (2) electrons (3) smaller (4) higher (5) metals

217


1. Whichof the followingphysicalpropertiesgenerally increases down the group butdecreasesacrosstheperiod?

A. Electricalconductivity

B. Meltingpoint

C. Atomicradius

D. Density

2. Whichofthefollowingoxideswouldformanacidicsolutionwithwater? Hint 1

A. Na2O(s)

B. MgO(s)

C. SiO2(s)

D. SO2(g)

3. Whichofthefollowingstatementsconcerningtheelements fromsodiumtochlorine in thePeriodicTableis/arecorrect?

(1) Aluminiumhas thehighestmeltingpointandthehighestelectricalconductivity.

(2) MeltingpointofP>MeltingpointofS>MeltingpointofCl

(3) Sodiumhasthehighestatomicradius.

A. (2)only

B. (3)only

C. (1)and(2)only

D. (1)and(3)only

4. Whichofthefollowingmetalionsisyellowincolour?

A. Fe3+ion

B. Fe2+ion

C. Mn3+ion

D. Mn2+ion

5. Considerthefollowingdiagram:

Figure 14.25

Whichof the followingmaybedescribedbythediagramabove?

A. SolubilitiesinwaterofAl,Si,PandS

B. AtomicradiiofP,S,ClandAr

C. ElectricalconductivitiesofNa,Mg,AlandSi

D. MeltingpointsofMg,Al,SiandP

6. Whichof the followingpolyatomic ions is /areNOTlikelytoexist? Hint 2

(1) TiO42-

(2) CrO4-

(3) VO43-

A. (3)only

B. (1)and(2)only

C. (1)and(3)only

D. (2)and(3)only

SectionA Multiple-choiceQuestions

222


Question CommandsQuestion command(s) Example(s)

What / Which ...

(Simple answer is usually required.)

What gas evolves?

Correct answer: Sulphur dioxide / SO2

What is the direction of electron flow in the external circuit?

Correct answer: From left to right

Which of the following compounds can be used to make an addition polymer?

H

C HC = C H2N NH2 OH

Correct answer: H

C HC = C

Suggest a formula ... The oxide of aluminium is insoluble in water, suggest the formula for this oxide.Correct answer: Al2O3

Incorrect answer: Aluminium oxideName ...

(Formula / Structure is NOT accepted.)

Name an element which is a metalloid.Correct answer: BoronIncorrect answer: B

Write the chemical equation ...

(Al though both chemical equation and ionic equation a r e a c c e p t e d , t h e b e s t answer should be a chemical equation.)

Write a chemical equation for the reaction when adding dilute hydrochloric acid to zinc granules.Correct answer: Zn + 2HCl → ZnCl2 + H2 (chemical equation)Poor answer: Zn + 2H+ → Zn2+ + H2 (ionic equation)

Write the chemical equation ... Write a chemical equation for the reaction between sodium and water. State symbols should be given.Correct answer: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) (Score 2 marks)Poor answer: 2Na + 2H2O → 2NaOH + H2 (Score 1 mark only) (Remarks: 1 mark for equation and 1 mark for state symbols)

Write an ionic equation ... Write an ionic equation for the reaction when adding hydrochloric acid to sodium carbonate.Correct answer: 2H+ + CO3

2- → H2O + CO2

Incorrect answer: 2HCl + Na2CO3 → H2O + CO2 + 2NaCl

2


Section A Multiple-choice Questions

1. B

2. C

For(2),WecanuseacidifiedKMnO4(aq)todeterminetheconcentrationofH2O2(aq).

5H2O2(aq)+KMnO4(aq)+3H2SO4(aq)→K2SO4(aq)+2MnSO4(aq)+5O2(g)+8H2O(l)

3. D

HClcanremovethealkaliquicklyandhencequenchthereactionmixture.

4. D

5. B

For(3),largergranuleshaveasmallertotalsurfacearea.Itwilldecreasetherateofreaction.

6. D

Sincethecatalystremainschemicallyunchangedattheendofthereaction,itsmassalsoremainsunchanged.

7. C

8. A

No.ofatomsin24dm3ofNH3=2424×4=4mol

No.ofatomsin24dm3ofN2=2424×2=2mol

No.ofatomsin48dm3ofNe=4824×1=2mol

No.ofatomsin72dm3ofAr=7224×1=3mol

9. B

CuO(s)+CO(g)→Cu(s)+CO2(g)

No.ofmolesofcarbonmonoxidereacted

=1224

=0.5mol

No.ofmolesofcopperformed=0.5mol

Massofcopperformed=0.5×63.5=31.75g

10. C

S(s)+O2(g)→SO2(g)

No.ofmolesofsulphurused=3.2132.1

=0.1mol

No.ofmolesofsulphurdioxideformed =0.1mol

Volumeofsulphurdioxideformed =0.1×24 =2.4dm3

Section B Short Questions and Structured / Essay-type Questions

1. (a) Measurethechangeinthevolume/pressureofgasproduced 1

(b) Colorimetricmethod 1 OR Titrimetricmethod (1)

(c) Colorimetricmethod 1

(d) Titrimetricanalysis 1

(e) Measurethechangeinthepressureofgasproduced 1

(f) Titrimetricanalysis/colorimetricmethod 1

2. Measurethechangeinthevolumeofgasproduced 1

Thereactiongivesoffgas. 1OR Measurethechangeintheconductivityofthe

solution 1 Thereactioninvolvesagreatchangeinthe

concentrationofions. 1OR Measurethechangeinthemassofthereaction

mixture 1 Thereactiongivesoffgas,andthemassofthe

reactionmixturedecreases. 1

Chapter 11 Rate of Reaction

14 patterns in the chemical world - hkep · patterns in the chemical world properties of the oxides...

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