1-6 Ionic and Covalent Substances

(1)

Topic

VI.

VI. Ionic

Ionic and

and covalent

covalent substances

substances

Reference

Reading

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 104–110 Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 104–110

Objectives

1.61.6 –

– describe describe the dthe differences ifferences in harin hardness, dness, melting pmelting point boint boiling poiling point, sooint, solubility anlubility and electrical d electrical conductivityconductivity between ionic and covalent substances

between ionic and covalent substances –

– explain the explain the differences differences in phyin physical propersical properties between ties between ionic and ionic and covalent sucovalent substances in bstances in terms of terms of the nature the nature of of forces holding the particles together:

forces holding the particles together: (a)

(a) in solid ionic substances, e.g. sodium chin solid ionic substances, e.g. sodium chloride and copper(loride and copper(II) sulphate, consist of giant lattices of ionsII) sulphate, consist of giant lattices of ions held together by strong electrostatic forces (giant ionic

held together by strong electrostatic forces (giant ionic structure)structure) (b)

(b) some solid covalent substances e.gsome solid covalent substances e.g. iodine, sulphur and dry ice, con. iodine, sulphur and dry ice, consist of a regular arrangement of sist of a regular arrangement of molecules held together by weak forces called van der Waals forces (simple molecular structure) molecules held together by weak forces called van der Waals forces (simple molecular structure) (c)

(c) some solid covalent susome solid covalent substances e.g. diamonbstances e.g. diamond and quartz, cond and quartz, consist of a giant lattice of atoms hesist of a giant lattice of atoms held togetherld together by strong covalent bonding (giant covalent

by strong covalent bonding (giant covalent structure)structure) –

– predict the predict the structures astructures and prond properties of perties of compoundcompounds when s when the grouthe group numbp numbers of ers of constituent elemconstituent elements are ents are givengiven

Notes

VI. VI. Ionic and covalenIonic and covalent substanct substanceses A.

A. Structures Structures of of different different substancessubstances

The most obvious difference between substance

The most obvious difference between substances with molecular structure and those with s with molecular structure and those with giant structure is giant structure is that thethat the latter ones have much higher melting point and boiling point.

latter ones have much higher melting point and boiling point. 1.

1. Different Different kinds kinds of of particlesparticles

All substances are comprised of tiny particles. Moreover, there are

All substances are comprised of tiny particles. Moreover, there are huge number of different substances in thehuge number of different substances in the world, logically, there must also be

world, logically, there must also be huge number of different particles. For simplicity, all huge number of different particles. For simplicity, all particles can beparticles can be classified into 3 categories :

classified into 3 categories : atom

atom – – smallest smallest particle particle of of an an element element e.g. e.g. oxygen oxygen atom atom O, O, chlorine chlorine atom atom ClCl molecule –

molecule – group group of of atoms e.g. atoms e.g. oxygen oxygen molecule O=O, molecule O=O, water molecule water molecule H–O–HH–O–H ion

(2)

--2.

2. Constituent Constituent particles particles of of different different substancessubstances

* Silicon(IV) oxide does not consist of individual molecules * Silicon(IV) oxide does not consist of individual molecules

The properties of a

The properties of a substance is related to the structure of the substance instead of the substance is related to the structure of the substance instead of the bonding only.bonding only. Structure = Particles + Bonding + Arrangement

Structure = Particles + Bonding + Arrangement

B.

B. Origin Origin of of different different physical physical propertiesproperties 1. Hardness

1. Hardness

Hardness is a measure of Hardness is a measure of how easy a solid can be how easy a solid can be broken.

broken.

Hardness depends on the Hardness depends on the strength of attractions strength of attractions holding the particles holding the particles together.

together.

2.

2. Melting Melting point point and and boiling boiling pointpoint

Similar to hardness, melting point and boiling point are measur

Similar to hardness, melting point and boiling point are measures of es of the strength of attractions among thethe strength of attractions among the particles.

particles.

If the attractions are strong, a higher temperature will be required to set the particles free. Thus, the melting point If the attractions are strong, a higher temperature will be required to set the particles free. Thus, the melting point and the boiling point would be

(3)

3.

3. Electrical Electrical conductivityconductivity Electrical conductivity depend

Electrical conductivity depends on the presence of s on the presence of free charge carriers.free charge carriers. There is two types of free charge carriers :

There is two types of free charge carriers : In metal - free electrons

In metal - free electrons

In liquid or aqueous electrolyte - mobile ions In liquid or aqueous electrolyte - mobile ions 4. Solubility

4. Solubility

A solid (solute) is soluble in a solvent only if the A solid (solute) is soluble in a solvent only if the attraction between the solvent particle and the solute attraction between the solvent particle and the solute particle is stronger than solute-solute attraction and particle is stronger than solute-solute attraction and

solvent-solvent-solvent atsolvent attraction.traction.

solvent-solute >> solute-solute & solvent-solvent solvent-solute >> solute-solute & solvent-solvent

If the attraction between the solvent and solute particles If the attraction between the solvent and solute particles is strong enough, the mobile solvent particles will is strong enough, the mobile solvent particles will bebe able to pull the solute particles from the

able to pull the solute particles from the solid structure.solid structure. Eventually, all the solute particles will

Eventually, all the solute particles will be pulled apartbe pulled apart and distributed evenly in the

and distributed evenly in the solvent.solvent.

The homogeneous mixture obtained is called a

The homogeneous mixture obtained is called a solutionsolution and the process is called dissolution.

and the process is called dissolution.

If the solute- solute attraction is stronger, the solvent particles will

If the solute- solute attraction is stronger, the solvent particles will not be able to pull not be able to pull the solute particles apart.the solute particles apart. If the solvent-solven

If the solvent-solvent attraction is stronger, the t attraction is stronger, the solvent particles will prefer to stick solvent particles will prefer to stick together instead of pulling thetogether instead of pulling the solute particles.

solute particles.

LIKE DISSOLVES LIKE LIKE DISSOLVES LIKE Common salt (Na

Common salt (Na++ClCl--(s)(s)) is only soluble in water but not in organic solvent.) is only soluble in water but not in organic solvent.

The dipole-ion interaction between the water The dipole-ion interaction between the water molecule and sodium ion or chloride ion is strong molecule and sodium ion or chloride ion is strong enough to pull the ions apart.

enough to pull the ions apart.

The attraction between ions and polar solvent molecules are rather weak (most of the organic solvent are The attraction between ions and polar solvent molecules are rather weak (most of the organic solvent are non-polar). Therefore, ionic compound is not soluble in non-polar solvent.

polar). Therefore, ionic compound is not soluble in non-polar solvent.

Nevertheless, not all ionic compounds are soluble in water. e.g. calcium sulphate, Ca

Nevertheless, not all ionic compounds are soluble in water. e.g. calcium sulphate, Ca2+2+SOSO442-2-(s)(s), is insoluble in, is insoluble in

water.

water. The The doubly charged doubly charged CaCa2+2+ion and SOion and SO44

2-ion make the 2-ionic bond too strong to be broken by the dipole-2-ion ion make the ionic bond too strong to be broken by the dipole-ion interaction.

interaction.

Similarly, plastic (a molecular solid) is soluble in o

Similarly, plastic (a molecular solid) is soluble in organic solvent but not in wrganic solvent but not in water. ater. In water, the attraction amongIn water, the attraction among water molecules is even stronger than t

water molecules is even stronger than t he attraction between water molecule and he attraction between water molecule and plastic molecule.plastic molecule.

A rule can be generalized from the above deduction - "Like Dissolves Like". A polar solute (ionic compound) is A rule can be generalized from the above deduction - "Like Dissolves Like". A polar solute (ionic compound) is only solu

(4)

C.

C. Physical prPhysical properties of operties of covalent ancovalent and ionic d ionic substancessubstances The physical properties of a

The physical properties of a substance mainly depends on the strength of the substance mainly depends on the strength of the forces among the particles.forces among the particles. I.

I. Ionic Ionic substancesubstance

For ionic compound, the attraction among the ions are strong electrostatic attractions. Because the attractions are For ionic compound, the attraction among the ions are strong electrostatic attractions. Because the attractions are strong, all ionic compounds have high melting point and boiling point. e.g. sodium chloride (m.p. 801 º

strong, all ionic compounds have high melting point and boiling point. e.g. sodium chloride (m.p. 801 º C)C) II.

II. Covalent Covalent substancesubstance

For covalent substance, there are

For covalent substance, there are two possibilities.two possibilities. 1.

1. For covalent For covalent substances with substances with giant covalent giant covalent structure (e.g. structure (e.g. diamond andiamond and quartz), d quartz), there is onthere is only strong ly strong covalentcovalent bond among the atoms. Therefore, all giant covalent structures also have high melting point and boiling bond among the atoms. Therefore, all giant covalent structures also have high melting point and boiling point.

point. e.g. diamon

e.g. diamond (m.p. 3550 ºd (m.p. 3550 º C) and quartz (m.C) and quartz (m.p. 1610ºp. 1610º C)C)

2.

2. But for But for those cothose covalent suvalent substance wbstance with molecuith molecular struclar structure (e.gture (e.g. water . water HH22O), there are two kinds of forcesO), there are two kinds of forces

acting among the particles. acting among the particles. a.

a. Strong Strong covalent covalent bond bond among among the atoms the atoms - strong - strong intramolecular forces intramolecular forces (intra - (intra - within).within). b.

b. Weak attracWeak attraction fortion forces among ces among the molecuthe molecules - weak les - weak intermolecular fintermolecular forces (inter orces (inter - betw- between).een). e.g. wate

e.g. water (m.p. 0 ºr (m.p. 0 º C)C)

strong intramolecular forces

(O-H covalent bond)

weak intermolecular forces

O

H

O

H

O

H

δ

--δ

δ

+

δ

+

δ

+

δ

--δ

δ

--This explain why water can be turned into steam easily but will not be decomposed into hydrogen and oxygen This explain why water can be turned into steam easily but will not be decomposed into hydrogen and oxygen by heating. (Decomposition of water molecule involves breaking of the strong O–H bond).

(5)

When the temperature is lowered to 0 º

When the temperature is lowered to 0 º C, water turns into ice. At low temperature, the kinetic energyC, water turns into ice. At low temperature, the kinetic energy posses

possessed by the water particle is not enough to sed by the water particle is not enough to offset the attraction forces among the offset the attraction forces among the water molecule. Thewater molecule. The water molecules stick together and become ice.

water molecules stick together and become ice.

Physical state of a

Physical state of a substancesubstance

Although the intermolecular forces are weak, molecular substance can be a solid at low temperature. The Although the intermolecular forces are weak, molecular substance can be a solid at low temperature. The molecules are arranged regularly in the solid to form a

molecules are arranged regularly in the solid to form a molecular crystal.molecular crystal.

The weak intermolecular forces holding the molecules in iodine, sulphur and dry ice

The weak intermolecular forces holding the molecules in iodine, sulphur and dry ice are called van der Waals'are called van der Waals' forces.

forces.

Structure of iodine crystal Structure of iodine crystal

D.

D. Difference Difference between ionbetween ionic compounic compound and d and covalent sucovalent substancebstance Structu

Structure re Giant Giant ionic ionic structure structure Giant Giant covalent covalent structure structure Simple Simple molecular molecular structurestructure Constituent

Constituent elements elements

metal

metal / / non-metal non-metal system system non-metal non-metal / / non-metalnon-metal system system non-metal / non-metal non-metal / non-metal system system Attraction Attraction involved involved

strong ionic bond among strong ionic bond among oppositely charged ion oppositely charged ion

strong

strong covalent covalent among among atoms atoms strong strong intramolecularintramolecular covalent bond but weak covalent bond but weak intermolecular forces intermolecular forces Boiling point and

Boiling point and melting point melting point

High High Low

Hardness

Hardness Hard Hard Hard Hard Usually Usually liquid liquid or or gasgas Solubilit

Solubility y Soluble Soluble in in water water butbut

insoluble in organic solvent insoluble in organic solvent

Insoluble in all kinds of Insoluble in all kinds of solvent

solvent

Usually soluble in organic Usually soluble in organic solvent but insoluble in solvent but insoluble in water. water. Electrical Electrical conductivity conductivity Conduct electricity in Conduct electricity in aqueous or molten state aqueous or molten state

Do not conduct electricity Do not conduct electricity except graphite

except graphite

Do not conduct electricity Do not conduct electricity

Glossary

simple simple molecular molecular structure structure macromolecule macromolecule giant giant ionic ionic structure structure giant giant covalent covalent structurestructure giant

giant metallic metallic structure structure hardness hardness melting point melting point boiling boiling point point solubilitysolubility homogeno

homogenous us mixture mixture solute solute solvent solvent solution solution dissolution dissolution free free charge charge carrierscarriers intramolecular

intramolecular forces forces intermolecular intermolecular forces forces kinetic kinetic energy energy molecular molecular crystalcrystal van

van der der Waals' Waals' forces forces polar polar molecule molecule non-polar non-polar molecule molecule like like dissolves dissolves likelike dipole-ion interaction

(6)

Past Paper

Questions

91 I 1 a v 91 I 1 a v 93 I 2 b iii 93 I 2 b iii 94 I 7 b i 94 I 7 b i 98 I 7 a i 98 I 7 a i 99 I 4 99 I 4 91 I 1 a v 91 I 1 a v 1a

1a The The following following is is a a part part of of the the Periodic Periodic Table:Table:

Group Group I

I II II III III IV IV V V VI VI VII VII 00 Second

Second Period Period a a bb

Third

Third Period Period c c d d e e f f gg

Referring to the letters indicated in the above table,

Referring to the letters indicated in the above table, answer the following questionsanswer the following questions v

v Element Element e can e can form form compouncompounds ds with with elements elements a ana and d c sepc separately.arately. (1)

(1) Draw the Draw the electronielectronic structures of c structures of these two compounds, showing these two compounds, showing the outermost electrons ONLY.the outermost electrons ONLY. (2)

(2) Which of these two coWhich of these two compounds has a hmpounds has a higher melting point ? Expigher melting point ? Explain your answer.lain your answer.

4 4 (1) (1) 1 + 1 marks 1 + 1 marks (Accept C for a, Na for c and S for e.)

(Accept C for a, Na for c and S for e.) (2)

(2) Compound foCompound formed between c and e has a higher m.p. because it is an ionic compound and the ions are heldrmed between c and e has a higher m.p. because it is an ionic compound and the ions are held by

by strong strong electrostatic electrostatic forces forces / / strong strong ionic ionic bond. bond. 1 1 mark mark Compound formed between a and e is a covalent compound and the molecules are held by weak van der Compound formed between a and e is a covalent compound and the molecules are held by weak van der Waals'

Waals' forces forces (intermolecular (intermolecular forces). forces). 1 1 mark mark C

C (1) The (1) The electronic electronic diagram diagram for for the the compound compound cc22e was incorrectly drawn by many candidates as below:e was incorrectly drawn by many candidates as below:

(2)

(2) A common misconcepA common misconception among candidates was that a covalent botion among candidates was that a covalent bond is weaker than an ionic bond. Thend is weaker than an ionic bond. The difference between melting points of the two compounds c

difference between melting points of the two compounds c22e and aee and ae22should be explained in terms of should be explained in terms of thethe

forces between ions in the former compound and those between molecules in the forces between ions in the former compound and those between molecules in the latter.latter. 93 I 2 b iii

93 I 2 b iii 2b

2b Physical Physical properties properties of of substances substances depend depend mainly mainly on on the the types types of of binding binding force force between between their their constituenconstituent t particlesparticles.. iii

iii Explain Explain why why tetrachloromethane tetrachloromethane does does not not conduct conduct electricity electricity in in liquid liquid state. state. 11 Tetrachloromethane which is a

Tetrachloromethane which is a covalent compound, does not possess any mobcovalent compound, does not possess any mobile electrons (ions) / exists asile electrons (ions) / exists as discrete

discrete molecules molecules in in liquid liquid state, state, therefore therefore is is not not an an electrical electrical conductor. conductor. 1 1 mark mark C

(7)

94 I 7 b i 94 I 7 b i 7b

7b The The table table below below lists lists some some physical physical properties properties of of lead, lead, bromine bromine and and lead(II) lead(II) bromide.bromide. Lead

Lead BromiBromine ne Lead(II) Lead(II) bromidebromide Melting

Melting point point 328 328 ºº C C -7 -7 ºº C C 370 370 ºº CC Electrical

Electrical conductivity conductivity in in the the solid solid state state Conducting Conducting Non-conducting Non-conducting Non-conduNon-conductingcting Electrical

Electrical conductivity conductivity in in the the liquid liquid state state Conducting Conducting Non-conducting Non-conducting —— i Explain

i Explain the the difference difference in in melting melting points points between between bromine bromine and and lead(II) lead(II) bromide. bromide. 22 Bromide exists as simple mo

Bromide exists as simple molecules with weak intermolecules with weak intermolecular attraction / van der Waal’lecular attraction / van der Waal’ s force.s force.∴∴it has low b.p.it has low b.p. (Do

(Do NOT NOT accept accept bromine bromine has has weak weak covalent covalent bond.) bond.) 1 1 mark mark In Lead(II) bromide , the attraction between ions is strong / ionic bond / PbBr

In Lead(II) bromide , the attraction between ions is strong / ionic bond / PbBr22 has strong electrostatic attraction.has strong electrostatic attraction.

∴

∴_it_{it has}_{has higher}_{higher a}_{a m.p.}_m.p. ₁_{1 mark}_mark

(Do NOT accept lead(II) bromide is an ionic compound / ionic

(Do NOT accept lead(II) bromide is an ionic compound / ionic structure)structure) C

C Many Many candidates candidates did did not not mention mention in in their their explanation explanation that that the the van van der der Waals' Waals' forces forces between between bromine bromine moleculesmolecules were weak.

were weak. 98 I 7 a i

98 I 7 a i 7a

7a Both carbon Both carbon and and silicon silicon are are Group Group IV IV elements elements in in the the Periodic Periodic Table. Table. The The diagrams diagrams below below show show the the structures structures of of dry ice (solid carbon dioxide) and quartz (a form of silicon dioxide):

dry ice (solid carbon dioxide) and quartz (a form of silicon dioxide):

i

i With refWith reference to erence to the structuthe structures of res of the two the two substances, substances, explain whexplain why quary quartz is tz is a solid a solid which mwhich melts at a elts at a highhigh temperature, while carbon dioxide is a gas at

temperature, while carbon dioxide is a gas at room temperature.room temperature.

3 3

99 I 4 99 I 4 4

4 With With the the help help of of electronic electronic diagrams, diagrams, describe describe the the formation formation of of magnesium magnesium chloride chloride and and tetrachloromethanetetrachloromethane from atoms of relevant elements. State, with

from atoms of relevant elements. State, with explanation, which of the two compounds has a higher meltingexplanation, which of the two compounds has a higher melting point.

(8)

90 25 90 25 D

D 25 25 Bromine Bromine has has a a low low melting melting point point becausebecause A.

A. it it is is a a non-metal.non-metal. B.

B. it is it is a mea member mber of of the hthe halogen alogen family.family. C.

C. the atoms the atoms in each in each bromine bromine molecule molecule are bonare bondedded together by a covalent bond.

together by a covalent bond. D.

D. the bromthe bromine molecules ine molecules are attracted are attracted together btogether by van y van derder Waals' forces.

Waals' forces. 92 2

92 2 D

D 2 2 X X and and Y Y are are elements. elements. The The melting melting points points of of their their chlorideschlorides are given below:

are given below:

Meltin

Melting poing point (ºt (º C)C) Chloride

Chloride of of X X 772772 Chloride

Chloride of of Y Y -68-68

Which of the following statements is correct ? Which of the following statements is correct ? A.

A. Both Both X X and and Y Y are are metals.metals. B.

B. The chloThe chloride of ride of Y is Y is a solid a solid at room at room temperature.temperature. C.

C. The chloThe chloride of ride of X condX conducts electricity ucts electricity in the in the solid state.solid state. D.

D. The The chloride of chloride of Y Y is is a a covalent compound.covalent compound. 94 46

94 46 B

B 46 46 Hydrogen Hydrogen chloride chloride has has a a lower lower melting melting point point than than sodiumsodium chloride.

chloride.

In each molecule of hydrogen chloride, a hydrogen atom In each molecule of hydrogen chloride, a hydrogen atom and a chlorine atom are joined together by a covalent bond. and a chlorine atom are joined together by a covalent bond. 95 39

95 39 A

A 39 39 Which Which of of the the following following substances substances can can conduct conduct electricelectricity ity ?? (1)

(1) molten molten zinc zinc chloridechloride (2)

(2) an aqueouan aqueous solution s solution of magnesiuof magnesium sulphatem sulphate (3)

(3) a mixture a mixture of ethanoof ethanol and wl and waterater A.

A. (1) (1) and and (2) (2) onlyonly B.

B. (1) (1) and and (3) (3) onlyonly C.

C. (2) (2) and and (3) (3) onlyonly D.

D. (1), (1), (2) (2) and and (3)(3) 96 45

96 45 B

B 45 45 The The melting melting point point of of hydrogen hydrogen chloride chloride is is lower lower than than that that of of potassium chloride.

potassium chloride.

Hydrogen chloride is a covalent compound whereas Hydrogen chloride is a covalent compound whereas potassium chloride is an

potassium chloride is an ionic compound.ionic compound. 96 50

96 50 C

C 50 50 Both Both dry dry ice ice and and quartz quartz exist exist in in the the form form of of discretediscrete molecules.

molecules.

Carbon and silicon atoms have the

Carbon and silicon atoms have the same number of same number of electrons in their outermost shells.

electrons in their outermost shells. 97 3

97 3 D

D 3 3 Argon Argon exists exists as as a a gas gas at at room room temperature temperature and and pressurepressure because

because A.

A. argon argon molecules molecules are are monoatomic.monoatomic. B.

B. argon argon is is chemically chemically inert.inert. C.

C. the outermosthe outermost electron t electron shell of shell of an argoan argon atom n atom has anhas an octet structure.

octet structure. D.

(9)

98 33 98 33 B

B 33 33 Consider Consider the the following following information:information: Substance Melting Substance Melting point/° point/° C C Electrical Electrical conductivity at conductivity at room temperature room temperature Solubility in Solubility in water water W

W -34 -34 poor poor slightly slightly solublesoluble X

X 44 44 poor poor insolubleinsoluble Y

Y 232 232 good good insolubleinsoluble Z

Z 782 782 poor poor very very solublesoluble Which of the above substances exists as a

Which of the above substances exists as a simple molecularsimple molecular solid at room temperature ?

solid at room temperature ? A. A. WW B. B. XX C. C. YY D. D. ZZ 99 11 99 11 A

A 11 11 The The table table below below shows shows the the ability ability of of four four substances substances W, W, X,X, Y and Z to conduct electricity.

Y and Z to conduct electricity. (In the table,

(In the table,√√and X respectively represent 'can and X respectively represent 'can conductconduct electricity' and 'cannot conduct electricity'.)

electricity' and 'cannot conduct electricity'.) Substance

Substance Solid Solid State State Liquid Liquid State State Aqueous Aqueous SolutionSolution W W XX √√ √√ X X X X XX √√ Y Y X X X X XX Z

Z √√ √√ (insoluble in water)(insoluble in water) Which of the substances is likely to be zinc chloride ? Which of the substances is likely to be zinc chloride ? A. A. WW B. B. XX C. C. YY D. D. ZZ 99 34 99 34 D

D 34 34 Iodine Iodine is is a a solid solid at at room room temperature temperature and and pressure. pressure. WhichWhich of the following statements concerning the structure of of the following statements concerning the structure of iodine is/are correct ?

iodine is/are correct ? (1)

(1) Iodine hIodine has a as a giant covalent giant covalent structure.structure. (2)

(2) Iodine moleIodine molecules are held togcules are held together by van dether by van der Waals'er Waals' forces.

forces. (3)

(3) Iodine atoIodine atoms are held ms are held together in ptogether in pairs by covairs by covalentalent bonds. bonds. A. A. (1) (1) onlyonly B. B. (2) (2) onlyonly C.

C. (1) (1) and and (3) (3) onlyonly D.