1-6 Ionic And Covalent Substances

VI. Ionic and covalent substances

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Topic

VI.

Ionic and covalent substances

Reference Reading

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

Objectives

1.6 – describe the differences in hardness, melting point boiling point, solubility and electrical conductivity between ionic and covalent substances – explain the differences in physical properties between ionic and covalent substances in terms of the nature of forces holding the particles together: (a) in solid ionic substances, e.g. sodium chloride and copper(II) sulphate, consist of giant lattices of ions held together by strong electrostatic forces (giant ionic structure) (b) some solid covalent substances e.g. iodine, sulphur and dry ice, consist of a regular arrangement of molecules held together by weak forces called van der Waals forces (simple molecular structure) (c) some solid covalent substances e.g. diamond and quartz, consist of a giant lattice of atoms held together by strong covalent bonding (giant covalent structure) – predict the structures and properties of compounds when the group numbers of constituent elements are given

Notes

VI. Ionic and covalent substances A. Structures of different substances

The most obvious difference between substances with molecular structure and those with giant structure is that the latter ones have much higher melting point and boiling point. 1.

Different kinds of particles

All substances are comprised of tiny particles. Moreover, there are huge number of different substances in the world, logically, there must also be huge number of different particles. For simplicity, all particles can be classified into 3 categories : atom – smallest particle of an element e.g. oxygen atom O, chlorine atom Cl molecule – group of atoms e.g. oxygen molecule O=O, water molecule H–O–H ion – charged atom or molecule e.g. oxide ion O2-, chloride ion Cl-, hydroxide ion H–O-

VI. Ionic and covalent substances 2.

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Constituent particles of different substances

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

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

B. Origin of different physical properties 1.

Hardness Hardness is a measure of how easy a solid can be broken. Hardness depends on the strength of attractions holding the particles together.

2.

Melting point and boiling point

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

VI. Ionic and covalent substances 3.

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Electrical conductivity

Electrical conductivity depends on the presence of free charge carriers. There is two types of free charge carriers : In metal - free electrons In liquid or aqueous electrolyte - mobile ions 4.

Solubility

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

If the attraction between the solvent and solute particles is strong enough, the mobile solvent particles will be able to pull the solute particles from the solid structure. Eventually, all the solute particles will be pulled apart and distributed evenly in the solvent. The homogeneous mixture obtained is called a solution and the process is called dissolution.

If the solute- solute attraction is stronger, the solvent particles will not be able to pull the solute particles apart. If the solvent-solvent attraction is stronger, the solvent particles will prefer to stick together instead of pulling the solute particles. LIKE DISSOLVES LIKE Common salt (Na+Cl-(s)) is only soluble in water but not in organic solvent.

The dipole-ion interaction between the water molecule and sodium ion or chloride ion is strong enough to pull the ions apart. The attraction between ions and non-polar solvent molecules are rather weak (most of the organic solvent are nonpolar). Therefore, ionic compound is not soluble in non-polar solvent. Nevertheless, not all ionic compounds are soluble in water. e.g. calcium sulphate, Ca2+SO42-(s), is insoluble in water. The doubly charged Ca2+ ion and SO42- ion make the ionic bond too strong to be broken by the dipole-ion interaction. Similarly, plastic (a molecular solid) is soluble in organic solvent but not in water. In water, the attraction among water molecules is even stronger than the attraction between water molecule and plastic molecule. A rule can be generalized from the above deduction - "Like Dissolves Like". A polar solute (ionic compound) is only soluble in polar solvent (water) while a non-polar solute is only soluble in non-polar solvent.

VI. Ionic and covalent substances

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C. Physical properties of covalent and ionic substances The physical properties of a substance mainly depends on the strength of the forces among the particles. I. Ionic substance 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 ºC) II. Covalent substance For covalent substance, there are two possibilities. 1. For covalent substances with giant covalent structure (e.g. diamond and quartz), there is only strong covalent bond among the atoms. Therefore, all giant covalent structures also have high melting point and boiling point. e.g. diamond (m.p. 3550 ºC) and quartz (m.p. 1610ºC)

2.

But for those covalent substance with molecular structure (e.g. water H2O), there are two kinds of forces acting among the particles. a. Strong covalent bond among the atoms - strong intramolecular forces (intra - within). b. Weak attraction forces among the molecules - weak intermolecular forces (inter - between). e.g. water (m.p. 0 ºC)

δ-

O

weak intermolecular forces

H

δ-

O H

δ+

H

δ+

H strong intramolecular forces (O-H covalent bond)

δ-

O H

δ+

H

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).

VI. Ionic and covalent substances

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When the temperature is lowered to 0 ºC, water turns into ice. At low temperature, the kinetic energy possessed by the water particle is not enough to offset the attraction forces among the water molecule. The water molecules stick together and become ice.

Physical state of a substance 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 molecular crystal. The weak intermolecular forces holding the molecules in iodine, sulphur and dry ice are called van der Waals' forces.

Structure of iodine crystal

D. Difference between ionic compound and covalent substance

Glossary

Structure Constituent elements Attraction involved

Giant ionic structure metal / non-metal system

Boiling point and melting point Hardness Solubility

High

High

Hard Soluble in water but insoluble in organic solvent

Hard Insoluble in all kinds of solvent

Electrical conductivity

Conduct electricity in aqueous or molten state

Do not conduct electricity except graphite

strong ionic bond among oppositely charged ion

Giant covalent structure non-metal / non-metal system strong covalent among atoms

Simple molecular structure non-metal / non-metal system strong intramolecular covalent bond but weak intermolecular forces Low Usually liquid or gas Usually soluble in organic solvent but insoluble in water. Do not conduct electricity

simple molecular structure macromolecule giant ionic structure giant covalent structure giant metallic structure hardness melting point boiling point solubility homogenous mixture solute solvent solution dissolution free charge carriers intramolecular forces intermolecular forces kinetic energy molecular crystal van der Waals' forces polar molecule non-polar molecule like dissolves like dipole-ion interaction

VI. Ionic and covalent substances

Past Paper Questions

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

91 I 1 a v 1a The following is a part of the Periodic Table: Group

v

C

I II III IV V VI VII Second Period a b Third Period c d e f Referring to the letters indicated in the above table, answer the following questions Element e can form compounds with elements a and c separately. (1) Draw the electronic structures of these two compounds, showing the outermost electrons ONLY. (2) Which of these two compounds has a higher melting point ? Explain your answer. (1)

0 g 4

1 + 1 marks (Accept C for a, Na for c and S for e.) (2) Compound formed between c and e has a higher m.p. because it is an ionic compound and the ions are held by strong electrostatic forces / strong ionic bond. 1 mark Compound formed between a and e is a covalent compound and the molecules are held by weak van der Waals' forces (intermolecular forces). 1 mark (1) The electronic diagram for the compound c2e was incorrectly drawn by many candidates as below:

(2) A common misconception among candidates was that a covalent bond is weaker than an ionic bond. The difference between melting points of the two compounds c2e and ae2 should be explained in terms of the forces between ions in the former compound and those between molecules in the latter. 93 I 2 b iii 2b Physical properties of substances depend mainly on the types of binding force between their constituent particles. iii Explain why tetrachloromethane does not conduct electricity in liquid state. Tetrachloromethane which is a covalent compound, does not possess any mobile electrons (ions) / exists as discrete molecules in liquid state, therefore is not an electrical conductor. 1 mark C Many candidates wrongly regarded 'liquid CCl4' as 'an aqueous solution of CCl4'.

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VI. Ionic and covalent substances Page 7 94 I 7 b i 7b The table below lists some physical properties of lead, bromine and lead(II) bromide. Lead Bromine Lead(II) bromide Melting point 328 ºC -7 ºC 370 ºC Electrical conductivity in the solid state Conducting Non-conducting Non-conducting Electrical conductivity in the liquid state Conducting Non-conducting — i Explain the difference in melting points between bromine and lead(II) bromide. 2 Bromide exists as simple molecules with weak intermolecular attraction / van der Waal’s force. ∴ it has low b.p. (Do NOT accept bromine has weak covalent bond.) 1 mark In Lead(II) bromide , the attraction between ions is strong / ionic bond / PbBr 2 has strong electrostatic attraction. ∴ it has higher a m.p. 1 mark (Do NOT accept lead(II) bromide is an ionic compound / ionic structure) C Many candidates did not mention in their explanation that the van der Waals' forces between bromine molecules were weak. 98 I 7 a i 7a Both carbon and silicon are Group IV elements in the Periodic Table. The diagrams below show the structures of dry ice (solid carbon dioxide) and quartz (a form of silicon dioxide):

i

99 I 4 4

With reference to the structures of the two substances, explain why quartz is a solid which melts at a high temperature, while carbon dioxide is a gas at room temperature.

With the help of electronic diagrams, describe the formation of magnesium chloride and tetrachloromethane from atoms of relevant elements. State, with explanation, which of the two compounds has a higher melting point.

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VI. Ionic and covalent substances 90 25 D 25 Bromine has a low melting point because A. it is a non-metal. B. it is a member of the halogen family. C. the atoms in each bromine molecule are bonded together by a covalent bond. D. the bromine molecules are attracted together by van der Waals' forces.

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92 2 D

94 46 B

95 39 A

96 45 B

96 50 C

2

X and Y are elements. The melting points of their chlorides are given below: Melting point (ºC) Chloride of X 772 Chloride of Y -68 Which of the following statements is correct ? A. Both X and Y are metals. B. The chloride of Y is a solid at room temperature. C. The chloride of X conducts electricity in the solid state. D. The chloride of Y is a covalent compound.

46 Hydrogen chloride has a lower melting point than sodium chloride.

39 Which of the following substances can conduct electricity ? (1) molten zinc chloride (2) an aqueous solution of magnesium sulphate (3) a mixture of ethanol and water A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

45 The melting point of hydrogen chloride is lower than that of Hydrogen chloride is a covalent compound whereas potassium chloride. potassium chloride is an ionic compound.

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

97 3 D

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

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Argon exists as a gas at room temperature and pressure because A. argon molecules are monoatomic. B. argon is chemically inert. C. the outermost electron shell of an argon atom has an octet structure. D. the attractive force between argon atoms is weak.

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

VI. Ionic and covalent substances 98 33 B 33 Consider the following information: Substance Melting Electrical Solubility in point/° conductivity at water C room temperature W -34 poor slightly soluble X 44 poor insoluble Y 232 good insoluble Z 782 poor very soluble Which of the above substances exists as a simple molecular solid at room temperature ? A. W B. X C. Y D. Z 99 11 A

99 34 D

11 The table below shows the ability of four substances W, X, Y and Z to conduct electricity. (In the table, √ and X respectively represent 'can conduct electricity' and 'cannot conduct electricity'.) Substance Solid State Liquid State Aqueous Solution W X √ √ X X X √ Y X X X Z (insoluble in water) √ √ Which of the substances is likely to be zinc chloride ? A. W B. X C. Y D. Z

34 Iodine is a solid at room temperature and pressure. Which of the following statements concerning the structure of iodine is/are correct ? (1) Iodine has a giant covalent structure. (2) Iodine molecules are held together by van der Waals' forces. (3) Iodine atoms are held together in pairs by covalent bonds. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only

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