Hong Kong Chinese Women’s Club College

Chemistry

Form 4 Bonding-Ex1

Answer in complete sentences wherever appropriate.

1. a) The table below lists some physical properties for potassium chloride and tetrachloromethane, CCl4.

potassium chloride / tetrachloromethane
Melting point /oC / 776 / - 23
Solubility in water at 25o C / g per 100 cm3 / 32 / Insoluble

i) Explain the difference in melting point between potassium chloride and tetrachloromethane in terms of the nature of forces between particles.

ii) Draw the electronic structures of potassium chloride and tetrachloromethane, indicating the arrangement of electrons in the outermost shells only.

iii) 32 g of potassium chloride, 32 g of tetrachloromethane and 100 cm3 of water

are shaken together in a stoppered flask at 25oC and then allowed to settle. After a few minutes, what would be observed ?

b) The various properties of the substances are given in the following table :

Substances / Melting point / Action with water / Electrical conductivity
Silicon(IV) oxide
SiO2 / 1700oC / Insoluble / Non-conductor
Sulphur dioxide
SO2 / - 75oC / Reacts with water
and dissolves / Non-conductor
Calcium oxide
CaO / 2600oC / Slightly soluble / Electrolysis occurs
Glucose
C6H12O6 / 146oC / Soluble / Non-conductor
Copper, Cu / 1083oC / Insoluble / Conducts well without
any decomposition

i) Classify the above substances into those having the following structures :

A) giant ionic structure,

B) giant covalent structure, and

C) simple molecular structure.

State reasons for your choice.

ii) Briefly explain why solid copper can conduct electricity at room temperature.

iii) Briefly explain molten calcium oxide can conduct electricity.

Hong Kong Chinese Women’s Club College

Chemistry

Form 4 Bonding-Ex1

Suggested Answers

1. a) i) Potassium chloride, an ionic compound, has high melting point as their opposite charged ions are held strongly by electrostatic forces. A large amount of energy is needed to melt the ionic compound - potassium chloride. Tetrachloromethane is a covalent compound which molecules are held by weak van der Waals’ forces. It thus has a low melting point.

ii) potassium chloride tetrachloromethane

iii) There are two separate (immiscible) layers. Tetrachloromethane forms the bottom oily layer. Potassium chloride dissolves completely in the upper colourless aqueous layer.

b) i) A) giant ionic structure : calcium oxide

Calcium oxide, an ionic compound with high melting point, and cannot conduct electricity in molten state.

B) giant covalent structure : silicon (IV) oxide.

Silicon(IV) oxide is in giant covalent lattice as it has high melting point and cannot conduct electricity either in solid or molten state.

C) Simple molecular structure : glucose and sulphur dioxide

They have low melting points. They cannot conduct electricity in molten state.

ii) Copper, a metal, can conduct electricity as it has mobile electrons.

iii) Solid calcium oxide does not have any free / mobile ions . Ions are fixed / held strongly by electrostatic forces and cannot move. When the ionic compound melts, ions become mobile and can conduct electric current.