HKCEE & HKAL Past Paper Questions: Part 12 Patterns in Chemical World 1. HKAL 1988 I Elements exhibit periodic behaviour as their atomic number increases. Illustrate graphically the periodicity of one such properties of the elements from hydrogen (Z=1) to argon

(Z=18). (2 marks)

2. HKAL 1990 II Q4b Account for the following observation. SiO2 is a solid with a high melting point, whereas CO2 is a gas at room temperature.

(4 marks)

SiO2: three-dimensional network solid with very strong Si-O bonds. (1M + 1M) CO2: exists as discrete molecules with only weak intermolecular van der Waals forces. (1M + 1M)

3. HKAL 1991 II Q4a (i) Briefly explain what is meant by periodicity in the chemical properties of the elements. (ii) Draw diagrams showing the variations in melting point (or boiling point) for the elements from lithium to

argon. (5 marks)

4. HKAL 1998 I Q3b Sketch the trends for the properties below, and account for the trend in each case. (i) melting point of the alkali metals, Li, Na and K (ii) boiling point of the period 3 elements, Na, Mg and Al.

(4 marks)

5. HKAL 1999 I Q1b Account for the following: At 298K and 1 atm pressure, carbon dioxide is a gas whereas silicon dioxide is a solid.

(1 mark)

6.

HKAL 2002The graph b

Explain wh(i) silico(ii) the bo (iii) there non- m(iv) the m

2 II Q2c below shows

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(9 mar

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7.

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(5 mar

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8. HKAL 2006 I Q1a The table below lists the melting points of three oxides of the Period 3 elements:

Oxide Na2O Al2O3 SO2

Melting point / 920 2040 75 Account for the large difference in the melting points of the three oxides.

(3 marks)

9. HKAL 2008 II Q1b Both sodium and chlorine are elements in Period 3 of the Periodic Table. (i) At room temperature and atmosphere pressure, ONa 2 is a solid with a very high melting point

whereas OCl2 is a gas. Account for this difference in property between ONa 2 and OCl2 . (ii) 22ONa is another oxide of sodium. Write the chemical equation for the reaction of ONa 2 with water

and that of 22ONa with water. (iii) 72OCl is another oxide of chlorine. Give the products formed when OCl2 and 72OCl are treated

separately with water. (6 marks)

10. HKAL 1990 I Q3d Illustrate, with examples using nickel, two characteristic properties of transition metals.

(2 marks)

Catalytic property: the use of nickel catalyst in the hydrogenation of oils to make margarine. Variable oxidation states: Ni2O3, NiO, NiO2 etc Form coloured ions: Ni2+(aq) is green