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NSS Chemistry Part 6 Microscopic World II (HKAL Past paper questions) 1. [HKAL 1985 I Q1a] Define the term dative covalent bond. Give one example to illustrate your answer.

(2 marks)

2. [HKAL 1985 I Q3a] The boiling point of CH4 is less than that of SiH4. However the boiling points of NH3 and H2O are greater

than those of PH3 and H2S respectively. Account for both of these observations.

(4 marks)

3. [HKAL 1986 I Q1 a, b] a. Account for the fact that both CH4 and NH3 have shapes based on the tetrahedron.

(2 marks) b. Deduce the shape of BF3.

(2 marks)

Dative covalent bond is a covalent bond in which both electrons in the bond are donated by one atom. Examples: H3O+, NH4

+, NH3.BF3

Since SiH4 and CH4 are non-polar and SiH4 has a greater molecular mass than CH4, the van der Waals’ forces of SiH4 is higher than that of CH4 and hence SiH4 has a higher boiling point. But for NH3 and H2O, since N atom and O atom are very electronegative, therefore, the intermolecular forces in NH3 and H2O are H-bond. But for PH3 and H2S, even P atom and S atom are electronegative, but they are less electronegative than N atom and O atom, therefore, the intermolecular forces in PH3 and H2S are dipole-dipole attractions, which are weaker than H-bond. Therefore, NH3 and H2O have higher boiling points than PH3 and H2S respectively.

a. The C atom in CH4 and N atom in NH3 have 4 electron pairs and according to VSEPR Theory, the 4 electron pairs will be arranged in a tetrahedral shape.

b. BF3 has 3 bond pairs around the central B atom, therefore, the shape is trigonal planar. c. N has a greater electronegativity than C.

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4. [HKAL 1986 I Q1c] Suggest a reason for the difference in dipole moments exhibited by the two molecules below:

NH2 CH3

Dipole moment: 1.48 D 0.37 D (2 marks)

5. [HKAL 1988] Based on the arrangement of the electron pairs, draw the three-dimensional structures of each of the

following covalent compounds: (i) PF5 (ii) SF6 (iii) BrF5

(3 marks)

6. [HKAL 1989] Comment on the trend shown by the boiling points of Group VI hydrides listed below:

H2O, 100oC; H2S, -60oC; H2Se, -41oC; H2Te, -2oC

(2 marks)

Generally, the boiling points in a series increases with the molecular mass and this would put the boiling point of water below -60oC. The boiling point of water is high because of the extensive H-bond between the water molecules.

It is because N has a greater electronegativity than C.

S

F

F

FF

F

F

PCl

Cl

Cl

Cl

Cl

F

FF

F

F

Br

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7. [HKAL 1990] Arrange the following substances in order of increasing melting point: NaF, F2, HF. Explain your order in

terms of the bonding involved. (4 marks)

8. [HKAL 1991] Draw the molecular shapes of: (i) PCl5(g) (ii) SCl4(g)

(2 marks)

Melting involves the overcoming of forces between the units of which the solid is made. Two of the given solids: HF & F2 would be expected to form molecular solids; the other, NaF would be expected to form ionic solid. The attractive forces holding an ionic solid together are strong ionic bonds, hence NaF would be expected to have high melting point. (1) The attraction between F2 molecules is quite weak and due only to van der Waal's forces would be expected to have a low melting point.(1) Even though HF forms molecular solid but because of the presence of hydrogen bonding between H and neighbouring F atom, which can hold the molecules together more strongly and therefore, expected to have much higher melting point than F2. (1) m.p.: F2 < HF < NaF (1) For reference: NaF(993°C), HF(-83.1°C), F2(-219.6°C)

PCl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

S:

(ii)(i)

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9. [HKAL 1991] Arrange the following substances in order of increasing boiling points: C2H5Cl, CH4 and C2H6. Explain

your order by comparing the relative magnitudes and nature of the intermolecular forces. (4 marks)

10. [HKAL 1992] a. Describe the bonding and intermolecular forces in ice and in SiO2 solid.

b. What type of interactions must be overcome to melt these solids? (4 marks)

11. [HKAL 1993] For each of the following molecules, draw a three-dimensional structure and state the molecular geometry. (i) SiF4 (ii) OF2

(4 marks)

Si

F

FF

F

tetrahedral

(i)

OF F

bent / V-shaped

(ii)

a. Ice consists of covalent H2O molecules held together by hydrogen bonds. (0.5+0.5) SiO2 solid is an infinite three dimensional network solid in which each Si atom is covalently bonded in a tetrahedral arrangement to four O atoms. (0.5+0.5) b. Some of the hydrogen bonds must be overcome in order to melt ice. (1) For SiO2, some strong Si-O covalent bonds have to be broken when melts. (1)

CH4 < C2H6 < C2H5Cl (1) C2H5Cl has electronegative Cl atom and hence it has a permanent dipole moment, (1) while C2H6 is non-polar. Thus C2H5Cl is expected to have stronger intermolecular forces than C2H6, and a higher boiling point. (1) CH4 also is non-polar and since its molecular mass is smaller than that of C2H6, therefore, the van der Waals’ forces of CH4 is weaker than that of C2H6 and hence CH4 has a smaller boiling point than C2H6. (1)

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12. [HKAL 1993] Consider the following compound, X:

Suggest expected values for the bond angles α , β and γ.

13. [HKAL 1994] For each of the following molecules, draw a three-dimensional structure showing the positions of the bond

electron pairs and lone electron pairs (if any). In each case, state the molecular geometry and whether the molecule possesses a non-zero dipole moment.

(i) BF3 (ii) ClF3

(4 marks)

15. [HKAL 1994] a. Explain the term "hydrogen bonding". b. Draw a diagram of the structure of a compound which has hydrogen bonds. Indicate the hydrogen

bond(s) clearly. c. Explain why (i) the boiling point of CH4 is lower than that of SiH4, and (ii) the boiling point of NH3 is higher than that of PH3.

(5 marks)

(i) Trigonal planar; no dipole moment (ii) T-shaped; possesses a net dipole moment

B

F

FF

Cl

F

F

F

xxxx

α : 109o, β: 180o, γ:120o

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16. [HKAL 1995] For each of the following species, draw a three-dimensional structure showing the bond electron pairs and

lone electron pairs of the central atom. State the shape of the species in each case.

(i) ICl4- (ii) SCl2

(3 marks)

a.

b.

c.

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17. [HKAL 1996 I Q2b] For each of the following chemical species, draw a three-dimensional structure showing the bond electron

pairs and lone pair(s) of the central atom underlined. State the shape of the species in each case. (i) ClO3 (ii) NOF

(3 marks)

18. [HKAL 1996 I Q3c]

(ii) Suggest expected values for the bond angles α, β and γ.

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19. [HKAL 1996 II Q2b] Briefly describe each of the following intermolecular forces and illustrate your answer with an appropriate

example in each case. dipole-dipole interactions hydrogen bonding van der Waals’ forces

(4 marks)

20. [HKAL 1997 I Q1d] Explain why a. the boiling point of HF is higher than that of HCl; b. the boiling point of HI is higher than that of HBr.

(2 marks)

a.

b.

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21. [HKAL 1997 II Q1a] (i) Explain the terms “dipole” and “dipole moment”, using HBr as an example. (ii) Explain why the dipole moment of HF is greater than that of HI. (iii) State the effect of an electric field on molecules of the following compounds and explain the effect

in terms of dipole moment.

Cl

Cl

Cl

Cl

(7 marks)

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22. [HKAL 1998 I Q2b] Which compound, H2O or F2O, would you expect to have a higher boiling point? Explain your answer.

(2 marks)

23. [HKAL 1998 II Q1c] (i) Draw the three-dimensional structure of BF3.

(ii) BF3 reacts with NH3 to form an adduct, BF3•NH3. Account for the formation of the adduct and draw the three-dimensional structure.

(4 marks)

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24. [HKAL 1999 II Q3c] Consider the hydrides of three Period 3 elements: SiH4, PH3 and H2S (i) For each hydride, draw a three-dimensional structure showing the bond pairs and lone electron pair(s),

if any, of the central atom. (iii) State, with explanation, whether SiH4 or H2S has a higher boiling point.

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25. [HKAL 2000 I Q1] a. The diagram below show the arrangements of atoms, ions or molecules in four crystalline substances:

graphite, ice, iodine and sodium chloride. (i) Write the name of the substance for each structure in the space provided. (ii) Label, on the diagrams, the types of interactions that are present in these substances.

(6 marks)

#b. Explain why nitrogen forms only one chloride, NCl3, whereas phosphorus forms two chlorides,

PCl3 and PCl5. (2 marks)

c. Account for the order of boiling point for the two series of compounds below: H2O > C2H5-OH > C2H5-O-C2H5 H2S < C2H5-SH < C2H5-S-C2H5

(3 marks)

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a. (i), (ii)

c.

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26. [HKAL 2001 I Q2c] Draw the three-dimensional structure for each molecule below, showing the lone electron pair(s), if any, of

the central atom. State the shape of each molecule. a. BF3 b. ClF3

(3 marks)

27. [HKAL 2002 II Q1d] Ammonia (NH3) and phosphine (PH3) are hydrides of nitrogen and phosphorus respectively. Account for

each of the following phenomena: (i) The bond angle between two N-H bonds in NH3 (about 107o ) is greater that that between two P-H

bonds in PH3 (about 94o ). (ii) NH3 is very soluble in water but PH3 is sparingly soluble.

(4 marks)

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28. [HKAL 2003 II Q3b] The “octet rule” is commonly used in elementary chemistry course to account for the formation of chemical

bonds. (i) What is the octet rule? (ii) With appropriate examples, state two limitations of the octet rules.

(3 marks)

29. [HKAL 2005 II Q2c] Account for the following: Sulphur dioxide possesses a net dipole moment while carbon dioxide does not.

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30. [HKAL 2006 II Q2b] (i) Explain why ice is less dense than water.

#(ii) Given that the triple point of water occurs at 6.03 × 10-3 aim and 0.0098°C, sketch a labelled pressure – temperature phase diagram of water.

(iii) Explain why it is possible to skate smoothly on ice at temperatures below 0°C. (7 marks)

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31. [HKAL 2008 I Q1a] (i) Draw a “dot-and-cross” diagram to show the arrangement of the outermost electrons in the species

NH2-(g), and predict the shape of this species.

(ii) Arrange the H-N-H bond angles in descending order for the three species: NH2

-(g), NH3(g) and NH4

+(g). Explain your ordering. (iii) Excess NH3(g) reacts with Cl2(g) in two steps to give N2(g) and NH4Cl(s). It is known that NH3(g)

functions as a reducing agent and as base in the reaction. For each step, write the chemical equation and state the function of NH3(g)

(7 marks)

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32. [HKAL 2008 I Q3a(ii)] Which one of the following molecules has a zero dipole moment? A. BF3 B. PH3 C. SO2 D. HCl

33. [HKAL 2008 II Q4c] Both diamond and graphite are allotropes of carbon. A unit cell of diamond and a part of the structure of

graphite are shown below:

#(i) Calculate the number of carbon atoms in one unit cell of diamond. (ii) Diamond and graphite show a marked difference in electrical conductivity. Account for their

difference in electrical conductivity in terms of bonding and structure. (iii) Buckminsterfullerene )C( 60 is another allotrope of carbon.

Suggest and explain how you would differentiate two samples of black powder, one of

buckminsterfullerene and the other of graphite, by (I) a physical method, and #(II) a spectroscopic method.

(8 marks)

A

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34. [HKAL 2009 I Q1a] Draw the respective electronic structures of BF3 and NF3. Hence, deduce the shape of each species.

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35. [HKAL 2009 I 3a(ii)] Which one of the following structural features of graphite accounts for its use as a lubricant ? A. each carbon atom with a coordination number of 3 B. the van der Waals forces between layers of atoms C. the hexagonal arrangement of carbon atoms in each layer D. the presence of delocalised electrons Ans: B