Answers to Assignment 3

7/27/2019 Answers to Assignment 3

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Assignment 3: Chemical Bonding (Part 2) H1 Chemistry (8872)

Lee Yong Kiang Justin 1 2012/13

YISHUN JUNIOR COLLEGE

H1 CHEMISTRY 8872

JC1 (2012)

SUGGESTED ANSWERS TO ASSIGNMENT 3

Section A: Multiple Choice Questions (8 marks)

1 2 3 4 5 6 7 8

A B D A B A D B

Explanation

1 Non-polar molecules are held together by instantaneous dipole-induced dipole interactions

only.

A

C

Cl

C

C l

C l C l

Although the molecule contains polar C Clbonds, it does not have

net dipole moment since the molecule is symmetrical. Hence, this

molecule is non-polar.

B

C

F

C

C l

F C l

This is one of the three possible isomers.

F is more electronegative than Cl. Hence, this molecule contains a

net dipole moment since it is unsymmetrical.

C

CC l

C l

H

H

The unsymmetrical molecule contains polar CCl bonds. Hence,

it contains net dipole moment and is polar.

D

CC l

H

CH3

H

The unsymmetrical molecule contains a polar C Cl bond.

Hence, it contains net dipole moment and is polar.

2 HF and HI exist as simple molecules. More energy is required to overcome the stronger

hydrogen bonding between HF molecules than the permanent dipole-permanent dipole

interactions between HI molecules. So the boiling point of HF is higher than that of HI.

3 In both states, iodine has a simple molecular structure with instantaneous dipole-induced

dipole interactions between I2 molecules. During sublimation, some of these interactions are

overcome by the energy absorbed and the molecules merely become further apart. II bonds

within the I2 molecules are not broken.

4 N2H4 molecules are capable of forming hydrogen bonding with H2O molecules. Compared

with NH3 molecules, N2H4 molecules are able to form more extensive hydrogen bonding with

H2O molecules and hence N2H4 is more soluble in water.


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5 Substance T has to be graphite since the liquid state of graphite is not known.

Both substances Rand S are ionic compounds since they can only conduct electricity in their

liquid states. It should also be noted that since the melting and boiling points of S is

approximately four times those ofR, the electrostatic attractions between the ions should also

compare in the same order. So, S is MgO and Ris NaClsuch that the product of ionic charges

in S is four times that in R.

6 1 Iodine exists as a giant lattice composed of I2 molecules held together by instantaneous

dipole-induced dipole interactions.

2 Sodium exists as a giant metallic lattice with electrostatic attractions between Na+ ions and

the sea of delocalised electrons.

3 Sodium iodide exists as a giant ionic lattice with electrostatic attractions between

oppositely charged Na+ and I ions.

7 1 Boron nitride can act as lubricant since little force is required to overcome the weak van der

Waals forces between the layers of atoms.

2 There is insufficient information to deduce the colour of the compound.

3 Boron nitride is not hard due to the weak van der Waals forces between the layers of atoms.Rather, boron nitride is soft and slippery.

8 1 Silicon carbide ceramic has a very high melting point of 2730oC. This is because a large

amount of energy is required to break the strong covalent bonds between Si and C atoms.

2 Unlike steel, which can be oxidised easily by atmospheric oxygen to form rust, silicon

carbide does not oxidise readily.

3 Silicon carbide does not deform easily under heat since a large amount of energy is required

to overcome the extensive covalent bonds between Si and C atoms.

Additional important information

Ceramics are materials whichhigh melting points

are good electrical insulators

do not deform under high temperature and pressure

do not corrode easily

C atom

Si atom


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Section B: Structured Questions (17 marks)

1 (a) More energy is required to overcome the stronger hydrogen bondingbetween H2O

molecules than the permanent dipole-permanent dipole interactions between H2S

molecules.

(b) Carbon dioxide has a simple molecular structure with weak instantaneous dipole-induced dipole interactions between CO2 molecules which requires a small energy to

overcome.

Silicon(IV) oxide has a giant covalent structure with extensive covalent bonds

between Si and O atoms and requires a large amount of energy to break.

(c) In graphite, each carbon atom uses three valence electrons for bonding. The remaining

valence electron is mobile and can act as charge carriers.

In diamond, each carbon atom uses all four valence electrons for bonding. There are no

mobile charge carriers available.

(d)

H5

C2

C

O

O H

H

C C2

H5

O

O

C2H5COOH molecules can dimerise through the formation of hydrogen bonding.

(e) In isomer B, intramolecular hydrogen bonding occurs between the OH and NH2

groups in close proximity.

As a result, the extent of intermolecular hydrogen bonding between molecules of

isomer B is lower than that between molecules of isomer A. A smaller amount of

energy is required to overcome the weaker intermolecular hydrogen bonding between

molecules of isomerB.

2 (a) Within molecules of each compound, there are covalent bonds between the atoms.

Hydrogen bonding and instantaneous dipole-induced dipole (id-id) interactions between

NH3 molecules.

Permanent dipole-permanent dipole (pd-pd) interactions and id-id interactions betweenHClmolecules.

Only id-id interactions between CO2 molecules.

NH3 has the highest boiling point since a larger amount of energy is required to overcome

the stronger hydrogen bonding between NH3 molecules than the id-id interactions

between molecules of the other two compounds.

CO2 has a higher boiling point than HCl since the id-id interactions between CO2

molecules are stronger than those between HCl molecules. The number of electrons

present in each CO2 molecule is greater.

+

+


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(b) NH3 molecules form hydrogen bonding with H2O molecules.

HClmolecules dissociates in water to form H+ and Cl ions, which can form ion-dipole

interactions with H2O molecules.

The interactions between CO2 molecules and H2O molecules do not release sufficient

energy to overcome the hydrogen bonding between H2O molecules.

Special notesolubility in water For ionic compounds to be soluble in water, the ion-dipole interactions between the ions and

water molecules must release sufficient energy to overcome electrostatic attractions between

oppositely charged ions.

For simple molecular substances to be soluble in water, the molecules should be able to formhydrogen bonding with water molecules.

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