The boiling points of the hydrogen halides are shown in the table. Hydrogen halide HF HCl HBr HI Boiling point / °C +20 –85 –67 –35 Explain the trend in boiling points from HCl to HI .

The trend in boiling points increases from HCl to HI. This is because on going down group 17 in the Periodic Table, the number of electrons increases, thereby increasing the Instantaneous dipole-induced dipole forces of the respective halogens down the group. The Instantaneous dipole-induced dipole forces (id-id forces) between the respective molecules are increased with an increasing number of electrons of the atoms constituting the molecule. So, more energy is needed to change the liquid into vapour and the boiling point is higher in the case of HI compared to the HCl and HBr. Instantaneous dipole-induce dipole forces (id-id forces) : The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short time, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ + end of the dipole in one molecule and the δ - end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. Instantaneous dipole-induced dipole forces are sometimes called temporary dipole-induced dipole forces. These forces are also called London dispersion forces. It is the weakest intermolecular attractive force. Instantaneous dipole-induced dipole forces (id-id forces) increase with: • increasing the number of electrons (and protons) in the molecule. • increasing the number of contact points between the molecules: contact points are places where the molecules come close together.

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Draw a diagram of a hydrogen sulphide molecule to show its shape. Show on your diagram the partial charges on each atom.

Important Questions on Chemical Bonding

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AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>BEFORE YOU START>Q 1

Explain what is meant by: metallic bonding.

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>Questions>Q 9

Answer the following, giving a full explanation in terms of metallic bonding:
Explain why aluminium has a higher melting point than sodium.

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>Questions>Q 9

Answer the following, giving a full explanation in terms of metallic bonding:

The thermal conductivity of stainless steel is 82 W m^-1 K^-1 . The thermal conductivity of copper is 400 W m^-1 K^-1. Why do some stainless steel saucepans have a copper base?

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>Questions>Q 9

Answer the following, giving a full explanation in terms of metallic bonding:

Why does aluminium conduct electricity better than sodium?

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>Questions>Q 11

The table lists the formulae and boiling points of some alkanes. Explain the trend in terms of instantaneous dipole-induced dipole forces.

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>Questions>Q 14

The boiling points of the hydrogen halides are shown in the table.

Hydrogen halide	HF	HCl	HBr	HI
Boiling point / °C	+20	–85	–67	–35

Explain the trend in boiling points from $HCl$ to $HI$ .

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>EXAM-STYLE QUESTIONS>Q 7

Draw a diagram of a hydrogen sulphide molecule to show its shape. Show on your diagram, an arrow showing the exact direction of the dipole in the molecule as a whole.

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 4 - Chemical Bonding>EXAM-STYLE QUESTIONS>Q 7

Oxygen,

O

, sulphur,

S

, and selenium,

Se

, are in the same group in the periodic table. Explain why hydrogen selenide,

H_{2} Se

, has a higher boiling point than hydrogen sulphide,

H_{2} S

Draw a diagram of a hydrogen sulphide molecule to show its shape. Show on your diagram the partial charges on each atom.

Important Questions on Chemical Bonding

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