Main group section 2

Question 1

Describe the trends down group 15

Answer 1

• The sum of the first 3 ionisation energies falls
• The lower oxidation state (+3) is increasingly stable compared to the higher (+5)
• Elements are increasingly electropositive down the group
• Radii increase as n increases

Question 2

Describe the trend in the oxides of group 15 elements

Answer 2

The oxide of P is acidic, but down the group oxides become more basic

Question 3

Describe the bond strengths of EH₃ down group 15

Answer 3

Bonds get steadily weaker. There is sigma bonding only

Question 4

Describe the bond strengths of E(CH₃)₃ down group 15

Answer 4

Bonds get steadily weaker. There is sigma bonding only

Question 5

Describe the bond strengths of EF₃ down group 15

Answer 5

There is no clear pattern

Question 6

Describe the bond strengths of ECl₃ down group 15

Answer 6

There is no clear pattern

Question 7

Compare the bond strength of EF₃ with ECl₃

Answer 7

Bonding weakens as the halogens become heavier. There is no clear trend down the respective groups but the weaker sigma bonding is compensated by increased π bonding.

Question 8

Describe 3 differences in the chemistry between nitrogen and phosphorus

Answer 8

1. There is very strong pπ-pπ bonds in N but weak in P
2. pπ - dπ bonding is rare in N but important in P
3. There is no valence expansion in N but there is valence expansion in P

Question 9

Describe valence expansion

Answer 9

An expanded valence shell is one where more than 8 of an atom’s electrons participate in covalent bonding.

Question 10

Describe phossy jaw

Answer 10

Phossy jaw was a problem caused by the vapour white phosphorus.

Question 11

Define an allotrope

Answer 11

Allotropes are different forms of the same element, eg diamond and graphite are allotropes of carbon

Question 12

Describe allotropes of Phosphorus

Answer 12

• White/ yellow phosphorus is a waxy solid. It forms discrete P₄ molecules, is soluble, volatile and pyrophoric.
• Red phosphorus is stable in air, insoluble and not volatile. It is prepared by heating P₄ in a vacuum.
• Other allotropes are formed by heating to higher temperatures and pressures. The most stable allotrope is black phosphorus

Question 13

Describe the difference between P₂ and N₂, and P₄ and N₄

Answer 13

N₂ is more preferable than P₂ and P₄ is more preferatble than N₄, for thermodynamic reasons.

Question 14

Describe how elemental arsenic is obtained

Answer 14

Elemental arsenic is obtained by the thermolysis of FeAsS ore to give FeS and As

Question 15

Describe thermolysis

Answer 15

Thermolysis, or thermal decomposition, is chemical decomposition caused by heat

Question 16

Describe how elemental antimony is obtained

Answer 16

Reacting Sb₂S₃ with Fe gives FeS and Sb

Question 17

Compare NH₃ with PH₃

Answer 17

NH₃ is highly soluble in water, PH₃ is not

NH₃ gives an alkaline solution, PH₃ gives a neutral solution