Topic 4

Question 1

Give the general equation for the thermal decomposition of group 2 metal carbonates

Answer 1

Group 2 metal carbonates decompose to give a metal oxide and carbon dioxide

MCO3 —> MO + CO2

(Where M is a group 2 metal and MO is a group 2 metal oxide)

Question 2

Give the general equation for the thermal decomposition of group 2 metal nitrates

Answer 2

-Group 2 metal nitrates decompose to give a metal oxide, nitrogen dioxide and oxygen

M(NO3)2 (s) —> 2 MO (s)+ 4NO2 (g)+ O2 (g)
(Where M is a group 2 metal and MO is a group 2 metal oxide)

note : nitrogen dioxide gas is a brown gas

Question 3

How does thermal stability of metal carbonates/nitrates change down as you go down the group

Answer 3

-they become more thermally stable
-as the charge density of the metal cations decreases (all have a 2+ charge, but ionic radius increases down group)
-so the cations have a weaker polarising power, and the anion cloud isn’t distorted as much and bonds in the anion aren’t weakened as much

Question 4

Compare the thermal stability of group 1 carbonates with group 2 carbonates

Answer 4

-Group 1 carbonates are more thermally stable than group 2 carbonates

• Group 1 carbonates (except lithium) are thermally stable under a Bunsen burner

-As the group 1 cations are only +1 charged, therefore they have a lower charger density than group 1 cations, and a weaker polarising power

Question 5

Give the general equation for the thermal decomposition of group 1 nitrates

Answer 5

-group 1 metal nitrates decompose to form a metal nitrite and oxygen gas
2MNO3 —> 2MNO2 (s) + O2 (g)

(however lithium is an exception to this

Question 6

Why do lithium carbonate and lithium nitrate decompose like group 2 metal nitrates/carbonates and not group 1 metal nitrates/carbonates

Answer 6

-Although the lithium ion only has a +1 charge, it has a very small ionic radius (electron configuration of Li+ = 1s2)

-Which gives it a much higher charge density than the other group 1 cations,
so it can polarise the carbonate and nitrate anion clouds

Question 7

Give the equations for the decomposition of lithium carbonate and nitrate

Answer 7

Li2(CO3) —> Li2O (s) + O2

LiNO3 —> Li2O (s) + NO2 (g) + O2(g)