4.1 4 Thermal Stability Of Group 2 Compounds

Question 1

Thermal stability

Answer 1

How stable a compound is when heated - how much decomposes

Question 2

Decomposition

Answer 2

Isn’t necessarily just a change in state of just a change in arrangement e.g ionic compound just go from a giant lattice to freely moving ions in a liquid but are still just the ions

Question 3

Factors to consider why compound decompose

Answer 3

Charge on ion
Ionic radius
Complexity of ion

Question 4

When group 2 carbonates and nitrates are heated

Answer 4

Do not melt instead they decompose
Why:
1. Larger more complex nitrate ion change into smaller more stable nitrite ion (NO2-) or oxide ion (O2-) by decomposing and releasing oxygen gas or nitrogen dioxide gas
2. Larger more complex carbonate ion can change into the smaller more stable oxide ion O2- by decomposing and releasing carbon dioxide gas CO2
3. The stabilities of nitrate and carbonate anions are influenced by the size and the charge of the cations present ( smaller and more charge affect more )

Question 5

Trends in cations in group 1 and 2

Answer 5

All 1 have 1+ and all 2 have two plus

Both cations get larger as you go down

Question 6

Nitrates of group 1 and group 2

Answer 6

White solids when heated all decompose to nitrites or oxides and give off nitrogen dioxide (brown fumes) or/and oxygen
If nitrate contains water of crystallization steam also observed

Question 7

If no brown fumes observed

Answer 7

Lesser decomposition
Metal nitrate = metal nitrite + oxygen
Nitrates are sometimes differentiated using oxidation numbers nitrate (V) for nitrate and nitrate (111) for nitrite

Question 8

If brown fumes are observed

Answer 8

Greater decomposition

Metal nitrate = metal oxide + nitrogen dioxide + oxygen

Question 9

Observations when samples of nitrates in test tubes over a Bunsen flame

Answer 9

Lithium nitrate + group 2 nitrate = brown fumes

Other group 1 nitrate nada

Question 10

Examples of nitrate decomposition
Lithium nitrate
Sodium nitrate
Beryllium nitrate

Answer 10

4LiNO3=2 Li2O+4NO2+O2
2NaNO3=2NaNO2+ O2
2Be(NO3)2=2BeO+4NO2+O2

Question 11

Carbonates of group 1 and group 2

Answer 11

White solids when heated decompose to oxides and give off carbon dioxide or do not decompose
Gas given off is colorless and both white solids no observations made

Question 12

Samples of carbonates heated in test tubes over a Bunsen flame

Answer 12

Lithium carbonate + group 2 carbonates decompose
Li2CO3=Li2O+ CO2
CaCO3= CaO+CO2

Question 13

Explanation

Answer 13

Metals higher up in the group have a smaller ionic radii but the same charge . This gives them a higher charge density so a great polarizing effect . Greater pull on the oxygen on the carbonate ion distorting the bond length. This makes the bond easier to break and therefore decrease the stability of the metal carbonate . Same for nitrates