Thermal stability

Question 1

What does thermal stability refer to?

Answer 1

How stable a compound is when heated (i.e the extent of decomposition)

Question 2

What happens to NaCl when it is heated?

Answer 2

The ions change from being arranged in a giant lattice to moving freely in a liquid. There is no decomposition occurring in this change of state

Question 3

Why do group 2 nitrates and carbonates act differently when heated compared to NaCl?

Answer 3

• They have double the charge on the cation
• The ionic radius of group 2 cations is smaller than group 1 cations in the same period
• Nitrate and carbonate anions are more complex than Cl-

Question 4

How does the complexity of the anion impact decomposition?

Answer 4

• Complex nitrates (NO3-) can decompose into nitrite ions or oxide ions, and release oxygen or nitrogen dioxide and oxygen
• Complex carbonates (CO3^2-) can decompose into oxides and release CO2
• Simple Cl- cannot decompose into anything

Question 5

What is the trend in ionic radii between group 1 and group 2?

Answer 5

Group 2 ions have a smaller ionic radii than group 1 ions in the same period

Question 6

What are all nitrates of group 1 and 2 elements?

Answer 6

They are all white solids

Question 7

What is the equation for lesser decomposition of nitrates?

Answer 7

Metal nitrate -> Metal nitrite + Oxygen

Question 8

What is the equation for greater decomposition?

Answer 8

Metal nitrate -> Metal oxide + Nitrogen dioxide + Oxygen

Question 9

If brown fumes are given off, what does this indicate in terms of decomposition?

Answer 9

Brown fumes are nitrogen dioxide, meaning if brown fumes are observed, greater decomposition must have occurred

Question 10

What causes thermal stability to decrease?

Answer 10

The greater the level of polarisation of the anion (i.e the more distorted the electrons in the anion), the greater the level of thermal decomposition.
The polarising effect of cations increase with an increasing charge and decreasing ionic radii (overall an increasing charge density)

Question 11

What is the difference in thermal stability between group 1 and 2?

Answer 11

Group 1 compounds are more thermally stable than group 2 compounds

Question 12

Why are group 2 compounds less thermally stable than group 1 compounds?

Answer 12

Group 2 compounds have a higher charge (2+ compared to 1+), and have a smaller ionic radii in the same period. This means they have a higher charge density, resulting in greater levels of polarisation of the anion

Question 13

Why does thermal stability increase down group 1 and 2?

Answer 13

As you go down the group, ionic radii increases while charge stays the same, meaning there is a decreasing charge density. This results in decreasing levels of polarisation of the anion

Question 14

What nitrates undergo further decomposition?

Answer 14

All group 2 nitrates, and lithium nitrate, undergo greater decomposition

Question 15

What nitrates undergo lesser decomposition?

Answer 15

All group 1 nitrates, except lithium nitrate, undergo lesser decomposition

Question 16

Do all group 1 and 2 nitrates decompose?

Answer 16

Yes, all group 1 and 2 nitrates decompose, however whether it is further or less decomposition changes dependent on thermal stability

Question 17

Do all group 1 and 2 carbonates decompose?

Answer 17

No, group 1 and 2 carbonates either decompose pr do not decompose. Where you would see lesser decomposition in nitrates, no decomposition would be seen in carbonates

Question 18

What carbonates undergo decomposition?

Answer 18

All group 2 carbonates, and lithium carbonate, undergo decomposition

Question 19

What carbonates do not undergo decomposition?

Answer 19

All group 1 carbonates, except lithium carbonate, undergo decomposition

Question 20

What do carbonates decompose to?

Answer 20

Metal carbonate -> Metal oxide + CO2

Question 21

What observations can be made in the decomposition of carbonates?

Answer 21

No visible observation can be made, as both metal carbonates and oxides are white solids, and CO2 given off is colourless

Question 22

Why does lithium carbonate undergo decomposition unlike the rest of group 1?

Answer 22

Although it only has a 1+ charge, it has the smallest ionic radii in group 1, meaning it has a high enough charge density to result in decomposition. As a result, lithium compounds decompose like a group 2 rather than group 1