Thermal Stability

What is thermal decomposition?

Thermal decomposition is the process when a substance breaks down by heating.

What is thermal stability?

Thermal stability indicates how stable a compound is when it is heated.

What is the thermal stability like for compounds that decompose easily with a Bunsen burner?

It is low.

What affects the thermal stability of a compound?

The charge density of the cation.

What two factors determine charge density?

1. Charge – higher charge = higher charge density 2. Radius – smaller radius = higher charge density

Are compounds with higher cation charge density more or less thermally stable?

Less thermally stable.

Why are cations with higher charge density less thermally stable?

They have more polarising power.

What does this polarising power do to the anion?

It weakens the covalent bonds within the anion, making it easier to break down.

Are Group 1 carbonates and nitrates more or less stable than Group 2 equivalents?

More thermally stable.

What trend is seen in thermal stability as you go down Group 1 or Group 2?

It increases.

What are carbonate and nitrate ions like in size?

They are large anions.

Why does size of the anion matter?

Large anions are more easily distorted by polarising cations.

Why do Group 1 metals have lower charge density than Group 2 metals?

Because they have a +1 charge and a larger ionic radius.

Do Group 1 carbonates decompose when heated?

No, except lithium carbonate.

Why is lithium an exception?

Lithium has a smaller radius and behaves more like Group 2 metals.

What is the decomposition reaction for lithium carbonate?

Li₂CO₃ → Li₂O + CO₂

Are Group 1 nitrates more or less stable than Group 1 carbonates?

Less stable.

What is the general decomposition reaction for Group 1 nitrates?

2MNO₃ → 2MNO₂ + O₂

What does lithium nitrate decompose into?

Li₂O + NO₂ + O₂

Are Group 2 compounds more or less thermally stable than Group 1?

Less stable due to higher cation charge density.

What do all Group 2 carbonates decompose into when heated?

Metal oxide (MO) + carbon dioxide (CO₂).

What is the general decomposition reaction for Group 2 carbonates?

MCO₃ → MO + CO₂

What is the general decomposition reaction for Group 2 nitrates?

2M(NO₃)₂ → 2MO + 4NO₂ + O₂

Do Group 1 carbonates undergo thermal decomposition?

No, except lithium carbonate.

What does lithium carbonate decompose into when heated?

Lithium oxide (Li₂O) and carbon dioxide (CO₂).

Do Group 2 carbonates undergo thermal decomposition?

Yes, all decompose into MO and CO₂.

What do Group 1 nitrates (except lithium) decompose into?

Metal nitrite (MNO₂) and oxygen (O₂).

What does lithium nitrate decompose into?

Lithium oxide (Li₂O), nitrogen dioxide (NO₂), and oxygen (O₂).

What do Group 2 nitrates decompose into?

Metal oxide (MO), oxygen (O₂), and acidic nitrogen dioxide (NO₂) gas.

How is carbonate thermal stability tested?

Using limewater, which turns milky white when CO₂ is released.

What must be controlled during carbonate thermal stability testing?

Surface area of the compound and flame strength.

How is nitrate thermal stability tested?

Measuring time for oxygen to relight a glowing splint.

How else can nitrate thermal stability be tested?

Measuring time until brown nitrogen dioxide (NO₂) gas appears.