Thermal Stability of Compounds

The Elements of Group 1 and 2

Define thermal stability.

A measure of the extent to which a compound decomposes when heated.

What does high thermal stability mean?

The compound is difficult to decompose by heating and requires a higher temperature to decompose.

What does low thermal stability mean?

The compound decomposes more easily when heated.

What two properties of a cation affect the thermal stability of nitrates and carbonates?

The charge and ionic radius of the cation.

How does cation charge affect its influence on an anion?

A higher positive charge gives the cation a greater influence on the anion.

How does cation size affect its influence on an anion?

A smaller cation has a greater influence on the anion.

Which cations have the greatest influence on nitrate and carbonate ions?

Small, highly charged cations.

Which Group 1 and 2 cation has the greatest influence on anions?

Be²⁺ because it has a +2 charge and a very small ionic radius.

Which Group 1 cation has the greatest influence on anions?

Li⁺ because it is the smallest Group 1 cation.

Which Group 1 cation has the least influence on anions?

Cs⁺ because it has a +1 charge and the largest ionic radius.

Why do Group 2 cations generally have a greater influence on anions than Group 1 cations?

Group 2 cations have a +2 charge and are generally smaller than Group 1 cations in the same period.

What happens to the thermal stability of Group 1 nitrates down the group?

It increases down the group.

What happens to the thermal stability of Group 2 nitrates down the group?

It increases down the group.

Why does thermal stability increase down Groups 1 and 2?

The cations become larger, so they have less influence on the anion and cause less distortion, making decomposition more difficult.

What are Group 1 and Group 2 nitrates like before heating?

They are white solids.

What does the production of brown fumes when heating a nitrate indicate?

Greater decomposition has occurred and nitrogen dioxide, NO₂, has been produced.

What gas causes the brown fumes produced during nitrate decomposition?

Nitrogen dioxide, NO₂.

What does the absence of brown fumes when heating a nitrate indicate?

Lesser decomposition has occurred.

What is produced during the lesser decomposition of a metal nitrate?

A metal nitrite and oxygen.

What is the general word equation for lesser decomposition of a nitrate?

Metal nitrate → metal nitrite + oxygen.

What is produced during the greater decomposition of a metal nitrate?

A metal oxide, nitrogen dioxide and oxygen.

What is the general word equation for greater decomposition of a nitrate?

Metal nitrate → metal oxide + nitrogen dioxide + oxygen.

How can oxygen produced during nitrate decomposition be identified?

A glowing splint relights.

Why can oxygen gas not be observed directly during nitrate decomposition?

Oxygen is colourless.

What oxidation-number name can be used for nitrate?

Nitrate(V).

What oxidation-number name can be used for nitrite?

Nitrate(III).

Which Group 1 nitrate undergoes greater decomposition when heated?

Lithium nitrate.

What happens when lithium nitrate is heated?

It decomposes to lithium oxide, nitrogen dioxide and oxygen.

What is the equation for the thermal decomposition of lithium nitrate?

4LiNO₃ → 2Li₂O + 4NO₂ + O₂

What happens when sodium nitrate is heated?

It decomposes to sodium nitrite and oxygen.

What is the equation for the thermal decomposition of sodium nitrate?

2NaNO₃ → 2NaNO₂ + O₂

How do potassium, rubidium and caesium nitrates decompose?

They form the corresponding metal nitrite and oxygen.

What is the general equation for the thermal decomposition of Group 1 nitrates except lithium nitrate?

2MNO₃ → 2MNO₂ + O₂

How do all Group 2 nitrates decompose when heated?

They form the metal oxide, nitrogen dioxide and oxygen.

What is the general equation for the thermal decomposition of a Group 2 nitrate?

2M(NO₃)₂ → 2MO + 4NO₂ + O₂

What is the equation for the thermal decomposition of beryllium nitrate?

2Be(NO₃)₂ → 2BeO + 4NO₂ + O₂

Which nitrates produce brown fumes when heated?

Lithium nitrate and all Group 2 nitrates.

Which Group 1 nitrates do not produce brown fumes when heated?

Sodium, potassium, rubidium and caesium nitrates.

Why does lithium nitrate behave differently from the other Group 1 nitrates?

Li⁺ is much smaller than the other Group 1 cations, so it has a greater influence on the nitrate ion.

Which Group 1 nitrate behaves similarly to Group 2 nitrates?

Lithium nitrate.

Are nitrate thermal decompositions redox reactions?

Yes, the decomposition reactions of nitrates are redox reactions.

What happens to the thermal stability of Group 1 carbonates down the group?

It increases down the group.

What happens to the thermal stability of Group 2 carbonates down the group?

It increases down the group.

What are Group 1 and Group 2 carbonates like before heating?

They are white solids.

What can carbonates form when they thermally decompose?

A metal oxide and carbon dioxide.

What is the general word equation for the thermal decomposition of a metal carbonate?

Metal carbonate → metal oxide + carbon dioxide.

Why are there few visible observations when carbonates decompose?

The carbonate and oxide are both white solids and the carbon dioxide produced is colourless.

Which Group 1 carbonate decomposes relatively easily on heating?

Lithium carbonate.

What happens when lithium carbonate is heated?

It decomposes into lithium oxide and carbon dioxide.

What is the equation for the thermal decomposition of lithium carbonate?

Li₂CO₃ → Li₂O + CO₂

What happens to the other Group 1 carbonates when heated under normal laboratory conditions?

They do not decompose.

Why does lithium carbonate behave differently from the other Group 1 carbonates?

Li⁺ is the smallest Group 1 cation, so it has a greater influence on the carbonate ion.

How do Group 2 carbonates behave when heated?

They decompose to form the metal oxide and carbon dioxide.

What is the general equation for the thermal decomposition of a Group 2 carbonate?

MCO₃ → MO + CO₂

What is the equation for the thermal decomposition of calcium carbonate?

CaCO₃ → CaO + CO₂

What happens to the ease of decomposition of Group 2 carbonates down the group?

They become more difficult to decompose down the group.

Why do Group 2 carbonates become more thermally stable down the group?

The M²⁺ ions become larger, so they have less influence on the carbonate ion and cause less distortion.

Which Group 2 carbonate is easiest to thermally decompose?

Beryllium carbonate.

Which Group 2 carbonate is most thermally stable?

Barium carbonate.

What is the key similarity between the thermal decomposition patterns of nitrates and carbonates?

Thermal stability generally increases down Groups 1 and 2, and lithium compounds behave more like Group 2 compounds than the other Group 1 compounds.

Why do lithium compounds often behave more like Group 2 compounds during thermal decomposition?

Li⁺ is very small and therefore has a strong influence on the anion.