5.5 Thermal Decomposition of Group 2 Carbonates and Nitrates

In the previous sections, we discussed the reactions of Group 2 elements with oxygen, water, and acids. Another important class of reactions is the thermal decomposition of Group 2 compounds, specifically the carbonates and nitrates. These reactions reveal a clear trend in thermal stability down the group and provide insight into the factors that influence the stability of ionic compounds.

Definition

• Thermal Decomposition: The breakdown of a compound into two or more different substances by the application of heat.
• Thermal Stability: The resistance of a compound to decomposition when heated. A more thermally stable compound requires a higher temperature to decompose.

Core Concept

The core concept is that the thermal stability of Group 2 carbonates and nitrates increases down the group. This means that beryllium carbonate (or magnesium carbonate) decomposes at a lower temperature than barium carbonate. The reason for this trend lies in the polarizing power of the metal cation. A small, highly charged cation can polarize (distort) the electron cloud of the large anion (like carbonate or nitrate), weakening the bond within the anion and making it easier to decompose. As the cation gets larger down the group, its polarizing power decreases, so the anion is less distorted, and the compound is more thermally stable.

Detailed Explanation

The textbook explains that the carbonates and nitrates of Group 2 elements decompose when heated.

1. Thermal Decomposition of Group 2 Carbonates

The carbonates break down to form the metal oxide and give off carbon dioxide gas: MCO_{3(s)} Heat→ MO_{(s)} + CO_{2(g)} For example: MgCO_{3(s)} Heat→ MgO_{(s)} + CO_{2(g)} The textbook notes that the temperature at which thermal decomposition takes place increases going down Group 2 carbonate.

2. Thermal Decomposition of Group 2 Nitrates

The Group 2 nitrates also undergo thermal decomposition. For example, calcium nitrate decomposes to form calcium oxide, nitrogen dioxide, and oxygen: 2Ca(NO_3)_{2(s)} Heat→ 2CaO_{(s)} + 4NO_{2(g)} + O_{2(g)} A brown gas is observed when a Group 2 nitrate is heated. This is toxic nitrogen dioxide (NO_2). Like carbonates, a higher temperature is needed to thermally decompose the nitrates as Group 2 is descended.

3. Trend in Thermal Stability

The textbook states that thermal stability of Group 2 carbonates or nitrates increases down the group. The relative ease of thermal decomposition is shown by the values of the enthalpy changes of reaction. The more positive the enthalpy change, the more energy will be needed to decompose the metal carbonates.

Table 5.2: Enthalpy change of reaction values for the decomposition of some Group 2 carbonates.

The data clearly show that the decomposition temperature and the enthalpy change of reaction both increase down the group. This means that it becomes harder to decompose the carbonate as the metal ion gets larger.

4. Explanation Using Ion Polarization

We can explain this trend using ideas about ion polarization:

• The carbonate ion (CO_3^{2-}) has a relatively large ionic radius, so it is easily polarized by a small, highly charged cation.
• The size of Group 2 cations increases down the group:
• The smaller cations (like Mg^{2+}) have a higher charge density (charge-to-radius ratio) and a greater polarizing power. They can distort the electron cloud of the carbonate ion, weakening the carbon-oxygen bonds and making it easier for the carbonate to break down into oxide and carbon dioxide.
• As the cation gets larger down the group, its polarizing power decreases. It causes less distortion of the carbonate ion, so the compound becomes more thermally stable, and a higher temperature is required for decomposition.

Intermediate Level

The thermal stability of Group 2 carbonates and nitrates increases down the group. This is because the polarizing power of the metal cation decreases as its size increases. The carbonate ion is large and can be distorted by a small, highly charged cation. This distortion weakens the C-O bonds, making it easier for the carbonate to decompose into the oxide and carbon dioxide. Magnesium ion is small and highly polarizing, so magnesium carbonate decomposes at a lower temperature. Barium ion is large and less polarizing, so barium carbonate is more stable and requires a higher temperature to decompose. The same reasoning applies to the nitrates. The trend is also reflected in the enthalpy change of decomposition, which becomes more positive down the group.

Important Points

• - Thermal Decomposition: Breakdown of a compound by heat.
• - Carbonates: Decompose to metal oxide + CO_2.
• - Nitrates: Decompose to metal oxide + NO_2 (brown gas) + O_2.
• - Trend: Thermal stability increases down Group 2.
• - Reason: Decrease in polarizing power of the cation as it gets larger.
• - Data: Decomposition temperature and enthalpy of decomposition increase down the group.
• - Polarizing Power: Smaller cations are more polarizing (e.g., Mg^{2+} > Ba^{2+}).

Remember

• - Thermal stability increases down Group 2.
• - Decomposition temperature: Be < Mg < Ca < Sr < Ba.
• - Reason: The polarizing power of the cation decreases down the group.
• - Carbonate decomposition: Metal oxide + CO_2.
• - Nitrate decomposition: Metal oxide + NO_2 (brown gas) + O_2.
• - Small cation = more polarizing = less stable carbonate/nitrate.
• - Large cation = less polarizing = more stable carbonate/nitrate.

Common Mistakes

• - Thinking that thermal stability decreases down the group. It increases.
• - Confusing the trend with the reactivity trend. Reactivity of the metals increases down the group, but the thermal stability of their carbonates and nitrates also increases. These are two different trends.
• - Forgetting the products of nitrate decomposition. They decompose to metal oxide, nitrogen dioxide, and oxygen. The brown gas is NO_2, not NO or N_2O.
• - Not understanding the role of polarizing power. The small size and high charge of the cation are key to polarizing the anion and reducing thermal stability.
• - Assuming that the trend is only due to lattice energy. While lattice energy plays a role, the polarization of the anion by the cation is the key factor.

Quick Revision

The thermal stability of Group 2 carbonates and nitrates increases down the group. This is because the polarizing power of the metal cation decreases as its size increases. A small, highly charged cation (like Mg^{2+}) can polarize the large anion (carbonate or nitrate), weakening it and making decomposition easier. As the cation gets larger (like Ba^{2+}), its polarizing power decreases, so the anion is less distorted, and the compound is more thermally stable. The decomposition temperatures and the enthalpy changes of decomposition both increase down the group. Carbonates decompose to metal oxide and carbon dioxide, while nitrates decompose to metal oxide, nitrogen dioxide, and oxygen.

60-Second Revision

The thermal stability of Group 2 carbonates and nitrates increases down the group. This is because the polarizing power of the metal cation decreases as it gets larger. A smaller cation (like Mg^{2+}) distorts the anion more, making decomposition easier. A larger cation (like Ba^{2+}) distorts it less, so the compound is more stable. Carbonates decompose to metal oxide + CO_2. Nitrates decompose to metal oxide + NO_2 (brown gas) + O_2.

Frequently Asked Questions (FAQs)

Q1. What is thermal decomposition? Answer: Thermal decomposition is the breakdown of a compound into two or more different substances by the application of heat. For example, heating calcium carbonate decomposes it into calcium oxide and carbon dioxide.

Q2. Describe and explain the trend observed in the thermal stabilities of the carbonates of Group 2. Answer: The thermal stability of Group 2 carbonates increases down the group. This means that higher temperatures are required to decompose the carbonates as you go from beryllium to barium. The reason is that the polarizing power of the metal cation decreases as its size increases. A small cation like Mg^{2+} can distort the electron cloud of the carbonate ion, weakening the carbon-oxygen bonds and making decomposition easier. A larger cation like Ba^{2+} causes less distortion, making the carbonate more stable.

• Q3. Write an equation to represent the thermal decomposition of calcium nitrate.
• Answer: The thermal decomposition of calcium nitrate is:
• 2Ca(NO_3)_{2(s)} Heat→ 2CaO_{(s)} + 4NO_{2(g)} + O_{2(g)}

Q4. What is the trend in thermal stability of Group 2 nitrates? Answer: The thermal stability of Group 2 nitrates increases down the group, similar to the carbonates. The decomposition temperature and the enthalpy change of decomposition increase from magnesium nitrate to barium nitrate.

Q5. Explain qualitatively the trend in the thermal stability of nitrates and carbonates including the effect of ionic radius on the polarization of the large anion. Answer: The trend is that thermal stability increases down the group. This is because the ionic radius of the metal cation increases down the group. A smaller cation (like Mg^{2+}) has a higher charge density and can polarize the large anion (carbonate or nitrate) more effectively. This polarization weakens the bonds within the anion, making it easier to decompose. As the cation gets larger (like Ba^{2+}), its polarizing power decreases, so it causes less distortion of the anion. This makes the compound more thermally stable, and a higher temperature is required for decomposition.

• Q6. What is the brown gas observed when Group 2 nitrates are heated?
• Answer: The brown gas is nitrogen dioxide (NO_2). It is produced during the thermal decomposition of the nitrates:
• 2M(NO_3)_2 → 2MO + 4NO_2 + O_2

Q7. Why is the enthalpy change of decomposition more positive down the group? Answer: The enthalpy change of decomposition becomes more positive (more endothermic) down the group because the carbonates become more stable. More energy is required to break them down, which is reflected in the higher enthalpy change. This is due to the decreasing polarizing power of the cation, making the carbonate harder to decompose.

Q8. Which Group 2 carbonate is the most thermally stable? Answer: Barium carbonate (BaCO_3) is the most thermally stable among the Group 2 carbonates listed in the textbook. It has the highest decomposition temperature (1360°C) and the most positive enthalpy change of decomposition (+268 kJ/mol).

• Q9. What are the products of the thermal decomposition of magnesium carbonate?
• Answer: Magnesium carbonate decomposes to form magnesium oxide and carbon dioxide:
• MgCO_3 Heat→ MgO + CO_2

Multiple Choice Questions (MCQs)

• Q1. What is the trend in thermal stability of Group 2 carbonates?
• A. It increases down the group.
• B. It decreases down the group.
• C. It remains constant.
• D. It first increases then decreases.
• Correct Answer: A
• Explanation: The thermal stability of Group 2 carbonates increases down the group because the polarizing power of the cation decreases.

• Q2. Why does the thermal stability of Group 2 carbonates increase down the group?
• A. Because the cation size decreases.
• B. Because the polarizing power of the cation decreases.
• C. Because the anion size decreases.
• D. Because the lattice energy increases.
• Correct Answer: B
• Explanation: As the cation gets larger down the group, its polarizing power decreases, so it distorts the carbonate ion less, making it more stable.

• Q3. Which Group 2 carbonate decomposes at the lowest temperature?
• A. Barium carbonate
• B. Strontium carbonate
• C. Calcium carbonate
• D. Magnesium carbonate
• Correct Answer: D
• Explanation: Magnesium carbonate has the smallest cation and the highest polarizing power, so it decomposes at the lowest temperature (540°C).

• Q4. What is the product of the thermal decomposition of a Group 2 carbonate?
• A. Metal oxide and oxygen
• B. Metal oxide and carbon dioxide
• C. Metal hydroxide and carbon dioxide
• D. Metal and carbon dioxide
• Correct Answer: B
• Explanation: Group 2 carbonates decompose to form the metal oxide and carbon dioxide gas.

• Q5. What is the brown gas produced when Group 2 nitrates are heated?
• A. NO
• B. NO_2
• C. N_2O
• D. N_2
• Correct Answer: B
• Explanation: The brown gas is nitrogen dioxide (NO_2).

• Q6. Which of the following is the correct order of thermal stability of Group 2 carbonates?
• A. MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3
• B. BaCO_3 < SrCO_3 < CaCO_3 < MgCO_3
• C. CaCO_3 < MgCO_3 < SrCO_3 < BaCO_3
• D. SrCO_3 < CaCO_3 < MgCO_3 < BaCO_3
• Correct Answer: A
• Explanation: Thermal stability increases down the group, so the order is Mg < Ca < Sr < Ba.

• Q7. What is the enthalpy change of decomposition for magnesium carbonate?
• A. +117 kJ/mol
• B. +176 kJ/mol
• C. +238 kJ/mol
• D. +268 kJ/mol
• Correct Answer: A
• Explanation: According to Table 5.2, the enthalpy change of decomposition for magnesium carbonate is +117 kJ/mol.

• Q8. The thermal stability of Group 2 nitrates:
• A. Increases down the group.
• B. Decreases down the group.
• C. Remains constant.
• D. Is unpredictable.
• Correct Answer: A
• Explanation: Like carbonates, the thermal stability of Group 2 nitrates increases down the group.

• Q9. Which factor is responsible for the increase in thermal stability down Group 2?
• A. Decrease in cation polarizing power.
• B. Increase in cation polarizing power.
• C. Increase in anion size.
• D. Decrease in lattice energy.
• Correct Answer: A
• Explanation: The polarizing power of the cation decreases as it gets larger, so it distorts the anion less, making the compound more stable.

• Q10. What is the decomposition temperature of calcium carbonate?
• A. 540°C
• B. 900°C
• C. 1280°C
• D. 1360°C
• Correct Answer: B
• Explanation: According to Table 5.2, calcium carbonate decomposes at 900°C.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in thermal stability of Group 2 carbonates?
• A. Increases down the group.
• B. Decreases down the group.
• C. Remains constant.
• D. First increases then decreases.
• Correct Answer: A
• Explanation: The thermal stability of Group 2 carbonates increases down the group because the polarizing power of the cation decreases.

• Q2. Why does the thermal stability of Group 2 carbonates increase down the group?
• A. Because the cation size decreases.
• B. Because the polarizing power of the cation decreases.
• C. Because the anion size decreases.
• D. Because the lattice energy increases.
• Correct Answer: B
• Explanation: As the cation gets larger down the group, its polarizing power decreases, so it distorts the carbonate ion less, making it more stable.

• Q3. Which Group 2 carbonate decomposes at the lowest temperature?
• A. Barium carbonate
• B. Strontium carbonate
• C. Calcium carbonate
• D. Magnesium carbonate
• Correct Answer: D
• Explanation: Magnesium carbonate has the smallest cation and the highest polarizing power, so it decomposes at the lowest temperature (540°C).

• Q4. What is the product of the thermal decomposition of a Group 2 carbonate?
• A. Metal oxide and oxygen
• B. Metal oxide and carbon dioxide
• C. Metal hydroxide and carbon dioxide
• D. Metal and carbon dioxide
• Correct Answer: B
• Explanation: Group 2 carbonates decompose to form the metal oxide and carbon dioxide gas.

• Q5. What is the brown gas produced when Group 2 nitrates are heated?
• A. NO
• B. NO_2
• C. N_2O
• D. N_2
• Correct Answer: B
• Explanation: The brown gas is nitrogen dioxide (NO_2).

• Q6. Which of the following is the correct order of thermal stability of Group 2 carbonates?
• A. MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3
• B. BaCO_3 < SrCO_3 < CaCO_3 < MgCO_3
• C. CaCO_3 < MgCO_3 < SrCO_3 < BaCO_3
• D. SrCO_3 < CaCO_3 < MgCO_3 < BaCO_3
• Correct Answer: A
• Explanation: Thermal stability increases down the group, so the order is Mg < Ca < Sr < Ba.

• Q7. What is the enthalpy change of decomposition for magnesium carbonate?
• A. +117 kJ/mol
• B. +176 kJ/mol
• C. +238 kJ/mol
• D. +268 kJ/mol
• Correct Answer: A
• Explanation: According to Table 5.2, the enthalpy change of decomposition for magnesium carbonate is +117 kJ/mol.

• Q8. The thermal stability of Group 2 nitrates:
• A. Increases down the group.
• B. Decreases down the group.
• C. Remains constant.
• D. Is unpredictable.
• Correct Answer: A
• Explanation: Like carbonates, the thermal stability of Group 2 nitrates increases down the group.

• Q9. Which factor is responsible for the increase in thermal stability down Group 2?
• A. Decrease in cation polarizing power.
• B. Increase in cation polarizing power.
• C. Increase in anion size.
• D. Decrease in lattice energy.
• Correct Answer: A
• Explanation: The polarizing power of the cation decreases as it gets larger, so it distorts the anion less, making the compound more stable.

• Q10. What is the decomposition temperature of calcium carbonate?
• A. 540°C
• B. 900°C
• C. 1280°C
• D. 1360°C
• Correct Answer: B
• Explanation: According to Table 5.2, calcium carbonate decomposes at 900°C.