5.5.1 Trend in thermal stabilities

In the previous section, we learned that Group 2 carbonates and nitrates decompose when heated. We also noted that the temperature required for this decomposition increases as you go down the group. This section provides a more detailed explanation of the trend in thermal stability, focusing on the underlying reasons and the supporting data.

Main Idea

The main idea is that the thermal stability of Group 2 carbonates and nitrates increases down the group. This means that barium carbonate is more thermally stable (harder to decompose) than magnesium carbonate. The trend can be explained by the polarizing power of the metal cation, which decreases as the cation gets larger.

Core Concept

The core concept is ion polarization. A small, highly charged cation can distort (polarize) the electron cloud of a large anion like carbonate (CO_3^{2-}) or nitrate (NO_3^-). This distortion weakens the bonds within the anion, making it easier for the anion to break apart into simpler molecules (like CO_2 or NO_2 + O_2). As the cation gets larger down the group, its charge density decreases, so its polarizing power decreases. Therefore, it causes less distortion of the anion, and the compound becomes more thermally stable.

Detailed Explanation

The 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 show that both the decomposition temperature and the enthalpy change of decomposition increase down the group. This indicates that the carbonates become more stable and harder to decompose as the metal ion gets larger.

Explanation Using Ion Polarization

We can explain this trend using ideas about ion polarisation:

• 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:

How Polarization Leads to Decomposition:

When a cation approaches the carbonate ion, it attracts the electron cloud of the carbonate ion towards itself. This distortion is called polarization. The more polarizing the cation, the more it distorts the carbonate ion. This distortion weakens the carbon-oxygen bonds, making it easier for the carbonate ion to break apart into carbon dioxide and oxide ions.

• Small cation (e.g., Mg^{2+}): High charge density, high polarizing power → strong distortion of the carbonate ion → weaker C-O bonds → easier decomposition (lower temperature).
• Large cation (e.g., Ba^{2+}): Low charge density, low polarizing power → less distortion of the carbonate ion → stronger C-O bonds → harder decomposition (higher temperature).

The same reasoning applies to the nitrates. The nitrate ion (NO_3^-) is also large and can be polarized by the cation. The smaller cations cause more distortion, making the nitrate easier to decompose.

Summary of the Trend

Intermediate Level

The thermal stability of Group 2 carbonates and nitrates increases down the group due to the decreasing polarizing power of the metal cation. Polarizing power is the ability of a cation to distort the electron cloud of an anion. It depends on the charge density of the cation, which is the ratio of its charge to its size. As you go down the group, the cation gets larger, so its charge density decreases. This means it has a lower polarizing power and causes less distortion of the carbonate or nitrate ion. Less distortion means the bonds within the anion are stronger, so the compound is more stable and requires a higher temperature to decompose. This is reflected in the more positive enthalpy change of decomposition down the group.

Important Points

• - Thermal Stability: Increases down the group.
• - Decomposition Temperature: Increases down the group.
• - Enthalpy of Decomposition: Becomes more positive (more endothermic) down the group.
• - Polarizing Power: Decreases down the group as cation size increases.
• - Reason: Smaller cations polarize the anion more, weakening it and making decomposition easier.
• - Data: MgCO_3 (540°C, +117 kJ/mol) → BaCO_3 (1360°C, +268 kJ/mol).
• - Same trend for nitrates.

Remember

• - Thermal stability increases down Group 2.
• - Order: Mg < Ca < Sr < Ba (for stability).
• - Reason: Polarizing power decreases as cation size increases.
• - Small cation = more polarizing = less stable.
• - Large cation = less polarizing = more stable.
• - Decomposition temperature increases down the group.
• - Enthalpy change becomes more positive down the group.

Common Mistakes

• - Confusing thermal stability with reactivity. The metals become more reactive down the group, but their carbonates and nitrates become more thermally stable.
• - Thinking that a larger cation is more polarizing. It is the opposite: smaller cations with higher charge density are more polarizing.
• - Forgetting the role of the anion. The carbonate and nitrate ions are large and easily polarized. This is why the polarizing power of the cation is so important.
• - Assuming the trend is due to lattice energy alone. While lattice energy plays a role, the polarization of the anion is the primary reason for the trend in thermal stability.
• - Not relating the trend to the data. The enthalpy values in Table 5.2 clearly show the trend, and they should be used to support the explanation.

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 cation like Mg^{2+} can distort the electron cloud of the carbonate or nitrate ion, weakening its bonds and making decomposition easier. A larger cation like Ba^{2+} causes less distortion, making the compound more stable. The data in Table 5.2 show that both the decomposition temperature and the enthalpy change of decomposition increase down the group, confirming this trend.

60-Second Revision

The thermal stability of Group 2 carbonates and nitrates increases down the group due to the decreasing polarizing power of the metal cation. Smaller cations (like Mg^{2+}) polarize the anion more, making decomposition easier. Larger cations (like Ba^{2+}) polarize it less, making the compound more stable. The decomposition temperature and enthalpy change of decomposition increase down the group.

Frequently Asked Questions (FAQs)

Q1. Explain qualitatively the variation in thermal stability of the carbonates of Group 2. Answer: The thermal stability of Group 2 carbonates increases down the group. This means that barium carbonate is more stable and harder to decompose than magnesium carbonate. The reason is that the polarizing power of the metal cation decreases as its size increases. The small Mg^{2+} ion strongly polarizes the carbonate ion, weakening its bonds and making it easier to decompose. The larger Ba^{2+} ion polarizes the carbonate ion less, so it is more stable.

Q2. What is the trend in the decomposition temperature of Group 2 carbonates? Answer: The decomposition temperature increases down the group. For example, magnesium carbonate decomposes at 540°C, while barium carbonate decomposes at 1360°C. This is because the larger cations form more thermally stable carbonates, which require more heat to decompose.

Q3. How does the enthalpy change of decomposition change down Group 2? Answer: The enthalpy change of decomposition becomes more positive down the group. For magnesium carbonate, it is +117 kJ/mol, while for barium carbonate, it is +268 kJ/mol. A more positive value means that more energy is required for decomposition, indicating greater thermal stability.

Q4. Why does ion polarization affect thermal stability? Answer: Ion polarization is the distortion of an anion's electron cloud by a cation. When a cation polarizes a carbonate or nitrate ion, it weakens the bonds within the anion. This weakening makes it easier for the anion to break apart into simpler molecules (like CO_2 or NO_2 and O_2). The more polarized the anion, the easier it is to decompose the compound.

Q5. 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).

Q6. Which Group 2 carbonate is the least thermally stable? Answer: Magnesium carbonate (MgCO_3) is the least thermally stable among the Group 2 carbonates listed. It has the lowest decomposition temperature (540°C) and the smallest enthalpy change of decomposition (+117 kJ/mol).

Q7. Does the same trend apply to Group 2 nitrates? Answer: Yes, the same trend applies to Group 2 nitrates. Their thermal stability also increases down the group, for the same reason: the decreasing polarizing power of the metal cation. Barium nitrate is more stable than magnesium nitrate.

Q8. How can the trend in thermal stability be explained using the concept of polarizing power? Answer: Polarizing power is the ability of a cation to distort an anion's electron cloud. It is higher for small, highly charged cations. As you go down the group, the cation size increases, so its polarizing power decreases. A lower polarizing power means less distortion of the carbonate or nitrate ion, so the compound is more stable and requires a higher temperature to decompose.

Q9. What is the relationship between the size of the cation and the thermal stability of the compound? Answer: The larger the cation, the more thermally stable the compound. This is because larger cations have lower polarizing power and cause less distortion of the anion. This makes the compound harder to decompose.

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 has the highest decomposition temperature?
• A. Magnesium carbonate
• B. Calcium carbonate
• C. Strontium carbonate
• D. Barium carbonate
• Correct Answer: D
• Explanation: According to Table 5.2, barium carbonate decomposes at the highest temperature (1360°C).

• Q4. What is the enthalpy change of decomposition for calcium carbonate?
• A. +117 kJ/mol
• B. +176 kJ/mol
• C. +238 kJ/mol
• D. +268 kJ/mol
• Correct Answer: B
• Explanation: According to Table 5.2, the enthalpy change of decomposition for calcium carbonate is +176 kJ/mol.

• Q5. Which of the following is the correct order of thermal stability (from least to most stable)?
• A. BaCO_3 < SrCO_3 < CaCO_3 < MgCO_3
• B. MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3
• C. CaCO_3 < MgCO_3 < SrCO_3 < BaCO_3
• D. SrCO_3 < CaCO_3 < MgCO_3 < BaCO_3
• Correct Answer: B
• Explanation: Thermal stability increases down the group, so the least stable is MgCO_3 and the most stable is BaCO_3.

• Q6. What is the primary reason for the increase in thermal stability down the group?
• A. Increase in cation polarizing power.
• B. Decrease in cation polarizing power.
• C. Increase in anion size.
• D. Decrease in lattice energy.
• Correct Answer: B
• Explanation: As the cation gets larger, its polarizing power decreases, so it causes less distortion of the anion, making the compound more stable.

• Q7. Which ion is more easily polarized by a cation?
• A. A small anion
• B. A large anion
• C. A cation
• D. A neutral atom
• Correct Answer: B
• Explanation: Large anions, like carbonate and nitrate, have a more diffuse electron cloud and are more easily polarized.

• Q8. The decomposition of Group 2 carbonates becomes more endothermic down the group. This indicates that the carbonates become:
• A. Less stable.
• B. More stable.
• C. More reactive.
• D. Less reactive.
• Correct Answer: B
• Explanation: A more endothermic decomposition means more energy is required to break down the compound, indicating greater thermal stability.

• Q9. Which of the following statements is true about the trend in thermal stability?
• A. It decreases down the group.
• B. It is highest for magnesium carbonate.
• C. It is highest for barium carbonate.
• D. It is independent of cation size.
• Correct Answer: C
• Explanation: Barium carbonate has the largest cation and is the most thermally stable, as shown by its high decomposition temperature.

• Q10. What is the polarizing power of a cation dependent on?
• A. Its charge and size.
• B. Its color and density.
• C. Its melting point.
• D. Its boiling point.
• Correct Answer: A
• Explanation: Polarizing power depends on charge density, which is the ratio of charge to size. Smaller, more highly charged cations are more polarizing.

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 has the highest decomposition temperature?
• A. Magnesium carbonate
• B. Calcium carbonate
• C. Strontium carbonate
• D. Barium carbonate
• Correct Answer: D
• Explanation: According to Table 5.2, barium carbonate decomposes at the highest temperature (1360°C).

• Q4. What is the enthalpy change of decomposition for calcium carbonate?
• A. +117 kJ/mol
• B. +176 kJ/mol
• C. +238 kJ/mol
• D. +268 kJ/mol
• Correct Answer: B
• Explanation: According to Table 5.2, the enthalpy change of decomposition for calcium carbonate is +176 kJ/mol.

• Q5. Which of the following is the correct order of thermal stability (from least to most stable)?
• A. BaCO_3 < SrCO_3 < CaCO_3 < MgCO_3
• B. MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3
• C. CaCO_3 < MgCO_3 < SrCO_3 < BaCO_3
• D. SrCO_3 < CaCO_3 < MgCO_3 < BaCO_3
• Correct Answer: B
• Explanation: Thermal stability increases down the group, so the least stable is MgCO_3 and the most stable is BaCO_3.

• Q6. What is the primary reason for the increase in thermal stability down the group?
• A. Increase in cation polarizing power.
• B. Decrease in cation polarizing power.
• C. Increase in anion size.
• D. Decrease in lattice energy.
• Correct Answer: B
• Explanation: As the cation gets larger, its polarizing power decreases, so it causes less distortion of the anion, making the compound more stable.

• Q7. Which ion is more easily polarized by a cation?
• A. A small anion
• B. A large anion
• C. A cation
• D. A neutral atom
• Correct Answer: B
• Explanation: Large anions, like carbonate and nitrate, have a more diffuse electron cloud and are more easily polarized.

• Q8. The decomposition of Group 2 carbonates becomes more endothermic down the group. This indicates that the carbonates become:
• A. Less stable.
• B. More stable.
• C. More reactive.
• D. Less reactive.
• Correct Answer: B
• Explanation: A more endothermic decomposition means more energy is required to break down the compound, indicating greater thermal stability.

• Q9. Which of the following statements is true about the trend in thermal stability?
• A. It decreases down the group.
• B. It is highest for magnesium carbonate.
• C. It is highest for barium carbonate.
• D. It is independent of cation size.
• Correct Answer: C
• Explanation: Barium carbonate has the largest cation and is the most thermally stable, as shown by its high decomposition temperature.

• Q10. What is the polarizing power of a cation dependent on?
• A. Its charge and size.
• B. Its color and density.
• C. Its melting point.
• D. Its boiling point.
• Correct Answer: A
• Explanation: Polarizing power depends on charge density, which is the ratio of charge to size. Smaller, more highly charged cations are more polarizing.