5.4 Group 2 Compounds: Uses and Thermal Decomposition

1. Uses of Group 2 Compounds

Group 2 elements and their compounds have diverse applications in construction, agriculture, and medicine.

1.1 Industrial Uses of Limestone

Limestone is primarily composed of calcium carbonate ($\mathrm{CaC}{\mathrm{O}}_3$). It serves as a fundamental building material and a key component in cement manufacturing.

• Building Material: Limestone rocks are cut and shaped into blocks for construction. Mortar, historically made from lime and sand, now commonly uses cement, which itself is derived from limestone. Marble, another form of calcium carbonate, is used for decorative tiles and architectural elements.

• Cement Manufacturing: The first step in cement production involves roasting limestone in a lime kiln at high temperatures. This thermal decomposition converts calcium carbonate into calcium oxide (lime or quicklime) and releases carbon dioxide.

  ${\mathrm{CaCO}}_{3(s)}\stackrel{\text{Heat}}{\to }{\mathrm{CaO}}_{(s)}+{\mathrm{CO}}_{2(g)}$

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The calcium oxide produced is then roasted with clay to form cement. When cement is mixed with sand and aggregate, it forms concrete, the most widely used building material worldwide. Concrete's tensile strength can be enhanced by embedding steel reinforcement bars within it.

1.2 Agricultural Uses of Slaked Lime

Calcium hydroxide ($\mathrm{Ca}(\mathrm{OH}{)}_2$), commonly known as slaked lime, is essential in agriculture.

• Soil pH Adjustment: It is applied to increase the pH of acidic soils. As a basic compound, calcium hydroxide neutralises soil acids, raising the pH to levels more suitable for crop growth.

  ${\mathrm{Ca}(\mathrm{OH})}_{2(s)}+2{\mathrm{H}}_{(aq)}^{+}\to {\mathrm{Ca}}_{(aq)}^{2+}+2{{\mathrm{H}}_2\mathrm{O}}_{(l)}$

1.3 Medical Applications of Group 2 Compounds

Several Group 2 compounds have important medical uses.

• Barium Sulfate ($\mathrm{BaS}{\mathrm{O}}_4$):
  • Diagnostic Use: Barium sulfate absorbs X-rays effectively, making it valuable for diagnosing disorders of the intestines and stomach. Patients consume a suspension of barium sulfate (a "barium meal") to coat the digestive tract lining, rendering it visible on X-ray images.

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• Safety Feature: Although barium is a heavy metal, barium sulfate is insoluble in water and is not absorbed into the bloodstream from the gut. This insolubility ensures its safety for internal diagnostic procedures.

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• Antacids:
  • Group 2 hydroxides and carbonates are basic compounds, making them suitable as antacids to treat acid indigestion (excess hydrochloric acid in the stomach) through neutralisation reactions.
  • Examples:
    • Calcium carbonate ($\mathrm{CaC}{\mathrm{O}}_3$) and magnesium carbonate ($\mathrm{MgC}{\mathrm{O}}_3$) are active ingredients in antacids such as Gaviscon and Rennies.
    • Magnesium hydroxide ($\mathrm{Mg}(\mathrm{OH}{)}_2$) is the active component in Milk of Magnesia.
  • Neutralisation Reactions: ${\mathrm{CaCO}}_{3(s)}+2{\mathrm{HCl}}_{(aq)}\to {\mathrm{CaCl}}_{2(aq)}+{\mathrm{CO}}_{2(g)}+{{\mathrm{H}}_2\mathrm{O}}_{(l)}$ ${\mathrm{Mg}(\mathrm{OH})}_{2(s)}+2{\mathrm{HCl}}_{(aq)}\to {\mathrm{MgCl}}_{2(aq)}+2{{\mathrm{H}}_2\mathrm{O}}_{(l)}$

2. Thermal Decomposition of Group 2 Carbonates and Nitrates

2.1 Definition and General Reactions

Thermal decomposition is the breakdown of a compound into two or more different substances by the application of heat.

Thermal Decomposition of Group 2 Carbonates: When heated, Group 2 carbonates decompose to form a metal oxide and carbon dioxide gas. Example for magnesium carbonate: ${\mathrm{MgCO}}_{3(s)}\stackrel{\text{Heat}}{\to }{\mathrm{MgO}}_{(s)}+{\mathrm{CO}}_{2(g)}$

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The temperature required for thermal decomposition of Group 2 carbonates increases as you descend the group.

Thermal Decomposition of Group 2 Nitrates: Group 2 nitrates also undergo thermal decomposition when heated, producing a metal oxide, nitrogen dioxide gas, and oxygen gas. Example for calcium nitrate: $2{\mathrm{Ca}(\mathrm{N}{\mathrm{O}}_3)}_{2(s)}\stackrel{\text{Heat}}{\to }2{\mathrm{CaO}}_{(s)}+4{\mathrm{NO}}_{2(g)}+{\mathrm{O}}_{2(g)}$

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A noticeable observation during the decomposition of Group 2 nitrates is the evolution of a brown gas, which is toxic nitrogen dioxide ($\mathrm{N}{\mathrm{O}}_2$). Similar to carbonates, higher temperatures are needed to thermally decompose nitrates as Group 2 is descended.

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2.2 Trend in Thermal Stabilities

The thermal stability of Group 2 carbonates and nitrates increases down the group. This trend is explained by enthalpy changes and ion polarisation.

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2.2.1 Enthalpy Changes and Stability

The relative ease of thermal decomposition is reflected by the enthalpy change of the reaction. A more positive enthalpy change indicates that more energy is required to decompose the compound, signifying greater thermal stability.

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

As shown in the table, decomposition temperature and enthalpy change increase down the group, indicating higher thermal stability for heavier Group 2 carbonates.

2.2.2 Explanation via Ion Polarisation

This trend is explained using the concept of ion polarisation:

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1. Carbonate Ion Characteristics: The carbonate ion ($\mathrm{C}{\mathrm{O}}_3^{2-}$) has a relatively large ionic radius, making its electron cloud easily distorted or "polarised" by nearby cations.
2. Group 2 Cation Size: The size of Group 2 cations increases down the group: ${\mathrm{Mg}}^{2+}<{\mathrm{Ca}}^{2+}<{\mathrm{Sr}}^{2+}<{\mathrm{Ba}}^{2+}$
3. Polarising Power: A smaller cation with a higher charge density is more effective at polarising an anion. Smaller Group 2 cations (such as ${\mathrm{Mg}}^{2+}$) have a stronger polarising effect on the large carbonate ion compared to larger cations (such as ${\mathrm{Ba}}^{2+}$). They attract electron density from the carbonate ion, distorting its electron cloud and weakening the carbon-oxygen bonds.
4. Order of Polarisation: The degree of polarisation of the carbonate ion by Group 2 cations follows the order: ${\mathrm{Mg}}^{2+}>{\mathrm{Ca}}^{2+}>{\mathrm{Sr}}^{2+}>{\mathrm{Ba}}^{2+}$
5. Effect on Stability: Greater polarisation of the carbonate ion makes it easier to weaken a carbon-oxygen bond, facilitating decomposition upon heating to form carbon dioxide and the metal oxide.
6. Order of Thermal Stability: Consequently, the thermal stability of Group 2 carbonates is: ${\mathrm{BaCO}}_3>{\mathrm{SrCO}}_3>{\mathrm{CaCO}}_3>{\mathrm{MgCO}}_3$ Magnesium carbonate is the least stable and decomposes at the lowest temperature.

A similar pattern is observed with the thermal decomposition of Group 2 nitrates. The order of stability is: ${\mathrm{Ba}(\mathrm{N}{\mathrm{O}}_3)}_2>{\mathrm{Sr}(\mathrm{N}{\mathrm{O}}_3)}_2>{\mathrm{Ca}(\mathrm{N}{\mathrm{O}}_3)}_2>{\mathrm{Mg}(\mathrm{N}{\mathrm{O}}_3)}_2$

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Possible Questions and Answers

Q1.i: Which compound will decompose at the lowest temperature: Calcium carbonate, Strontium carbonate, or Barium carbonate? A1.i: Calcium carbonate ($\mathrm{CaC}{\mathrm{O}}_3$). Thermal stability increases down the group, so calcium carbonate is less stable than strontium carbonate and barium carbonate.

Q1.ii: Which compound will decompose at the lowest temperature: Barium nitrate, Calcium nitrate, or Magnesium nitrate? A1.ii: Magnesium nitrate ($\mathrm{Mg}(\mathrm{N}{\mathrm{O}}_3{)}_2$). Thermal stability increases down the group, so magnesium nitrate is the least stable among the three.

Q2.i: Write a balanced chemical equation, including state symbols, for the thermal decomposition of Strontium carbonate. A2.i: ${\mathrm{SrCO}}_{3(s)}\stackrel{\text{Heat}}{\to }{\mathrm{SrO}}_{(s)}+{\mathrm{CO}}_{2(g)}$

Q2.ii: Write a balanced chemical equation, including state symbols, for the thermal decomposition of Barium nitrate. A2.ii: $2{\mathrm{Ba}(\mathrm{N}{\mathrm{O}}_3)}_{2(s)}\stackrel{\text{Heat}}{\to }2{\mathrm{BaO}}_{(s)}+4{\mathrm{NO}}_{2(g)}+{\mathrm{O}}_{2(g)}$

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Summary

• Diverse Applications: Group 2 compounds are integral to construction, agriculture, and medicine.
• Industrial Uses: Limestone ($\mathrm{CaC}{\mathrm{O}}_3$) is a key raw material for cement production via thermal decomposition to quicklime ($\mathrm{CaO}$). Cement is used to make concrete.
• Agricultural Uses: Slaked lime ($\mathrm{Ca}(\mathrm{OH}{)}_2$) neutralises acidic soils, raising their pH to improve fertility.
• Medical Uses: Barium sulfate ($\mathrm{BaS}{\mathrm{O}}_4$) is used in diagnostic imaging due to its X-ray absorption and insolubility. Group 2 carbonates and hydroxides (such as $\mathrm{CaC}{\mathrm{O}}_3$, $\mathrm{Mg}(\mathrm{OH}{)}_2$) act as antacids, neutralising stomach acid.
• Thermal Decomposition:
  • Group 2 carbonates decompose into a metal oxide and carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$).
  • Group 2 nitrates decompose into a metal oxide, nitrogen dioxide ($\mathrm{N}{\mathrm{O}}_2$), and oxygen (${\mathrm{O}}_2$). Nitrogen dioxide is a brown, toxic gas.
• Thermal Stability Trend: The thermal stability of Group 2 carbonates and nitrates increases down the group.
• Explanation: This trend is attributed to ion polarisation. Smaller, more highly charged cations (higher up the group, such as ${\mathrm{Mg}}^{2+}$) have greater ability to polarise the large carbonate or nitrate anion. This polarisation weakens internal bonds within the anion, making decomposition easier at lower temperatures. Larger cations (lower down the group, such as ${\mathrm{Ba}}^{2+}$) have less polarising power, resulting in stronger bonds and higher thermal stability.

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References

Derived from FBISE textbook