Problem 117 - A Complete Resource Book in Chemistry for JEE Main

Question

The following compounds are to be arranged in order of their increasing thermal stabilities. Identify the correct order. 1\. \(\mathrm{K}_{2} \mathrm{CO}_{3}\) 2\. \(\mathrm{MgCO}_{3}\) 3. \(\mathrm{CaCO}_{3}\) 4\. \(\mathrm{BeCO}_{3}\) (a) \(1<2<3<4\) (b) \(4<2<3<1\) (c) \(4<2<1<3\) (d) \(2<4<3<1\)

Step-by-Step Solution

Answer

The correct order, from least to most thermally stable, is found in option (b): 4<2<3<1.

Step 1: Understanding Thermal Stability

Thermal stability of carbonates generally increases down the group in the periodic table. As you move from Be to Ca to Sr in the group, the size of the metal ion increases, leading to a stronger lattice enthalpy and higher thermal stability.

Step 2: Analyze the Metal Ions in Compounds

List the metal ions: Be in \(\mathrm{BeCO}_{3}\), Mg in \(\mathrm{MgCO}_{3}\), Ca in \(\mathrm{CaCO}_{3}\), K in \(\mathrm{K}_{2}\mathrm{CO}_{3}\). They belong to the groups in increasing order as Be, Mg, Ca, and K.

Step 3: Arrange the Compounds by Group Trend

Since thermal stability increases as you go down a group, arrange the carbonates in order: BeCO_3 < MgCO_3 < CaCO_3 < K_2CO_3.

Step 4: Match with Given Options

Compare the order found in Step 3 to the options given. The correct order is found in option (b): 4<2<3<1.

Key Concepts

Periodic Table Trends

The periodic table is a systematic arrangement of elements, organized by atomic number, electron configuration, and recurring chemical properties. One of the profound trends observed in the periodic table is the change in physical and chemical properties as you move down a group or across a period.
Particularly, the size of atoms tends to increase as you move down a group due to the addition of electron shells. This increase in atomic radius affects properties such as ionization energy, electronegativity, and, importantly, lattice enthalpy.
Thermal stability of compounds like carbonates tends to increase down a group. This is because larger metal ions, farther down a group, form less polarized and thus more stable compounds. Understanding these periodic trends can help in predicting and explaining the properties of different elements and their compounds.

Lattice Enthalpy

Lattice enthalpy is an essential concept in chemistry that refers to the energy required to break apart an ionic solid into its constituent ions in the gaseous state. It's a measure of the strength of the forces holding the ions together in a solid lattice.
Lattice enthalpy is influenced by two main factors:

• Size of the ions: Smaller ions can pack more closely, leading to stronger attractions and higher lattice enthalpies.
• Charge of the ions: Higher charged ions attract each other more strongly, resulting in greater lattice enthalpy.

Down a group in the periodic table, metal ions increase in size, which generally reduces the lattice enthalpy. However, the overall thermal stability may still increase due to the lower polarizing ability of larger ions, making the compound less susceptible to decomposition upon heating.

Group 2 Elements

Group 2 elements, also known as the alkaline earth metals, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements are characterized by having two electrons in their outermost energy level, which they readily lose to form +2 charged cations.
Several key properties of Group 2 elements include:

• They are metallic and form basic oxides and hydroxides.
• Their reactivity increases down the group due to decreasing ionization energies.
• They form ionic compounds, including carbonates, which tend to become more thermally stable down the group.

Understanding their trends helps predict behaviors such as why \( ext{CaCO}_3\) (calcium carbonate) is more thermally stable compared to \( ext{BeCO}_3\) (beryllium carbonate).

Carbonate Compounds

Carbonate compounds, which contain the \( ext{CO}_3^{2-}\) ion, are salts of carbonic acid. In the context of metal carbonates, these compounds often display varying thermal stabilities based largely on the metal ion they pair with.
Some general properties of carbonate compounds include:

• They often decompose on heating to release carbon dioxide gas and form oxides.
• Metal carbonates of Group 1 elements are typically more thermally stable than those of Group 2, due to the larger ionic radii leading to lower lattice energies and higher stability.
• The stability trend often increases as the metal's ionic size increases, following the same group trend as other properties of Group 2 elements.

The understanding of these traits helps in predicting the order of thermal stability among carbonates in a group, such as those in the provided exercise.