Question

The relative thermal stabilities of alkali metal halides are such that: (a) \(\mathrm{CsCl}>\mathrm{RbCl}>\mathrm{KCl}<\mathrm{NaCl}>\mathrm{LiCl}\) (b) \(\mathrm{CsCl}>\mathrm{RbCl}<\mathrm{KCl}>\mathrm{NaCl}<\mathrm{LiCl}\) (c) \(\mathrm{LiCl}>\mathrm{NaCl}>\mathrm{KCl}>\mathrm{RbCl}>\mathrm{CsCl}\) (d) \(\mathrm{Cs} \mathrm{Cl}>\mathrm{RbCl}>\mathrm{KCl}>\mathrm{NaCl}>\mathrm{LiCl}\)

Short answer

The correct answer is (c) \(\mathrm{LiCl} > \mathrm{NaCl} > \mathrm{KCl} > \mathrm{RbCl} > \mathrm{CsCl}\).

Step-by-step solution

Understand the Concept of Thermal Stability

Thermal stability of alkali metal halides refers to their resistance to decomposition upon heating. Generally, the lattice stability, which depends on the size of ions and the strength of the ionic bond, determines the thermal stability. Smaller ions, which form stronger ionic bonds, often result in greater thermal stability.

Analyze the Trend in Ionic Sizes

Alkali metals increase in size as you move down the group from lithium to cesium. Similarly, the chloride ion is the same in all compounds. Larger alkali metal ions result in weaker ionic bonds because the distance between ions is greater, decreasing lattice energy and reducing stability.

Identify the Thermal Stability Pattern

Recognize that thermal stability is ranked in order of decreasing lattice energy. Therefore, smaller alkali metal cations form more stable halides. Since \( ext{LiCl}\) has the smallest cation, it should be most stable, and \( ext{CsCl}\) should be the least stable.

Determine the Correct Order

Based on the general principle that smaller ions provide greater thermal stability, the sequence is from most stable to least stable as follows: \( ext{LiCl} > ext{NaCl} > ext{KCl} > ext{RbCl} > ext{CsCl}\).

Match With Given Options

Compare your determined order of thermal stability to the provided options. Option (c) \( ext{LiCl} > ext{NaCl} > ext{KCl} > ext{RbCl} > ext{CsCl}\) matches the pattern identified.

Key concepts

Lattice Energy

Lattice energy is a critical concept when exploring the thermal stability of alkali metal halides. This energy is defined as the amount of energy required to break the ionic bonds between the cations and anions in a crystalline lattice to form gaseous ions. The higher the lattice energy, the more energy is required to decompose the compound, indicating higher thermal stability.
Factors influencing lattice energy include the size of the ions and their charges. Typically,

• Smaller ions contribute to a larger lattice energy because they can get closer together, resulting in a more potent cohesive force.
• The greater the charge on the ions, the stronger the attraction between them, which also increases lattice energy.

Therefore, in alkali metal halides, smaller alkali metal ions and chloride ions contribute to higher lattice energy and thus greater thermal stability.

Ionic Bond Strength

Understanding ionic bond strength is vital to grasping why certain alkali metal halides are more thermally stable than others. Ionic bonds are formed through the electrostatic force of attraction between oppositely charged ions in a compound. The strength of this bond depends on the charges and sizes of the involved ions.
Stronger ionic bonds result in more stable compounds, as more energy is needed to break these bonds. Important factors include:

• Ionic size: Smaller ions form stronger bonds because they can pack closely together, enhancing the electrostatic forces.
• Ionic charge: Higher charges on ions result in stronger attraction forces, thus increasing bond strength.

In alkali metal halides, stronger ionic bonds, typically seen with smaller ions, correlate with greater thermal stability.

Ionic Sizes

The size of ions is a significant factor in determining both lattice energy and ionic bond strength, ultimately affecting the thermal stability of alkali metal halides. As a general rule, smaller ions in compounds result in

• Higher lattice energy.
• Stronger ionic bonds.

Smaller ions can come closer together, maximizing the attractive forces between them. In the context of alkali metals:

• As you move down the group in the periodic table from lithium to cesium, the size of the alkali metal ions increases.
• Chloride ions remain constant in size because they are not changing in different alkali metal halides.

Therefore, lithium chloride, containing the smallest alkali metal ion, exhibits more significant lattice energy and bond strength compared to larger cesium chloride, thus being more thermally stable.

Alkali Metals

Alkali metals, located in group 1 of the periodic table, include lithium, sodium, potassium, rubidium, and cesium. These elements exhibit particular trends that are critical in understanding the nature of their compounds.
Trends that impact the properties of alkali metal halides include:

• Increasing atomic and ionic sizes with a downward movement in the periodic table.
• Decreasing ionization energy and, generally, the ability to form ionic compounds.

The increasing size of these metal ions, as you move from lithium down to cesium, affects both the lattice energies and thermal stability of their halides. Because smaller ions like lithium can form more stable bonds with halogens through stronger electrostatic attractions, lithium chloride is consequently more thermally stable compared to compounds involving larger alkali metals.

Decomposition Resistance

The resistance to decomposition, especially when subjected to heat, is paramount when discussing the thermal stability of ionic compounds like alkali metal halides. Decomposition resistance indicates how well the compound maintains its structure without breaking down into simpler substances.
This resistance is closely tied to the compound’s lattice energy and ionic bond strength:

• Higher lattice energy translates to greater decomposition resistance, as more energy is required to overcome ionic attractions.
• Stronger ionic bonds imply that more heat energy is needed to break these connections under high temperatures.

In the case of alkali metal halides, compounds containing smaller alkali ions with a significant lattice energy tend to portray higher resistance to decomposition, making them thermally more stable when heated.