Question

The correct order of the thermal stability of hydrogen halides \((\mathrm{H}-\mathrm{X})\) is \(\quad[2005]\) (a) \(\mathrm{HCl}>\mathrm{HF}>\mathrm{HBr}>\mathrm{HI}\) (b) \(\mathrm{HI}>\mathrm{HCl}>\mathrm{HF}>\mathrm{HBr}\) (c) \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}\) (d) \(\mathrm{HF}>\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}\)

Short answer

(d) \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\)

Step-by-step solution

Understand the question

The problem requires us to determine the correct order of thermal stability of hydrogen halides, which are compounds formed by hydrogen and halogens (F, Cl, Br, I). We have to choose from the given options which one correctly represents the order from most stable to least stable at high temperatures.

Review thermal stability concept

Thermal stability in hydrogen halides is related to the bond strength between hydrogen and the halogen. Generally, a stronger bond requires more energy to break, thus increasing the compound's thermal stability. Typically, bond strength diminishes as the size of the halogen increases.

Analyze bond strength among hydrogen halides

The bond strength is strongest in \(\mathrm{HF}\) due to the small size of fluorine allowing for a closer and stronger bond with hydrogen. As we progress from \(\mathrm{HF}\) to \(\mathrm{HI}\), the bond becomes weaker because iodine is much larger and forms a weaker bond with hydrogen.

Arrange hydrogen halides based on bond strength

Based on the bond strengths, we have \(\mathrm{HF}\) as the strongest and \(\mathrm{HI}\) as the weakest. Therefore, the thermal stability order is \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\).

Choose the correct option

Compare the determined order \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\) with the provided options. Option (d) \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\) matches our derived order, so it is the correct answer.

Key concepts

Hydrogen Bond Strength

The strength of a chemical bond is a critical factor in determining thermal stability. In the case of hydrogen halides, this refers to how strongly hydrogen atoms are bonded to halogen atoms. A strong bond means that more energy is required to break the bond, making the compound more stable at high temperatures. Hydrogen fluoride (HF) exhibits the strongest bond among the hydrogen halides because fluorine, the smallest halogen, can approach hydrogen more closely. This proximity enhances the bond strength due to the higher electron density sharing between hydrogen and fluorine. The stronger the bond, the better the thermal stability of the compound. Hence, HF, with its strong hydrogen-fluorine bond, ranks highest in thermal stability among hydrogen halides.

Halogen Size Effect

The size of halogen atoms is an essential factor that influences the bond strength in hydrogen halides. As you move down the halogen group in the periodic table, from fluorine to iodine, the atomic size increases. This increase in size impacts the bond with hydrogen as follows:

• Fluorine is the smallest halogen, creating a very short and strong bond with hydrogen.
• As you move to chlorine and further to bromine, the distance between hydrogen and the halogen increases, weakening the bond.
• Iodine, being the largest, forms the weakest bond due to its larger atomic radius, resulting in poorer overlap of orbitals with hydrogen.

Therefore, the halogen size affects bond strength and thermal stability, with smaller halogens leading to stronger bonds and greater thermal stability.

Bond Energy

Bond energy is essentially the measure of bond strength. It represents the amount of energy required to break one mole of bonds in gaseous molecules. For hydrogen halides, bond energy can be directly linked to thermal stability. Higher bond energy means the molecule is more stable because it requires more energy to be broken. HF has the highest bond energy due to the strong bond resulting from the small size of fluorine. Here’s a general trend for bond energies in hydrogen halides:

• HF has the highest bond energy, contributing to its high thermal stability.
• Bond energy decreases as we move from HCl, HBr, to HI, due to the increasing size of the halogen.

This trend is instrumental in explaining why HF is more thermally stable than HCl, HBr, and especially HI, which has the lowest bond energy and, thereby, the lowest thermal stability.