Investigation on the variation in melting point and boiling point of hydrides of group 15, 16, and 17 with respect to their % ionic character

Connecting topics within a discipline has always been my fascination and a major reason to be interested about a subject. To be honest, I was always interested to do an experiment in Chemistry, collect data and base my Internal Assessment on a topic which relates or establishes a correlation between two variables. The very moment the school was physically closed due to nationwide lockdown and we were not able to access the school laboratory, I was really sad about the fact that doing an experimental IA is not feasible any more. However, my teacher inspired to do something on the similar line using secondary data. Thus, the challenge was to identify two variables from two different domains and collect secondary data about these two variables from secondary sources and find a correlation between them. It was during the time when I was studying Topic-4 (Chemical Bonding) when the idea about the two variables- melting point and percentage ionic character came into my mind. This is because when I learnt that covalent molecules may also have ionic character if they have a polar bond was really fascinating for me. I wanted to explore how this existence of partial ionic character in this molecule may affect the macroscopic properties of a molecule. The macroscopic properties involved are – melting point, boiling point, viscosity, density and so on. The microscopic properties involved in this process is the extent of ionic character in a covalent molecule which can be indicated by the term – percentage ionic character.

How does the melting point and boiling point of hydrides of Group-15 (NH₃, PH₃, AsH₃ and BiH₃), Group-16 (H₂O, H₂S, H₂Se and H₂Te) and Group-17 (HF, HCl, HBr and HI) depends on the percentage ionic character of the X-H bonds (X = Group-15/16/17 element) they contain, calculated using Hannay-Smith formula?

In a covalent bond, it is assumed that the electron cloud is distributed equally between the two atoms that are bonded. However, if either of the two atoms is more electronegative than the other, there is a development of a partial negative charge region on the more electronegative atom and development of a partial positive charge region on the less electronegative atom.

As shown in figure 1, X atom is more electronegative than H and hence, it attracts the electron cloud towards itself. As a result, there is a development of partial negative charge on X and consequently, a development of positive charge on H. Due to this phenomenon, a covalent bond gains polarity, behaves as a dipole and acquires ionic character.

The % ionic character of a covalent compound can be determined using Hannay – Smith formula. It states that the ionic character depends upon the difference in electronegativity of the constituent atoms of the compound as shown below:

Ionic Character % = 16 × ∆ EN + 3.5 × (∆ EN)²

∆ EN = Difference of electronegativity

Thus, as the difference of electronegativity between the two atoms making a covalent bond increases, the bond becomes more polar. As a result, the extent of the ionic character increases.

A fixed temperature at which any substance transforms from its solid state to liquid state is known as its melting point. It is an intensive property of a substance, i.e., it does not depend upon the quantity or mass of the substance. Melting point depends upon the stability of the substance. If a compound is very stable, then a significant amount of thermal energy is required to increase the kinetic energy of the solid molecules in order to overcome their intermolecular force of attraction. As a result, the temperature at which its physical state will change, increases.

A fixed temperature at which any substance transforms from its liquid state to gaseous state is known as its boiling point. It is an intensive property of a substance, i.e., it does not depend upon the quantity or mass of the substance. Boiling point depends upon the stability of the substance. If a compound is very stable, then a significant amount of thermal energy is required to increase the kinetic energy of the liquid molecules in order to overcome their intermolecular force of attraction. As a result, the temperature at which its physical state will change, increases.

Types of intermolecular forces existing between the hydrides of Group-15, 16 and 17 – H bond, London dispersion forces and dipole-dipole forces:

There are three different types of intermolecular forces that exist between the atoms those are covalently bonded. They are –

• London Dispersion Force,
• Dipole – Dipole Interaction
• H – bonding.

London dispersion force exists in all covalent molecules. It depends upon the surface area of the atom increases or the number of electrons. London dispersion force is directly proportional to the surface area and the number of electrons present in the atoms that are covalently bonded to form a molecule.

Dipole – dipole interaction force exists only in polar covalent molecules.

H – Bonding exists if there is a covalently bonded O-H, F-H, or N-H in the compound. Here, intermolecular attractive force is observed between the H – atom of one molecule with the O – atom, N – atom, or F – atom of any other molecule. Due to very high affinity of oxygen, fluorine and nitrogen towards hydrogen, such bonds (H – bond) is formed.

Among all the three different types of inter molecular forces discussed above, H bonding is considered as the most predominant one.

Hydrogen bonding can be of two types – intra molecular H bonding and inter molecular H bonding. The former is H bonding between two groups within the same molecule while the latter is the type of H bonding between two different molecules.

The inter molecular H boding brings the molecules together and thus creates more association between them. As a result, the molecules are coming closer and that eventually increases the melting point/ boiling point of the molecule.

Ionic Character: The ionic character of hydrides is considered as the independent variable in this exploration. It will be calculated using the Hannay – Smith formula written below.

Ionic Character (%) = 16 × ∆ EN + 3.5 × (∆ EN)²

∆ EN = Difference of electronegativity

The objective of this investigation is to understand how the lack of covalency in a compound impacts the intermolecular forces and thus the intrinsic properties of a compound which depends on the intermolecular forces. The set of compounds chosen for the investigation are all hydrides of Group 15, 16 and 17. The elements from Period 2 to Period 4 has been chosen because the elements of these groups belonging to period 5 and 6 radioactive in nature and do not form hydrides. For example, in group 17, the halogens - F, Cl, Br and I has been chosen while At, which belongs to period 5 of Group 17 has not been chosen.

All the hydrides of the elements belonging to this group will contain a X – H bond where X represents the electronegative element taken from Group 15, 16 and 17. Due to the difference in electronegativity between X and H, the electron cloud in the X – H bond will not be uniformly distributed. As a result, the X – H will become polar in nature. This will introduce partial ionic character in the hydrides which are otherwise covalent in nature. The % ionic character is thus an indirect measure of the polarity present within these molecules.

• Melting Point (℃):
• Boiling Point (℃):

These two properties have been chosen because they are intensive and macroscopic in nature. So, they do not depend on the mass of the material but is greatly influenced by the strength of the intermolecular force present within the compound.

• Percentage ionic character has been calculated using the same formula in all cases.
• All the compounds chosen are hydrides and thus all members of the samples are of the same category.

*The data for BiH₃ is not available.

*The data for BiH₃ is not available.