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.

• In graph 1, the variation in Boiling Point of four hydrides each from Group 15, Group 16 and Group 17 of Modern Periodic Table has been plotted compared to their percentage ionic character. The ionic character (in %) of each hydride is plotted along the X – Axis as it is the independent variable and the Boiling Point (measured in ℃) of each hydride is plotted along the Y – Axis.
• It has been observed in that graph that with an increase in percentage ionic character of hydrides from 0.16 % to 39.57 %, the boiling point of hydrides has increased from -62.60℃ to 19.53℃. Overall, a linearly increasing trend has been obtained between boiling point of hydrides (measured in ℃) compared to their ionic character (in %).
• The above trend can be scientifically justified using the concepts of intermolecular forces. As the difference of electronegativity between X – H increases, where X represents the electronegative element from Group 15/ 16/ 17, the X – H becomes more polar. This increases the ionic character of the molecule. As a result, the electrostatic force of attraction between the molecules become stronger. This makes it more difficult to increase the intermolecular distance and convert this compound from liquid state to gaseous state. So, more thermal energy is needed to be supplied to overcome this intermolecular force of attraction and separate the molecules. Thus, the boiling point increases.
• Moreover, as the % ionic character increases, the dipole – dipole interaction between the molecules become stronger which can also be held responsible for the increase in boiling point.
• However, there are significant number of outliers obtained in the graph. Ammonia (NH₃), and water (H₂O) are having significant positive deviation from the trendline. It can be explained by the effect inter-molecular hydrogen bonding. In the above mentioned two compounds, H-atom is bonded with N in ammonia and O in water. As they show strong affinity towards hydrogen, intermolecular hydrogen bond is formed in the above two compounds. As a result, to change the physical state of ammonia and water, comparatively more amount of energy is required to break the intermolecular hydrogen bonds compared to any other compound which does not have inter-molecular hydrogen bonds. Consequently, the boiling point of ammonia and water are found to be higher than the obtained trend.
• On the other hand, Phosphine (PH₃), Hydrogen Chloride (HCl), and Hydrogen Bromide (HBr) are showing significant negative deviation from the obtained trend. However, the scientific justification of the above claim has not been deduced.

• In graph 2, the variation in Melting Point of four hydrides each from Group 15, Group 16 and Group 17 of Modern Periodic Table has been plotted compared to their percentage ionic character. The ionic character (in %) of each hydride is plotted along the X – Axis as it is the independent variable and the Melting Point (measured in ℃) of each hydride is plotted along the Y – Axis.
• It has been observed in that graph that with an increase in percentage ionic character of hydrides from 0.16 % to 39.57 %, the boiling point of hydrides has increased from -111.23℃ to -83.63℃. Overall, a linearly increasing trend has been obtained between boiling point (measured in ℃)  of hydrides compared to their ionic character (in %).
• The above trend can be scientifically justified using the concepts of intermolecular forces. As the difference of electronegativity between X – H increases, where X represents the electronegative element from Group 15/ 16/ 17, the X – H becomes more polar. This increases the ionic character of the molecule. As a result, the electrostatic force of attraction between the molecules become stronger. This makes it more difficult to increase the intermolecular distance and convert this compound from solid state to liquid state. So, more thermal energy is needed to be supplied to overcome this intermolecular force of attraction and separate the molecules. Thus, the melting point increases.
• Moreover, as the % ionic character increases, the dipole – dipole interaction between the molecules become stronger which can also be held responsible for the increase in melting point.
• However, there are significant number of outliers obtained in the graph. Ammonia (NH3), and water (H2O) are having significant positive deviation from the trendline. It can be explained by the effect inter-molecular hydrogen bonding. In the above mentioned two compounds, H-atom is bonded with N in ammonia and O in water. As they show strong affinity towards hydrogen, intermolecular hydrogen bond is formed in the above two compounds. As a result, to change the physical state of ammonia and water, comparatively more amount of energy is required to break the intermolecular hydrogen bonds compared to any other compound which does not have inter-molecular hydrogen bonds. Consequently, the melting point of ammonia and water are found to be higher than the obtained trend.
• On the other hand, Phosphine (PH3), Hydrogen Chloride (HCl), and Hydrogen Bromide (HBr) are showing significant negative deviation from the obtained trend. However, the scientific justification of the above claim has not been deduced.

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?

An overall increase in melting point and boiling point of hydrides of Group-15, 16, and 17 with respect to their % ionic character is observed in the exploration. This is because, with an increase in ionic character of a compound, the electrostatic force of attraction between the molecules become stronger. This makes it more difficult to increase the intermolecular distance and convert physical state of the compound, from solid phase to liquid phase or from liquid phase to gaseous phase. So, more thermal energy is needed to be supplied to overcome this intermolecular force of attraction and separate the molecules. Thus, the melting point or boiling point increases.

• In Graph-1, with an increase in percentage ionic character of hydrides from 0.16 % to 39.57 %, the boiling point of hydrides has increased from -62.60℃ to 19.53℃.
• Ammonia (NH₃), and water (H₂O) are having significant positive deviation from the trendline. It can be explained by the effect inter-molecular hydrogen bonding. In the above mentioned two compounds, H-atom is bonded with N in ammonia and O in water. As they show strong affinity towards hydrogen, intermolecular hydrogen bond is formed in the above two compounds. As a result, to change the physical state of ammonia and water, comparatively more amount of energy is required to break the intermolecular hydrogen bonds compared to any other compound which does not have inter-molecular hydrogen bonds. Consequently, the boiling point of ammonia and water are found to be higher than the obtained trend.
• Phosphine (PH₃), Hydrogen Chloride (HCl), and Hydrogen Bromide (HBr) are showing significant negative deviation from the obtained trend. However, the scientific justification of the above claim has not been deduced.
• In Graph-2, with an increase in percentage ionic character of hydrides from 0.16 % to 39.57 %, the boiling point of hydrides has increased from -111.23℃ to -83.63℃.
• Ammonia (NH₃), and water (H₂O) are having significant positive deviation from the trendline. It can be explained by the effect inter-molecular hydrogen bonding. In the above mentioned two compounds, H-atom is bonded with N in ammonia and O in water. As they show strong affinity towards hydrogen, intermolecular hydrogen bond is formed in the above two compounds. As a result, to change the physical state of ammonia and water, comparatively more amount of energy is required to break the intermolecular hydrogen bonds compared to any other compound which does not have inter-molecular hydrogen bonds. Consequently, the melting point of ammonia and water are found to be higher than the obtained trend.
• Phosphine (PH₃), Hydrogen Chloride (HCl), and Hydrogen Bromide (HBr) are showing significant negative deviation from the obtained trend. However, the scientific justification of the above claim has not been deduced.

• In both Graph 1 and Graph 2, multiple outliers have been detected. H₂O and NH₃ are showing significant positive deviation from the trendline; however, Phosphine (PH₃), Hydrogen Chloride (HCl), and Hydrogen Bromide (HBr) is showing moderate negative deviation from the trendline. This limits the reliability of the trend that both the boiling point and the melting point increases with the increase in % ionic character.
• Hydrogen is often considered as a rogue element because it shows physical and chemical properties similar to both Alkali metals in Group 1 and the Halogens in Group 17. In the current investigation, the % ionic character has been calculated using the X – H bond. Thus, the dual behavior of the H atom will invariably make the data processing less coherent.
• All the elements taken from Group 15, 16 and 17 are looked upon as non-metals and thus way more electronegative than Hydrogen. However, certain elements like Arsenic are metalloids and shows properties of both metals and non-metals. This fact makes the data processing less coherent and questions the reliability of the conclusion made.
• The value of melting point of BiH₃ is not available in either of the three sources mentioned in the data collection section of this exploration. Moreover, the melting point is not available in any other sources as well. Due to lack of availability of secondary data of all the compounds chosen for this exploration, the investigation becomes less coherent.

• All the sources used to collect the secondary data are authentic and reliable. None of them are commercial websites who would have an intention of present non authenticated data for some personal purposes. This use of tremble websites as a source of secondary data makes the data more reliable.
• The data has been collected from three different sources and an arithmetic average has been calculated. This idea is quite similar to the Triangulation research methodology where three different sources are considered and the data set common to all of them is taken. This use of multiple sources to collect the data makes the result more reliable.
• The standard deviation has been calculated for all the raw data and the value obtained is very small and close to zero. This indicates there is no significant discrepancy between the data collected from different sources. This makes the data more precise.

As an extension to this investigation. I would like to study more properties that may depend on difference of electronegativity or partial ionic character. The properties that may be considered are viscosity, density, bond length, bond energy and many more. All these properties depend on the strength of intermolecular attraction or the polarity of the covalent bonds present within the molecules. Thus, studying the effect of difference of electronegativity on these properties will allow us to understand how changes in the composition of the molecule may affect its physical or chemical behavior and thus have an impact on the use of it. For example,if I need to synthesize a liquid with low boiling point, I can use my knowledge about the relationship between electronegativity and boiling point to choose the correct element for my synthesis.

• CDC - NIOSH Pocket Guide to Chemical Hazards - Hydrogen Chloride. https://www.cdc.gov/niosh/npg/npgd0332.html. Accessed 11 Feb. 2021.
• ‘Dipole-Dipole Interactions’. Chemistry LibreTexts, 2 Oct. 2013, https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Intermolecular_Forces/Specific_Interactions/Dipole-Dipole_Interactions.
• ‘Electronegativity Chart of Elements — List of Electronegativity’. Dynamic Periodic Table of Elements and Chemistry, https://periodictable.me/electronegativity-chart/. Accessed 11 Feb. 2021.
• Helmenstine, Todd. ‘List of Electronegativity Values of the Elements’. Science Notes and Projects, 9 May 2015, https://sciencenotes.org/list-of-electronegativity-values-of-the-elements/.
• HYDROCHLORIC ACID, SOLUTION | CAMEO Chemicals | NOAA. https://cameochemicals.noaa.gov/chemical/3598. Accessed 11 Feb. 2021.
• Hydrogen Bonding. https://www.chem.purdue.edu/gchelp/liquids/hbond.html. Accessed11 Feb. 2021.
• London Dispersion Forces. https://www.chem.purdue.edu/gchelp/liquids/disperse.html. Accessed 11 Feb. 2021.
• Melting Point, Freezing Point, Boiling Point. https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/melting.php. Accessed 11 Feb. 2021.
• ‘Percentage Ionic Character’. Percentage Ionic Character, https://learnchemistrybyinamjazbi.blogspot.com/2015/03/percentage-ionic-character.html. Accessed 11 Feb. 2021.
• Polar Covalent Bond - an Overview | ScienceDirect Topics. https://www.sciencedirect.com/topics/chemistry/polar-covalent-bond. Accessed 11 Feb. 2021.