Chemical Bonding - Application of IMF (part 2)

This write up illustrate another application of intermolecular forces which will explain why certain molecular substances are able to dissolve in a particular solvent while the other molecular substances are unable to.

The basic tenet of solubility of molecular substance is "likes dissolve likes". In this phrase, it highlights that in order for a molecular substance to dissolve in a particular solvent, the intermolecular forces formed between the solvent molecule and the solute molecule must be stronger than/ or identical the intermolecular forces between solvent molecules only and between solute molecules only.

Thus, it is expected that polar molecular substance dissolves in polar solvents as similar type of intermolecular forces is formed. And for the same reason, non-polar molecular substances dissolve in non-polar solvent.

However, in the case of solubility of substance in water, we need to be abit more careful. A substance can be soluble in water if it can form hydrogen bonding with water. It need not be able to have hydrogen bonding on its own, but it definitely must be able to form hydrogen bonding with water. This is illustrated by the example below.

In addition, in order to dissolve in water, some molecular substance may choose to ionise (some examples include HCl). The formation of ion-dipole moment (this is actually interaction between ion and molecular dipole moment) results in the molecular substance to dissolve favourably in water.

The formation of ion-dipole interaction is also the reason the reason to why ionic compounds prefer to dissolve in water than in a solvent which only has pd-pd (e.g. CHCl₃). This is because solvents that can form hydrogen bonding forms the strongest ion-dipole interaction as compared to other solvents which can only exhibit pd-pd.

Lastly, a mind map is available to help you summarise this prose. This map will be a useful guide to conceptualising how the intermolecular forces is applied to predict solubility of a molecular solute.

^{-- -- -- -- --
Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Bonding - Application of IMF (part I)

The knowledge of intermolecular forces can be used to explain for the different boiling point (or melting point) trend of compounds which exist as a simple discrete molecule. These covalent compounds break intermolecular forces when they undergo a change in physical state (e.g. Solid to liquid or liquid to gas) and in this process these compound's covalent bond (between atoms) remain intact.

In an earlier post, I briefly described to you the different intermolecular forces. In this write up, I will aim to show how the knowledge of intermolecular forces can be applied to account for the different boiling point trend (or melting) in a series of simple discrete molecules.

Do note that melting point can also be affected by how the molecules are packed. Smaller molecules can be packed more densely and hence when melting them, more intermolecular forces are broken and thus giving a larger melting point despite its intermolecular forces is weaker.

(A) Comparing id-id against pd-pd against Hydrogen bonding

We will compare the different types of intermolecular forces when (1) The Mr of the molecules is relatively similar and (2) The shape of the molecule is relatively the same.

In (1) this implies that the number of electrons in the series of compounds is the similar and hence id-id (which is dependent on number of electrons) will be similar. In (2) it will ensure that the electron cloud size is similar (hence id-id) is similar.

When this happens, Hydrogen bonding is the strongest while id-id is the weakest.

(B) Comparing the Mr of the molecule

When the a pair of molecules respective Mr is very different. This implies that the number of electrons the present in the pair is also very different. Hence, the size of the electron cloud will be different too.

Hence, this result in the larger molecule to be polarised easily and hence its induce dipole moment is larger and hence its id-id interaction is stronger and therefore having a higher boiling point.

(C) Comparing the shape of the molecule (e.g. Branched or unbranched)

Unbranched molecules such as CH₃CH₂CH₂CH₃ have an elongated electron cloud which is easier to be polarised. Hence, its induced dipole moment will be large and hence, id-id interaction is stronger. While unbranched molecules such as (CH₃)₄C have a spherical electron cloud which is less easily polarised and hence smaller induced dipole and thus weaker id-id.

(D) If Mr, shape of molecules and IMF are similar/identical

For both molecules satisfy this subtitle and both have hydrogen bonding. Then there are two main factors. (1) Extensive-ness of hydrogen bonding. For example H₂O has more extensive hydrogen bonding than HF and NH₃ because it has the best (most equal) lone pair to hydrogen ratio. (2) Using the definition of Hydrogen bonding as illustrated by this diagram, HF has a stronger hydrogen bonding than NH₃ because F is more electronegative than N and hence making the d+ on H in HF to be larger.

When both molecules satisfy this subtitle and both have pd-pd interactions. There are also two main factors. (1) Electronegativity of the atom. More electronegative atom results in a larger dipole moment. (2) More electronegative atoms which create a dipole moment in the same direction. This will result in the molecule to have a larger overall net dipole moment.

Needless to say, when both have only id-id interaction, the boiling point should be the same.

As this is a length entry. A mind map, which shows you how to sequence your thoughts to construct the concise and comprehensive argument is found here.

^{-- -- -- -- --
Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Bonding - Intermolecular Forces (mindmap)

The mind map below helps to conceptualise the points found in the article on intermolecular forces.

You may want to click here to see the entire map.

^{-- -- -- -- --
Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Bonding - Intermolecular Forces

Substances that contains covalent bond between their constituent atoms have two possible structures: Either (1) Simple discrete molecules or (2) Macromolecular structure (as known as Giant Molecular Structure). Substances that exist as simple discrete molecules have covalent bond between the atoms, while they have intermolecular forces between the molecules. This is a fact which you must be able to distinguish.

A simple guide I have often used to determine whether a substance exists as either (1) simple discrete molecule or (2) macromolecular, would be to see if I am able to draw the dot and cross diagram of the substance. If I can draw a finite dot and cross diagram showing sharing of electrons, that substance will exist as a simple discrete molecule. The following illustration show examples of finite dot and cross diagrams and their corresponding Lewis structure.

When we can establish that a substance exist as a simple discrete molecule, we will need to determine the shape of its molecule. The shape of the molecule play a role in determining the intermolecular forces that exist between the molecules. The pictures below provide a guide to how to determine the shape of the molecule.

There are two broad types of intermolecular forces, van der Waals Forces and Hydrogen-bonding. The former can be divided to (1) Induced dipole - induced dipole (id-id) interactions and (2) Permanent dipole - permanent dipole (pd-pd) interactions.

All molecules contain id-id interaction as this form of interaction is due to a temporal unequal distribution of the molecule's electron cloud, which result in a side of the molecule to have smaller electron density, while the other side has more.

Non-polar molecules have only id-id interaction between their respective molecules.

While polar molecule can have either pd-pd interactions or Hydrogen-bonding as the pre-dominant (note the I used pre-dominant instead of "only") intermolecular forces.

Hydrogen-bonding is a unique case of pd-pd (thus, Hydrogen-bonding is an intermolecular force of attraction). Polar molecules which has a highly electronegative atom such as N, F, and/or O and this electronegative atom is attached to O will show Hydrogen-bonding.

With the knowledge of shapes of molecules and intermolecular forces, we will soon be able to effectively discuss how to account certain physical properties of the substances.

^{-- -- -- -- --
Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}