IMF Notes 2018.docx

Different types of intermolecular forces

Recall: The covalent bond is a strong bond. It is the electrostatic attraction between the shared valance electrons (negatively charged) and the nucleus of the non-metal atoms (positively charged) and requires it a large amount of energy to overcome/break.

However it is important to remember that even though the covalent bond that holds atoms together within a molecule is strong there are only a weak forces that attracts molecules to other neighbouring molecules. This force is called an intermolecular force. This force is weak.

A contrast of forces

Intramolecular force (better called a covalent bond) - The electrostatic attraction that holds atoms together within a molecule it is the attraction between the positive nucleus and negatively charged shared valance electrons. A strong attraction!

Intermolecular force - The electrostatic attraction that holds neighbouring molecules together is between the partial positive charge (+ve dipole) on one molecule and the partial negative charge (-ve dipole) on another neighbouring molecule. A generally weaker attraction!

Types of Intermolecular forces

The intermolecular forces that exist between neighbouring molecules are classified into three groups. These are hydrogen bonding, dipole-dipole forces and dispersion forces. These intermolecular forces vary in strength but are still considered weak compared to a covalent bond.

INTERMOLECULAR FORCE TYPE 1 - DISPERSION

The dispersion force is a type of intermolecular force that exists between ALL molecules. It is due to the random movement of electrons within the molecule producing temporary dipoles. This is due to more electrons being located at one end of the molecule than the other at one point in time end due to random movement.

The Dispersion force is the only intermolecular force present between all molecules. But they are of greatest significance in non-polar molecules because they are the only intermolecular force present.

Dispersion forces become stronger when…

1. The number of electrons within the molecule increases
2. The shape of the molecule is more linear (less branched).

A temporary dipole resulting from the random movement of electrons.

Dispersion force. The attraction between two temporary dipoles formed on a non-polar molecule.

Dispersion forces are the only intermolecular force present between non-polar molecules

Examples of substances that only show dispersion forces are all non-polar substances such as

*Note: Dispersion forces are also present in all polar molecules as well.

How a dispersion force is formed?

The H₂ molecule is a non-polar molecule which means the electrons are evenly distributed because there is no part of the molecule that the electrons are more attracted to (because both H atoms have the same electronegativity). This means there are no parts of the molecule with a slightly negative charge due to having more electrons present.

BUT! As a result of random movement of electrons the electrons may at one point in time be more densely packed together at one side of the molecule. This is only temporary but it still caused one side of the molecule to be slightly negative and the other side of the molecule (with less electrons) is slightly positive . These partially charged ends are called temporary dipoles. (or instantaneous dipoles).

If one molecule with a temporary dipole moves near another molecule it can effect the electron distribution in the neighbouring molecule causing temporary dipoles to be induced in the neighbouring molecule.

The two oppositely charges dipoles located at either end of the two neighbouring molecules are then attracted to each other (electrostatic attraction). This force of attraction is called the dispersion force.

Dispersion forces increase with number of electrons

The boiling points of the noble gases are given in the table. You will notice that helium has the lowest boiling point and as you move down the group the boiling point increases.

The reason that the boiling points increase as you go down the group is that the number of electrons increases in each atom increases (Helium atoms have only 2 elecrons, Xenon atoms have 54 electrons). The more electrons you have, the bigger the possible temporary dipoles and therefore the bigger the dispersion forces.

Because of the larger temporary dipoles, xenon molecules are more strongly attracted to each other than neon molecules. Neon molecules will break away from each other at much lower temperatures than xenon molecules - neon has the lower boiling point.

This is the reason that (all other things being equal) bigger molecules have higher boiling points than small ones. Bigger molecules have more electrons and more distance/surface area over which temporary dipoles can develop - and so the bigger molecules are able to produce larger temporary dipoles and consequently the electrostatic dispersion force between the larger positive and negative temporary dipoles is stronger.

Examples – Alcohols and Hydrocarbons

It is clear to see in the graph that as the number of carbon atoms

increases the boiling point of the hydrocarbons and alcohols increases.

This is again because bigger molecules have more electrons and more distance over which temporary dipoles can develop - and so the bigger molecules are able to produce larger temporary dipoles and consequently the electrostatic dispersion force between the larger positive and negative temporary dipoles is stronger.

The Halogens

Likewise the halogens have increasing melting and boiling points as you move down the group on the periodic table. This is because as you move down the number of electrons in the molecules increases causing the dispersion forces between molecules to be stronger and consequently more energy is required to overcome this dispersion force.

Hydrogen Halides

But we do not always see a nice pattern like the ones above. Some other intermolecular forces must be impacting the boiling points of this family of molecules rather than just dispersion forces. (more to come)

Dispersion forces increase with more linear shape.

The shapes of molecules can also influence the size of the dispersion force between molecules. Given two molecules are both non-polar and have the same molecular mass they can possess very different boiling points.

Longer chain molecules can produce stronger temporary dipoles (thus higher boiling points) as opposed to more branched and clustered molecules which have weaker temporary dipoles (lower boiling points)

Consider the example across comparing pentane and neopentane (which we would call dimethylpropane). Both these molecules are non-polar, symmetrical molecules with the same molecular masses yet their boiling points differ.

Another example.

INTERMOLECULAR FORCE TYPE 2 – DIPOLE-DIPOLE

The dipole-dipole force is a type of intermolecular force that exists between polar molecules. It is the force of attraction between the partially charged negative end (a permanent dipole) of one polar molecule and the partially charged positive end (a permanent dipole) of another neighbouring polar molecule.

Don’t forget where dipole-dipole forces are present dispersion forces are also present in addition.

Dipole-Dipole forces vary in strength. They are strongest when the molecule is made up of atoms with significant difference is electronegativity. For example the H-Cl covalent bond is more polar than the H-Br covalent bond because the electronegativity difference between H and Cl is greater than that between H and Br. Therefore the dipole-dipole forces will be stronger between molecules with a H-Cl bond than one with a H-Br bond.

Dipole-Dipole forces are generally said to be stronger than dispersion forces, however dispersion forces can be stronger than dipole-dipole forces if the dispersion force is between molecules with a large number of electrons.

Dipole-Dipole forces are present between polar molecules

Examples of substances that have dipole-dipole forces are substances such as;

How a dipole-dipole force is formed?

Consider the polar molecule Dimethyl ether, shown across. This molecule is polar because the oxygen atom is the most electronegative atom in the molecule. For this reason the electrons are permanently more attracted to the oxygen end of the molecule. This results in a greater density of electrons in the oxygen end of the molecule and thus a permanent negative dipole. This also results in a positive dipole at the other end of the molecule.

Note: These dipoles are permanent due to the polar nature of the molecule.

When two dimethyl ether molecules come near each other the oppositely charged permanent dipoles are attracted to each other. This attraction between dipoles is called the dipole-dipole force.

Note: Dispersion forces are also present here but much weaker than the dipole-dipole force.

Dipole-Dipole forces increase when there is a greater difference in electronegativity of atoms.

When the atoms in the molecule have large differences in electronegativity the bond is said to be highly polar and (assuming the polar bonds do not cancel out due to shape of the molecule) the molecule is said to be more polar. The greater the polarity the stronger the attraction between the two permanent dipoles. (In other words the stronger the dipole-dipole force)

A problem to solve in class - The boiling points of CH₃Cl and CH₃I are 249.0 K and 315.6 K, respectively. Using your understanding of dispersion forces and dipole-dipole forces explain this observation.

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INTERMOLECULAR FORCE TYPE 3 – HYDROGEN BONDING

Hydrogen bonding is A SPECIAL CASE of dipole-dipole forces.

Hydrogen bonding is a type of intermolecular force that exists between certain, highly polar molecules. These molecules contain a H-F, H-N or a H-O covalent bonds. As a result of these very polar bonds the molecule is highly polar and consequently the attraction between two of these molecules is very strong.

Hydrogen bonding is the strongest of all the intermolecular forces. However dispersion forces can be stronger than some hydrogen bonds if the dispersion force is between molecules with a much larger number of electrons.

Hydrogen bonds are much stronger than dispersion and dipole-dipole forces and therefore it takes more energy to separate molecules bonded in this way. Hence NH₃, H₂O, HF, CH₃OH and many proteins, alcohols and organic acids have higher boiling points to molecules similar in size.

Hydrogen bonding is are present between certain polar molecules

Examples of substances that have hydrogen bonding are substances such as;

A couple of more complex examples

A problem to solve in class – Use the examples of hydrogen bonding shown below to construct a definition of hydrogen bonding yourself.

A Unique type of Intermolecular Interaction

Ion - dipole solvation of ions in aqueous solution

An ion-dipole force is an attractive force that results from the electrostatic attraction between an ion and a neutral molecule that has a dipole.

 Most commonly found in solutions. Especially important for solutions of ionic compounds in polar liquids.

 A positive ion (cation) attracts the partially negative end of a neutral polar molecule.

 A negative ion (anion) attracts the partially positive end of a neutral polar molecule.

What factors affect the strength of a Ion-dipole force

Ion-dipole attractions become stronger as either the charge on the ion increases, or as the magnitude of the dipole of the polar molecule increases.