1.4 c/e: Intramolecular and Intermolecular Forces

Intramolecular and Intermolecular Forces

+ Allotropy​

​

By Erin Hannan

Syllabus Dot Points

• 1.4c investigate elements that possess the physical property of allotropy

• 1.4 e explore the similarities and differences between the nature of intermolecular and intramolecular bonds and the strength of the forces associated with each, in order to explain the:

– physical properties of elements

– physical properties of compounds (ACSCH020, ACSCH055, ACSCH058)

​What we will cover

• What are inter and intramolecular forces

• Difference between the two

• Types of these forces

• Relative strength of these forces

• How this relative strength relates to strength of molecules and bonds

• Allotropy

What are intermolecular and intramolecular forces?

• ​Forces that keep molecules attracted to each other and the internal bonds within the molecules

• ​We have already learnt about intramolecular forces

  • ​ionic

  • covalent (polar and nonpolar)

  • metallic

​How to remember which is which?

​

Intramolecular forces = within a molecule

Intranet = internet system within a ​company

​Intermolecular Forces

There are 3 main types of intermolecular forces:

• ​dipole-dipole

• dispersion

• ​hydrogen bonding

​

These are all present in different molecules, and have big differences in relative strength. This informs us about the melting and boiling point of the molecules.

Dispersion Forces

• These are present in every bonded molecule.

• All atoms and therefore molecules are in perpetual and random motion. This means that at any given point in time, ​the electrons around an atom may be distributed asymmetrically, therefore creating a partial charge.

• This is called a temporary dipole. ​

• ​These only last an instant, but millions are happening all at once within a substance, so therefore, both polar and non-polar molecules can bond to each other through dispersion forces.

• However, these are weak forces, but increase in magnitude the more electrons an atom has.

​

​Can occur between two non-polar molecules, two polar molecules, or between a polar and non-polar molecule.

​

In the above diagram, we see dispersion forces between a P and a NP atom. The polar atom induces a dipole moment in the non-polar atom ​by disturbing its electron configuration.

Dipole dipole forces

In polar molecules, there are dipole moments (partial charges on certain atoms within the molecule). Dipole dipole forces are where positive partial charges of one molecule electrostatically bond with the partial negative charges of another molecule.

​Hydrogen Bonding

This is a special type of dipole dipole bond that only occurs covalently between hydrogen and a few other atoms. These atoms are nitrogen, oxygen, fluorine.

​These bonds give large negative and positive partial charges, meaning the bonds are very strong.


The small size of the H atom allows the other atom to get very close to the lone pairs on the other atom, therefore creating a very strong electrostatic attraction.

​Relative strength of these forces

​H-bonds = very strong --> will result in a high melting and boiling point

Only dispersion forces = weak --> result in a much lower melting and boiling point

​

This is what MANY questions about this part of the syllabus will be about! So being able to recognise where these bonds occur is important. ​

Practice question:

​ C₂H₆ has a boiling point of - 88.8 degrees, and CH₃​OH has a boiling point of +64.7 degrees.

Give a reason, referring the intermolecular bonding, as to why this might be: ​

Practice question:

​ C₂H₆ has a boiling point of - 88.8 degrees, and CH₃​OH has a boiling point of +64.7 degrees.

Give a reason, referring the intermolecular bonding, as to why this might be: ​

​Allotropy

Allotropes are forms of a singular element that have distinctly different physical properties. These differences arise due to differences in bonding.

​

Example: Carbon (diamond, graphite, buckyball) ​

Diamond and graphite are both covalent lattice structures . But in diamond an atom is bonded to 4 others, whereas in graphite it is only bonded to 3. This results in different lattice structures.

• graphite sits in flat layers with weak intermolecular forces, which results in it being a soft and slippery material. And it has free electrons, which allow it to conduct electricity

• diamond is structured in a full ​lattice, with strong covalent bonds throughout, which make it extrmeley stable and extremely hard

Other allotropes

Phosphorus (white, red, and black):

• white: ​very reactive, structure is a pyramid, leaves valence electrons on all atoms

• red: ​more of an 'opened out' structure, which utilises some of the valence electrons to bond with neighbouring P atoms. Not as reactive as white.

• black: ​buckled layers of P atoms where each atoms is bonded to 3 other P atoms. More stable and therefore less reactive.

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​Other allotropes

Sulfur: rhombic, monoclinic, plastic

• rhombic and monoclinic: both are 8 sulfurs attached in a ring forming slightly different crystalline shapes

• plastic: ​long chains of single S atoms attached to each other, this does not have a crystalline structure

Multiple Choice

Which type of intermolecular force is the strongest?

• LDF

• dipole-dipole

• Hydrogen bonding

• Covalent bonding

Multiple Choice

For hydrogen bonding to occur, a molecule must have a hydrogen bonded to

• carbon

• another hydrogen

• Fluorine, Chlorine or Oxygen

• Fluorine, Nitrogen or Oxygen

Multiple Choice

Does this molecule contain hydrogen bonding?

• yes

• no

Multiple Choice

What kind of force is the arrow pointing to?

• Intramolecular Force

• Intermolecular Force

Multiple Choice

Which substance has the weakest intermolecular forces?

• Substance A, boiling point of 75 °C

• Substance B, boiling point of 105 °C

• Substance C, boiling point of 25 °C

• Substance d, boiling point of 45 °C

Next up:

• End of Module 1!

• Going over any content from Module 1

• Preparing for Depth Study