Intermolecular forces - Compounds and Bonding

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Intermolecular forces

Lessons

Notes:
In this lesson, we will learn:
  • To apply the principles of electrostatic forces to understand intermolecular forces.
  • To explain how hydrogen bonding, London forces and dipole-dipole interactions occur.
  • To recall the relative strength of relative forces between molecules.

Notes:
  • The molecules of chemical compounds are held together by chemical bonds, or forces between the atoms the molecules are made of. As we saw earlier, these forces between atoms are called intramolecular forces and the two major types are covalent and ionic bonding.
  • As well as intramolecular forces, there are intermolecular forces that occur between the molecules of any chemical substances. Intermolecular forces are not as strong as intramolecular forces, but they influence a lot of properties in a chemical. For example, the melting point of a substance is greatly influenced by the intermolecular forces holding molecules close together.
  • To go further we need to understand what a dipole is, because all intermolecular forces are based on dipole interactions. A dipole (meaning ‘two poles’) is a charge difference inside part of a molecule, created by an excess charge. Whenever part of an atom or molecule is more positive than another part which is more negative, you have a dipole. This can happen in a few ways.
    • One way dipoles form is from a covalent bond in a molecule between two elements of different electronegativity.
      • Example below: carbon and chlorine can be covalently bonded together. Because chlorine is more electronegative than carbon, the electron pair they share in the bond is going to be held closer to chlorine (on average) than carbon. This means that this part of the molecule on average is going to be slightly more negative (because the negatively-charged electrons are closer) around the chlorine atom than the carbon atom, where the relative lack of electrons leaves carbon with a slight positive charge. Therefore we give carbon a δ\delta+ (slightly positive) and chlorine a δ\delta- (slightly negative) charge. These are ‘partial’ positive / negative charges.
      • intermolecular force example 1

      • Because the electronegativity of an atom does not change, this effect will always be present in a carbon-chlorine bond – dipoles like this are called permanent dipoles. Any bond between two atoms with a difference in electronegativity creates a dipole like this. We call the bond a polar bond because it has the positive/negative charged effect of a north/south pole.
  • A major type of intermolecular force are van der Waals forces:
    • One type of van der Waals forces are dipole-dipole interactions.
      • When a chemical compound has a permanent dipole (see above), the delta positive (δ\delta+) and delta negative (δ\delta-) charge has an effect on other molecules around them too. This includes other identical molecules!
      • Because each molecule has the same permanent dipole, they arrange with the opposite ends of other molecules to maximize their δ\delta+/δ\delta- attractive forces with as many molecules as possible across 3d space. See below for an example with HCl, where H holds a δ\delta+ and Cl holds a δ\delta- charge in their covalent bond together.
      • dipole-dipole interactions example 1

      • Because this effect is caused by dipoles across molecules interacting, it is called dipole-dipole interactions. This is the stronger of the two van der Waals type forces.
    • Another type of van der Waals forces are London dispersion forces (London forces).
      • Because electrons repel each other and are attracted to positive charge, if two atoms or molecules approach one another, the electrons in each will repel the other. (See below: the solid black is high electron density, spotted is less electrons / lower density).
      • London dispersion forces  1

      • The movement of electrons due to repulsion polarizes both atoms and leads to a dipole being created: see above.
      • London dispersion forces 2

      • This 'forced' polarization of two atoms/molecules that come too close to each other is called an induced dipole. This is the attraction in London dispersion forces.
      • Because this attractive force only exists when atoms/molecules are close together, and vanishes if they are moved apart, it is a temporary dipole, not a permanent one. For this reason we say London forces are temporary induced dipole forces. They are weaker than permanent dipole-dipole interactions (see above) and can exist between any atoms.
      • London forces are stronger on atoms that are more polarizable – meaning the charge (electrons) on the atom can be manipulated and moved around. This occurs much more easily in larger atoms with more electron shells (so the outer shell is further from the nucleus). So, smaller atoms with less electron shells are less polarizable and have weaker London forces. Think of the difference between trying to twist and change the shape of a hard, small marble and a large bean bag – the bean bag is a more ‘polarizable’ larger atom!
      • This explains why the melting point of helium is lowest in the noble gases, and melting point increases going down the group as London forces become stronger, needing more energy to overcome.
  • Another type of intermolecular force is known as hydrogen bonding. These are the strongest intermolecular forces.
    • Hydrogen bonding is a very strong form of dipole-dipole interactions that happens when hydrogen, H, covalently bonds to a very electronegative element ‘X’ (X = F, O or N). Because hydrogen is not very electronegative, these bonds make strong permanent dipoles; the δ\delta+ charge on hydrogen is relatively big, and the δ\delta- on the electronegative atom ‘X’ is equally strong.
    • This causes attractive forces; interactions between the negative lone pair electrons on X in one molecule and the slightly positive hydrogen atom in the other. Hydrogen bonds are very similar to the dipole-dipole interactions seen above, but even stronger..
    • The attractive forces of hydrogen bonding are cumulative – electronegative atoms with more than one lone pair can make more than one hydrogen bond, which is more stable than if it made just one hydrogen bond.
    • Hydrogen bond

    • Hydrogen bonding makes a molecule polar – it will affect its solubility in water (see our “Solution Chemistry” chapter!)
  • Intermolecular forces are cumulative – they have an 'adding up' effect, and can occur across all or any fraction of a molecule. The stronger and the more intermolecular forces there are, the more the physical properties are affected – hardness, melting and boiling point and whether a substance conducts electricity are all affected by the existence of intermolecular forces. Later on, in the Polarity lesson, you'll be able to predict these properties of substances based on intermolecular forces!
  • Intro Lesson
    Intermolecular forces
  • 1.
    Identify the intermolecular forces present in molecules.
    Look at the formula of the chemical substances listed below. Which type of intermolecular forces would you expect to see between the molecules?
  • 2.
    Recall the order of intermolecular force strength
    Look at each group of chemical substances below. Order them in how relatively strong their intermolecular forces are. Hint: Identify the intermolecular forces present first.
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Intermolecular forces

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