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| Questão | Responda |
| What are intermolecular forces? | Forces between molecules |
| What is the strength of intermolecular forces compared to covalent, ionic or metallic bonds? | Intermolecular forces are much weaker |
| Name the three types of intermolecular forces | Induced dipole-dipole interactions Permanent dipole-dipole interactions Hydrogen bonding |
| What are induced dipole-dipole interactions also known as? | London forces |
| How are induced dipole-dipole interactions (London forces) formed? | 1. Electrons in charge clouds are always moving really quickly 2. At any particular moment, the electrons in an atom are likely to be more to one side than the other 3. At this moment, the atom would have a temporary dipole 4. This can cause another temporary dipole int he opposite direction on a neighbouring atom 5. The two dipoles are then attracted to each other 6. The second dipole can cause another dipole in a third atom and so on |
| What determines the strength of London forces? | Larger molecules have larger electron clouds, meaning stronger London forces Molecules with greater surface areas have stronger London forces as they have a more exposed electron cloud |
| How are permanent dipole-dipole interactions formed? |
The δ+ and δ- charges on polar molecules cause weak electrostatic forces of attraction between molecules
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| TRUE OR FALSE: Permanent dipole-dipole interactions happen in addition to (not instead of) London forces | TRUE |
| In what elements can hydrogen bonding occur? | Hydrogen covalently bonded to fluorine, nitrogen or oxygen |
| How are hydrogen bonds formed? | Hydrogen has a high charge density because it's so small and fluorine, nitrogen and oxygen are very electronegative. The bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair of electrons on the fluorine, nitrogen or oxygen in another molecule. |
| What effect does hydrogen bonding have on the properties of substances? | Substances are soluble in water and have higher boiling and melting points than molecules of a similar size that are unable to form H bonds. E.g. water and ammonia have very high boiling points when compared to other hydrides in their groups |
| Why do molecules which are able to form hydrogen bonds have very high boiling/melting points? | Extra energy is needed to break the hydrogen bonds |
| Explain why ice is less dense than liquid water | In ice, molecules of H₂O are held together in a lattice by hydrogen bonds. Since hydrogen bonds are relatively long, the molecules in ice will be further apart than in liquid water - this makes ice less dense than liquid water |
| What is the main factor that determines the boiling point? | The strength of the London forces (unless the molecule can form hydrogen bonds) |
| Explain how the strength of the London forces between molecules determines the boiling point | If two molecules have a similar number of electrons, then the strength of the London forces will be similar. So if one of the substances has molecules that are more polar than the other, it will have stronger permanent dipole-dipole interactions and so a higher boiling point. For example, the boiling points of the Group 7 hybrids increases - the strength of the increasing strength of London forces has a higher effect than the decreasing strength of the permanent dipole-dipole interactions. |
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