Chapter 6: Intermolecular Forces Flashcards
What are intermolecular forces?
- Weak interactions between the dipoles of different molecules
What are the 3 types of intermolecular forces?
- Induced dipole-dipole interactions (London forces)
- Permanent dipole-dipole interactions
- Hydrogen bonding
What do intermolecular forces determine?
- Physical properties
- E.g. melting and boiling points
What do covalent bonds determine?
- The identity of molecules
- Their chemical reactions
List the 4 types of bonds in order of the weakest to the strongest.
- London forces
- Permanent dipole-dipole interactions
- Hydrogen bonding
- Single covalent bonds
What are London forces/ induced dipole-dipole interactions? What else can they be called?
- They are weak, attractive forces between induced dipoles in different molecules
- They exist between all molecules, regardless of what other forces are present
- They can also be called dispersion forces
How do London forces work?
- The natural fluctuation in electron density in atoms produces a changing dipole in a molecule
- At any moment, an instantaneous dipole exists, but its position is constantly changing
- The instantaneous dipole induces a dipole on a neighbouring molecule
- The induced dipole then induces further dipoles on neighbouring molecules, which then attract one another
- Induced dipoles are only temporary, so a dipole that was there may no longer exist shortly after, but this whole process would just happen all over again
What affects the strength of London forces, and why?
- The more electrons there are in each molecule, the stronger the London forces
This is because: - The more electrons there are in each molecule, the larger the instantaneous and induced dipoles
- The stronger the induced dipole-dipole interactions
- The stronger the attractive forces
How does the strength of London forces impact the melting/ boiling points of substances?
- The stronger the London forces are, the more energy is needed to overcome the intermolecular forces
- This increases the melting/ boiling point
What are permanent dipole-dipole interactions?
- Attractive forces between the permanent dipoles in different polar molecules
How are polar molecules bonded together within a substance, and how does this affect their melting/ boiling point?
- There are both London forces and permanent dipole-dipole interactions acting between the molecules
- As energy is needed to break the permanent dipole-dipole interactions as well as the London forces, polar molecules have higher melting/ boiling points
What do simple molecules form in the solid state? Describe their structure.
- A simple molecular lattice
- Molecules are held together by weak intermolecular forces
- The atoms within each molecule are bonded together strongly by covalent bonds
Describe the melting/boiling points of simple molecular lattices, and explain why they are like this.
- They have low melting/ boiling points
- This is because their weak intermolecular forces can be broken even with a small amount of energy
Are non-polar simple molecular substances soluble in non-polar solvents, and why?
- They usually are
- This is because intermolecular forces form between the molecules and the solvent
- These interactions weaken the intermolecular bonding in the simple molecular substance until they break, so the compound dissolves
Are non-polar simple molecular substances soluble in polar solvents?
- They usually aren’t
- When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the molecules in the solvent
- This is because the bonding within the polar solvent is too strong to be broken
Do polar simple molecular substances dissolve in polar solvents?
- They do, similarly to how an ionic lattice dissolves in water
- However, this depends on the strength of the dipole and can be hard to predict
What makes certain biological molecules capable of dissolving in both polar and non-polar solvents?
- They have hydrophilic and hydrophobic parts
- The hydrophilic part would be polar (and usually contain oxygen) and would be able to interact with water
- The hydrophobic part would be non-polar (and made of a carbon chain)
Can simple molecular substances conduct electricity? Why?
- They have no moving, charged particles
- Nothing can therefore complete an electrical circuit, so they don’t conduct electricity
What is a hydrogen bond? Required.
- A special type of permanent dipole-dipole interaction found between molecules containing:
- an electronegative atom (such as F, O and N) with a lone pair of electrons
- a hydrogen atom bonded to an electronegative atom
How are hydrogen bonds drawn?
- They are shown with a dashed line
- The line is drawn between the lone pair of electrons on the electronegative atom and the hydrogen atom on the other molecule
- The dipoles are drawn on the bonded atoms
- You then label the hydrogen bond
Why does water have anomalous properties?
- It has hydrogen bonds between its molecules
What is special about water’s density, and why does it have this property?
- Water in the solid form (ice) is less dense than water in the liquid form
- This is because hydrogen bonds hold water molecules in the solid state far apart in an open lattice structure
What is special about the melting and boiling points of water, and why are they notable?
- They are relatively high
- This is because hydrogen bonds are the strongest type of intermolecular force, so a lot more energy is needed to overcome them when compared to London forces, for example
What are 2 other special properties of water, and why does it have them?
- High surface tension
- High viscosity
- It has hydrogen bonding
Why might some substances with hydrogen bonding have higher melting/boiling points than others?
- The more hydrogen bonds a molecule can form, the higher its melting/ boiling point
