Intermolecular forces Flashcards
1
Q
What is the difference between intramolecular bonds and intermolecular bonds? What are the three types of intermolecular forces?
A
- Intra (means within) and these bonds exist inside the molecule
- Inter (means between) and these bonds exist between the molecules
- Intermolecular forces are responsible for physical properties (solubility, melting and boiling point)
- There three types of intermolecular forces: London dispersion forces, dipole-dipole forces and hydrogen bonding
2
Q
What are London dispersion forces?
A
- Are the weakest type of intermolecular force
- They exist between all atoms and molecules
- Used in non-polar substance too
- London dispersion forces consist of instantaneous dipoles and induced dipoles
- Instantaneous dipole is caused by changes in electron density
- Electrons may be concentrated on one side of an atom, giving the side a slight negative charge, the other side is slightly positive, this is a temporary dipole
- δ+, (for a partial positive charge ) and δ- (for a partial negative charge).
- Molecules with a temporary (instantaneous) dipole can induce a dipole in a neighbouring molecule, this is called induced dipole
Check book
3
Q
Explaining the instantaneous induced dipole- induced dipole forces.
A
- At a random time the instantaneous (temporary) becomes polar because the concentration of electrons shift to one side of the atom.
- When it became polar it causes the molecules close to it to become polar too, these adjacent molecules are called induced dipoles
- The induced dipoles and instantaneous dipoles form London dispersion forces
- London dispersion forces are instantaneous dipole-induced dipole interactions
4
Q
What does the strength of London dispersion forced depend on?
A
- The polarisability: The ease with which the electrons in an atom form a temporary or induced dipole
- When the molar mass of a molecule increases, so does the polarisability and the temporary or induced dipole that are formed are larger (stronger London dispersion forces)
- When the strength of the London forces between molecules increase, the boiling point also increases
AND - The surface area of the molecule
- Molecules that have a greater area between them have stronger London dispersion forces and higher boiling points
5
Q
What are the dipole-dipole forces?
A
- Only exist between polar molecules that have a permanent dipole
- E.g. HCl (the permanent dipole arises because of the difference in electronegativity between the atoms)
- Hydrogen has a partial positive charge while chlorine has a partial negative charge
- The force of attraction occurs between the partial δ- of the chlorine atom and partial δ+ charge of the hydrogen atom
- Dipole-dipole forces are represented by dashed lines
- Between two polar molecules
Check book
6
Q
What is hydrogen bonding?
A
- Occurs between molecules that have a nitrogen, oxygen or fluorine atom directly bonded to a hydrogen atom (N, O, F)
- A stronger type of dipole-dipole attraction
- It is responsible for a high boiling point of water
- The hydrogen bonds occur between the lone pairs of electrons on the oxygen atoms and the hydrogen atoms on a nearby water molecule
7
Q
What effect does hydrogen bonding have on the boiling points of different compounds?
A
- Group 16 hydrides show a large decrease in boiling point from H2O to H2S and an increase from H2Se, H2Te
- H2S has dipole-dipole forces too
- The large decrease in boiling point is due to the fact that water is able to form strong hydrogen bonds between its molecules
- The gradual increase in boiling point is due to the increase in molar mass results in stronger London dispersion forces between the molecules
- Group 16 hydrides are polar
- Similar trends seen in group 15 and 17 hydrides
- Group 15 starts with HF and group 17 with NH3
- Non-polar molecules only have weaker London dispersion forces unlike polar molecules who have a bent molecular geometry (high boiling point)
8
Q
What properties does water have due to the hydrogen bonding?
A
- Water has its highest density at 4°C and become less dense as it freezes so ice can float on water
- The hydrogen bonds holds the water molecules in a rigid structure when frozen
9
Q
What is the relative strengths of the intermolecular forces?
A
Strongest: hydrogen bonding
Middle: dipole-dipole forces
Weakest: London dispersion forces
hydrogen bonding > dipole–dipole forces > London dispersion forces
10
Q
What substances can water dissolve and why?
A
- Many ionic compounds are soluble in water die to its polar nature, some covalent substances are also soluble as they can form hydrogen bonds with water molecules
- Polar substances are soluble in polar solvents, same with non-polar
- A substances will dissolve in a solvent if the intermolecular forces in the solute and solvent are similar
11
Q
What forces are included in the term van der Waals?
A
- London dispersion forces, dipole-dipole forces and dipole-induced dipole are collectively known as van der Waal’s forces
12
Q
Why is ethanol soluble in water?
A
- C2H5OH is soluble in water as it can form hydrogen bonds with water molecules
- The bonding between them releases energy in the solution and energy is taken in when the hydrogen bonds break and as a result have pure water and pure ethanol
13
Q
Explain the link between intermolecular forces and boiling points.
A
- The stronger the intermolecular forces, the more energy must be supplied to break them hence higher boiling point
- When a hydrocarbon has a OH at the end, it has a higher boiling point as hydrogen bonds exist
14
Q
What substances contain which intermolecular forces?
A
- Substances containing H bonded to N,O or F: hydrogen bonding, dipole-dipole forces and London dispersion forces
- Polar substances: dipole-dipole forces and London dispersion forces
- Non-polar substances: London dispersion forces
15
Q
How to determine the order from lowest to highest boiling point.
A
- Consider the relative molecular masses (highest requires most energy to overcome)
- Determine weather they are polar or non-polar
- Check which type of intermolecular force they have
- Order them from weakest to strongest forces
