Intermolecular Forces (Edexcel International A Level (IAL) Chemistry): Flashcards
Exam code: YCH11
1/360Still learning
Know0
What are intermolecular forces?
Was this flashcard helpful?
Cards in this collection (36)
What are intermolecular forces?
Intermolecular forces are weak attractive forces that act between molecules, as opposed to intramolecular forces, which act within molecules.
What are the three types of intermolecular force, in order of increasing strength?
Induced dipole-dipole forces (London dispersion forces)
Permanent dipole-dipole forces
Hydrogen bonding
What are induced dipole-dipole forces?
Induced dipole-dipole forces (also called London dispersion forces or van der Waals forces) are temporary attractive forces between molecules caused by fluctuations in the electron charge cloud creating temporary dipoles.
How does a temporary dipole arise in a non-polar molecule?
The electron charge cloud constantly moves, so at any instant it may be unevenly distributed, creating a temporary dipole. This can then induce a dipole in a neighbouring molecule.
True or False?
Intermolecular forces are stronger than intramolecular forces.
False.
Intramolecular forces (such as covalent bonds) are stronger than intermolecular forces. For example, a hydrogen bond is about one tenth the strength of a covalent bond.
What conditions are required for hydrogen bonding to occur between molecules?
A molecule must contain a hydrogen atom covalently bonded to a highly electronegative atom: O, N or F.
The H becomes sufficiently δ+ to attract the lone pair of an O, N or F atom in a neighbouring molecule.
Permanent dipole-dipole forces arise when the δ+ end of one molecule is attracted to the .......... end of a .......... molecule.
Permanent dipole-dipole forces arise when the δ+ end of one molecule is attracted to the δ− end of a neighbouring molecule.
Why does propanone have a higher boiling point than butane, given that both have the same number of electrons?
Propanone is a polar molecule with permanent dipole-dipole forces, which are stronger than the induced dipole-dipole forces in non-polar butane. More energy is needed to separate propanone molecules.
True or False?
Induced dipole-dipole forces exist only between non-polar molecules.
False.
Induced dipole-dipole forces exist between all atoms and molecules, including polar ones, as all electron clouds can fluctuate.
Name three classes of compound that can form hydrogen bonds.
Alcohols (contain an O-H bond)
Amines / ammonia (contain an N-H bond)
Carboxylic acids (contain an O-H bond)
Hydrogen fluoride (H-F bond) also forms hydrogen bonds.
Why does water have unexpectedly high melting and boiling points compared to other Group 16 hydrides?
Water molecules form hydrogen bonds with each other. These are much stronger than the induced dipole-dipole forces in H2S, H2Se and H2Te, so significantly more energy is needed to separate water molecules.
True or False?
Ice is denser than liquid water.
False.
In ice, water molecules are arranged in an open lattice held apart by hydrogen bonds, making ice about 9% less dense than liquid water.
Why is ice less dense than liquid water?
In ice, water molecules form an open lattice structure held together by hydrogen bonds. The relatively long bond lengths space the molecules further apart than in liquid water, reducing the density.
What is surface tension?
Surface tension is the ability of a liquid surface to resist external forces, caused by the net inward pull of intermolecular forces on surface molecules.
How does hydrogen bonding give water a high surface tension?
Surface water molecules are pulled downwards and inwards by hydrogen bonds with molecules below them. This compresses the surface layer, increasing its resistance to external forces.
Ammonia can form a maximum of .......... hydrogen bond(s) per molecule, because it has .......... lone pair(s) on the nitrogen atom.
Ammonia can form a maximum of one hydrogen bond(s) per molecule, because it has one lone pair(s) on the nitrogen atom.
True or False?
The enthalpy of vaporisation of water is lower than expected based on the trend in Group 16 hydrides.
False.
Water's enthalpy of vaporisation is almost three times higher than expected, due to the additional energy needed to break hydrogen bonds between molecules.
State three anomalous properties of water caused by hydrogen bonding.
Unusually high melting point and boiling point
High surface tension
Ice is less dense than liquid water
How does molecular branching affect boiling point, and why?
Branching reduces the surface area of a molecule, decreasing contact between adjacent molecules. This weakens London dispersion forces and lowers the boiling point.
Why do boiling points of straight-chain alkanes increase with chain length?
Longer chains have more electrons and greater surface area, leading to stronger London dispersion forces between molecules. More energy is needed to separate them.
True or False?
A branched-chain alkane has a higher boiling point than its straight-chain isomer.
False.
Branching reduces surface area and weakens London dispersion forces, so a branched-chain alkane has a lower boiling point than its straight-chain isomer.
Why does propanol (bp 97°C) have a much higher boiling point than propane (bp -42°C)?
Propanol molecules form hydrogen bonds via their O-H groups, which are much stronger than the induced dipole-dipole forces in propane. More energy is needed to separate propanol molecules.
Branching .......... the surface area of a molecule, which .......... the London dispersion forces between molecules.
Branching reduces the surface area of a molecule, which weakens the London dispersion forces between molecules.
Why does HF have a higher boiling point than HCl despite having fewer electrons?
HF molecules form hydrogen bonds with each other, due to the highly polar H-F bond and the small, intense lone pairs on fluorine. These are stronger than the induced dipole-dipole forces in HCl.
How does the number of electrons in a molecule affect the strength of London dispersion forces?
More electrons increase the likelihood of temporary dipoles forming. This results in stronger and more frequent London dispersion forces and a higher boiling point.
True or False?
The boiling points of HCl, HBr and HI increase in the order HCl < HBr < HI.
True.
As the molecules increase in size, the number of electrons increases, strengthening London dispersion forces and raising the boiling point.
Why do noble gases have higher boiling points as you go down Group 18?
Each successive noble gas has more electrons, increasing the frequency and magnitude of temporary dipoles. This strengthens London dispersion forces and raises the boiling point.
What is the general principle that explains why substances dissolve in particular solvents?
'Like dissolves like': non-polar solutes dissolve in non-polar solvents and polar solutes dissolve in polar solvents, because the solute-solvent interactions must be comparable in strength to the forces within the solute.
Why does ethanol dissolve readily in water?
Ethanol contains an O-H bond and can form hydrogen bonds with water molecules. These solute-solvent interactions are strong enough to overcome the forces within each pure substance.
True or False?
Haloalkanes are completely soluble in water.
False.
Haloalkanes are only partially soluble in water. They contain a polar C-X bond but cannot form hydrogen bonds with water as they have no H-F, H-O or H-N bonds.
Why does the solubility of alcohols in water decrease as the carbon chain length increases?
As the chain gets longer, the non-polar hydrocarbon part of the molecule becomes a larger fraction of the structure. The polar O-H group makes up a smaller proportion, weakening the ability to form hydrogen bonds with water.
When an ionic compound dissolves in water, the δ+ end of the water molecule surrounds the .......... ion and the δ− end surrounds the .......... ion.
When an ionic compound dissolves in water, the δ+ end of the water molecule surrounds the negative (anion) ion and the δ− end surrounds the positive (cation) ion.
Why do giant covalent substances generally not dissolve in any solvent?
The energy needed to break the strong covalent bonds throughout the lattice is too great to be compensated for by solute-solvent interactions.
True or False?
In general, ionic compounds with a greater ionic charge are more soluble in water.
False.
In general, the greater the ionic charge, the less soluble an ionic compound is, because the electrostatic forces in the lattice are stronger and harder to overcome.
Why are haloalkanes only partially soluble in water despite containing a polar C-X bond?
Haloalkanes cannot form hydrogen bonds with water as they have no H-F, H-O or H-N bonds. The dipole-dipole interactions between haloalkane and water are not strong enough to fully overcome the hydrogen bonding within water.
What type of intermolecular force forms between a non-polar solute and a non-polar solvent when the solute dissolves?
Induced dipole-dipole forces (London dispersion forces) form between the non-polar solute and the non-polar solvent molecules.
Sign up to unlock flashcards
or