Intermolecular Interactions and Physical Properties

Intermolecular Interactions and Physical Properties

How does the boiling temperature of unbranched alkanes change as chain length increases?

It increases as the carbon chain becomes longer.

What is the main intermolecular force between alkane molecules?

London forces.

Why does the boiling temperature of alkanes increase with increasing molecular mass?

Larger molecules contain more electrons, causing greater fluctuations in electron density, stronger instantaneous and induced dipoles, and therefore stronger London forces.

How does increasing carbon chain length affect London forces in alkanes?

A longer chain provides more points of contact between neighbouring molecules, increasing the overall London forces.

Why do longer-chain alkanes have more points of contact?

Their molecules can fit closely together along a greater length of the carbon chain.

Why do stronger London forces result in a higher boiling temperature?

More energy is required to overcome the intermolecular forces and separate the molecules.

How does branching affect the boiling temperature of an alkane?

Increased branching lowers the boiling temperature.

Why do branched alkanes have lower boiling temperatures than their unbranched isomers?

Branching reduces the points of contact between neighbouring molecules, weakening the overall London forces.

How does branching affect how closely alkane molecules pack together?

More highly branched molecules do not pack together as closely as unbranched molecules.

Which has the highest boiling temperature: pentane, 2-methylbutane or 2,2-dimethylpropane?

Pentane, because it is the least branched and has the greatest points of contact between molecules.

What are the approximate boiling temperatures of pentane, 2-methylbutane and 2,2-dimethylpropane?

309 K, 301 K and 283 K respectively.

What is the general formula of an alcohol?

CₙH₂ₙ₊₁OH.

Why can alcohol molecules form hydrogen bonds with each other?

They contain an O–H group, allowing hydrogen bonding between the δ+ hydrogen and a lone pair on an oxygen atom of another molecule.

What intermolecular forces occur between alcohol molecules?

London forces, permanent dipole–dipole interactions and hydrogen bonds.

Why do alcohols have higher boiling temperatures than comparable alkanes?

Alcohols form hydrogen bonds in addition to London forces, so more energy is required to separate their molecules.

Why is it useful to compare an alcohol with an alkane containing a similar number of electrons?

Their London forces should be similar, so differences in boiling temperature can largely be attributed to hydrogen bonding in the alcohol.

Why does methanol have a much higher boiling temperature than ethane despite both having 18 electrons?

Methanol forms hydrogen bonds between its molecules whereas ethane experiences only London forces.

How does the boiling temperature of alcohols change as carbon chain length increases?

It generally increases.

Why does the boiling temperature of alcohols increase with carbon chain length?

The number of electrons and molecular surface area increase, strengthening London forces.

Is hydrogen bonding always the predominant intermolecular force in alcohols?

No. It is particularly important in short-chain alcohols, but London forces become increasingly important as the carbon chain becomes longer.

What happens to the relative importance of hydrogen bonding as alcohol chain length increases?

It decreases because London forces become increasingly strong.

What is enthalpy change of vaporisation?

The energy required to completely separate one mole of molecules in a liquid and convert it into a gas at the same temperature.

What does a larger enthalpy change of vaporisation indicate?

Stronger intermolecular forces between the molecules.

Why can enthalpy change of vaporisation be used to compare intermolecular forces?

More energy is needed to separate molecules when their intermolecular attractions are stronger.

What is the approximate enthalpy change of vaporisation of ethanol?

38.6 kJ mol⁻¹.

Approximately how much of ethanol's enthalpy change of vaporisation can be attributed to London forces?

15.7 kJ mol⁻¹.

Approximately how much of ethanol's enthalpy change of vaporisation is associated with hydrogen bonding?

22.9 kJ mol⁻¹, or about 59% of the total.

What happens to the percentage contribution of hydrogen bonding as alcohol chain length increases?

It decreases.

Why does the percentage contribution of hydrogen bonding decrease as alcohol chain length increases?

The London forces increase with molecular size while the molecule still contains only one O–H group.

What is the approximate percentage contribution of hydrogen bonding in propan-1-ol?

56%.

What is the approximate percentage contribution of hydrogen bonding in butan-1-ol?

50%.

What is the approximate percentage contribution of hydrogen bonding in pentan-1-ol?

45%.

What is the approximate percentage contribution of hydrogen bonding in hexan-1-ol?

41%.

What trend occurs in the boiling temperatures of HCl, HBr and HI?

The boiling temperature increases from HCl to HBr to HI.

Why does boiling temperature increase from HCl to HI?

The number of electrons increases, producing stronger London forces.

What intermolecular forces are present in hydrogen halides?

London forces and permanent dipole–dipole interactions; HF also forms significant hydrogen bonds.

Why are permanent dipole interactions relatively unimportant in HCl, HBr and HI?

The predominant intermolecular interaction is the London force.

Why does HF have a much higher boiling temperature than the other hydrogen halides?

HF forms strong hydrogen bonds because fluorine is highly electronegative.

Why does HF have a high boiling temperature despite having fewer electrons than HCl, HBr and HI?

Its London forces are weaker, but its strong hydrogen bonding produces much stronger overall intermolecular attraction.

What are two important anomalous properties of water?

Water has unusually high melting and boiling temperatures, and ice is less dense than liquid water.

Why does water have an unusually high melting temperature and boiling temperature?

Strong and extensive hydrogen bonding occurs between water molecules, so a large amount of energy is required to separate them.

What is the boiling temperature of water at 100 kPa?

373 K (100°C).

What is the melting temperature of water?

273 K (0°C).

Why is it useful to compare H₂O, NH₃ and HF?

They each contain 10 electrons per molecule, so their London forces are similar and differences in boiling temperature mainly result from differences in hydrogen bonding.

What are the approximate boiling temperatures of H₂O, NH₃ and HF?

H₂O = 373 K, NH₃ = 240 K and HF = 293 K.

What are the approximate hydrogen bond strengths in H₂O, NH₃ and HF?

H₂O = 22 kJ mol⁻¹, NH₃ = 17 kJ mol⁻¹ and HF = 29 kJ mol⁻¹.

Which has the strongest individual hydrogen bonds: H₂O, NH₃ or HF?

HF.

Why does water have a higher boiling temperature than HF even though HF has stronger individual hydrogen bonds?

Water can form a more extensive hydrogen-bonding network, with up to four hydrogen bonds per molecule, whereas HF forms fewer.

How many hydrogen bonds can a water molecule form?

Up to four: two through its hydrogen atoms and two through the two lone pairs on oxygen.

How many hydrogen bonds does HF effectively form per molecule?

Two.

Why is hydrogen bonding less extensive in ammonia than in water?

Each nitrogen atom has only one lone pair, so there are not enough lone pairs to form hydrogen bonds using all three N–H hydrogens.

On average, how many hydrogen bonds can each ammonia molecule form using its lone pair?

Approximately one.

Why does ammonia have a relatively low boiling temperature compared with water and HF?

Its hydrogen bonding is less extensive and the individual hydrogen bonds are relatively weak.

Why is ice less dense than liquid water?

Hydrogen bonds hold water molecules in an open structure in ice, leaving relatively large areas of empty space.

How are water molecules arranged in ice?

They form an open hydrogen-bonded structure containing rings of six water molecules.

What happens to the hydrogen-bonded structure when ice melts?

The rigid open structure is partially destroyed and the average distance between water molecules decreases.

Why does water become denser when ice melts?

The molecules move closer together as the open hydrogen-bonded structure collapses, so the same mass occupies a smaller volume.

Why does ice float on water?

Ice is less dense than liquid water because of its open hydrogen-bonded structure.

What two conditions must generally be met for a substance to dissolve?

The solute particles must be separated and surrounded by solvent particles, and the new solute–solvent attractions must be strong enough to overcome solute–solute and solvent–solvent attractions.

Why can many ionic solids dissolve in water?

Water forms strong attractions with the ions, and hydration releases energy that helps compensate for the energy needed to separate ions in the lattice.

What is hydration of an ion?

The process in which an ion becomes surrounded by water molecules.

What is hydration energy?

The energy released when ions become hydrated by water molecules.

What type of attraction forms between an ion and a polar water molecule?

An ion–dipole interaction.

How do water molecules orient around a positive ion such as Na⁺?

The δ− oxygen ends of the water molecules point towards the positive ion.

Why does the oxygen end of water attract Na⁺ ions?

Oxygen carries a partial negative charge, δ−, which attracts the positively charged Na⁺ ion.

How do water molecules orient around a negative ion such as Cl⁻?

The δ+ hydrogen ends of the water molecules point towards the negative ion.

Why do the hydrogen ends of water attract Cl⁻ ions?

The hydrogen atoms carry partial positive charges, δ+, which attract the negatively charged Cl⁻ ion.

What happens to sodium ions when sodium chloride dissolves in water?

Na⁺ ions are removed from the ionic lattice and become surrounded by water molecules with their δ− oxygen atoms facing the ions.

What happens to chloride ions when sodium chloride dissolves in water?

Cl⁻ ions are removed from the lattice and become surrounded by water molecules with their δ+ hydrogen atoms facing the ions.

What must happen energetically for an ionic compound to dissolve in water?

The energy released by hydration must sufficiently compensate for the energy required to separate the ions in the ionic lattice.

Why can alcohols dissolve in water?

Their O–H groups can form hydrogen bonds with water molecules.

How does an alcohol form a hydrogen bond with water?

A δ+ hydrogen attached to oxygen can interact with a lone pair on another oxygen atom.

Why do ethanol and water mix in all proportions?

Hydrogen bonding between ethanol and water is similar in strength to hydrogen bonding within pure ethanol and pure water.

How does the solubility of alcohols in water change as hydrocarbon chain length increases?

It decreases.

Why does alcohol solubility in water decrease as carbon chain length increases?

The non-polar hydrocarbon portion becomes larger and London forces become increasingly important compared with the single polar O–H group.

Why are non-polar molecules such as alkanes generally insoluble in water?

Attractions between alkane and water molecules are not strong enough to replace the strong hydrogen bonds between water molecules.

Why can some polar molecules still have low solubility in water?

They may be unable to form hydrogen bonds with water, or the hydrogen bonds they form may be too weak to compensate for breaking water–water hydrogen bonds.

Why is ethoxyethane almost insoluble in water even though it is polar?

Its attractions with water are not strong enough to replace the relatively strong hydrogen bonds between water molecules.

Why are halogenoalkanes generally not very soluble in water?

Their interactions with water are not strong enough to replace the hydrogen bonds between water molecules.

Why are halogenoalkanes more soluble in ethanol than in water?

The intermolecular interactions between halogenoalkanes and ethanol are more favourable than those between halogenoalkanes and water.

What is the general rule for choosing a solvent?

Like dissolves like.

What does "like dissolves like" mean?

Substances with similar intermolecular interactions and polarity tend to dissolve in one another.

What type of solvent is generally suitable for a non-polar substance?

A non-polar solvent.

Why are alkanes soluble in one another?

They are non-polar and experience similar London forces.

Why is crude oil an example of "like dissolves like"?

It consists largely of different non-polar hydrocarbons dissolved in one another.

Why does bromine dissolve readily in hexane?

Both bromine and hexane are non-polar and interact mainly through London forces.

Why can hexane be useful as a solvent when testing for unsaturation?

Many non-polar organic substances dissolve in hexane even though they are insoluble in water.

How should you explain a boiling-temperature trend in a homologous series?

Compare molecular size, number of electrons, points of contact and the resulting strength of London forces, then relate stronger forces to the greater energy needed for boiling.

How should you explain why branching lowers boiling temperature?

Branching makes molecules more compact, reduces points of contact between neighbouring molecules, weakens London forces and therefore reduces the energy required to separate the molecules.

How should you explain why an alcohol has a higher boiling temperature than a similar alkane?

Both have similar London forces, but the alcohol also forms hydrogen bonds, giving stronger overall intermolecular attraction and requiring more energy to separate its molecules.

How should you explain the unusually high boiling temperature of water?

Water forms an extensive network of strong hydrogen bonds, with each molecule able to form up to four hydrogen bonds, so a large amount of energy is required to separate the molecules.

How should you explain why an ionic solid dissolves in water?

Polar water molecules form strong ion–dipole attractions with the ions; hydration of the ions releases energy that helps overcome the attractions holding the ionic lattice together.