(Green) Intermolecular Forces

Question 1

what is the key word called for the forces acting within a molecule

Answer 1

The bonding within a molecule is intramolecular,

Question 2

what is the key word called for the forces acting outside a molecule

Answer 2

the forces between molecules are intermolecular

Question 3

what are the three types of intremolcular forces

Answer 3

· London forces or dispersion forces also called Instantaneous dipole – induced dipole forces or Van der Waals forces

· Permanent dipole – permanent dipole forces

· Hydrogen bonds (which are intermolecular forces NOT bonds!)

Question 4

Why would the melting points and boiling points of a substance give a measure of the strength of intermolecular forces?

Answer 4

· Forces of attraction between molecules need to be overcome for a substance to melt / boil

· Intermolecular forces are overcome by supplying heat energy

· Stronger intermolecular forces require more heat energy, hence M.Pt. higher

Question 5

how do london forces work

Answer 5

This type of force is always present and varies in strength depending on the number of electrons and shape of the molecule. The electrons in a molecule or atom are always in constant motion. Although on average the electron charge is symmetrically distributed in a neutral molecule, at any given instant it may not be. At such an instant there may be more negative charge in one area of a molecule than another and more positive charge in another area. This creates an instantaneous dipole.

The instantaneous charge distribution in one such molecule influences the charge distribution in nearby molecules. An induced charge distribution then results. A negative area produces a positive area in the neighbouring section of the next molecule as it repels the electrons, whilst a positive area induces a negative charge as it attracts the electrons. An electrostatic attractive force results.

Question 6

give three featurs of london forces

Answer 6

·The dipoles are not permanent, they arise and disappear all the time. The molecule remains neutral. The forces however are always attractive ones.

· The more electrons an atom/molecule possesses the stronger these forces will be. This is because the dipoles created can be greater.

· The shape of the molecule also affects the strength of attraction. Long thin molecules have larger instantaneous dipoles and can approach closer.

Question 7

Explain why the boiling point increases with increasing chain length.

Answer 7

Increased chain length means more electrons and longer molecule so there will be a larger instantaneous dipole which induces the opposite charge distribution in a neighbouring molecule, these then attract

Question 8

Do these molecules have the same number of carbon and hydrogen atoms?
So what can you say about the number of electrons of both?
Explain why their boiling points are so different.

pentane, 2-methylbutane , 2,2-dimethylpropane

Answer 8

Yes, they are isomers.

They have the same number of electrons.

Although they have the same number of electrons the shapes of the molecules influence the strength of these attractions.

Pentane has a higher boiling point because the London forces are greater.

The molecules are longer (and so set up bigger temporary dipoles) and they can lie closer together than the shorter, fatter 2,2-dimethylpropane molecules.

Question 9

what are the states of the halogen gases at room tempreature

Answer 9

fluorine : gas

chlorine : gas

Bromine : luiqid

iodine : solid

Question 10

Explain the different boiling points and change in state at room temp down the halogen group

Answer 10

number of electrons increases and radius of the molecule increases so the instantaneous dipole is larger.

This in turn induces larger dipoles in neighbouring molecules.

This results in stronger London forces which require more heat E to overcome.

So B.Pt increases down the group and halogens become solid.

Question 12

Underline the molecule(s) which are polar:
CCl 4
CHCl 3
CH2Cl2
CH3Cl

Answer 12

non polar :
CCl 4

polar :
CHCl3
CH2Cl2
CH3Cl

Question 13

draw out the displayed structural formula for CHCl3, giving the distribution of charge on each bond as δ+ and δ- where appropriate.

· Now draw a second molecule next to the first, oriented to show how a δ+ region in one molecule is attracted to a δ- region in the second.

· Show the resulting attractive force between the molecules as a dotted line

Answer 13

should have two molecules of CHCl3

with the hydrogen bonded with a dotted line from molecule 1 to a Chlorine on molecule 2

with negative dipoles on the chlorine molecules
and positive dipoles on the hydrogen molecule

Question 14

what is the force called between perminent dipoles called

Answer 14

PERMANENT DIPOLE - PERMANENT DIPOLE FORCES

Question 15

Account for the difference in boiling temperatures between the following pairs of molecules by considering both London forces and permanent dipole- permanent dipole interactions.

a) ethene which boils at -88oC and fluoromethane which boils at -78oC.

Answer 15

Ethene, CH2CH2 has 16 electrons and is a longer, non-polar molecule with weak London forces between molecules.

Fluoromethane, CH3F has 18 electrons but is a more spherical molecule which is polar so in addition to the London forces it also has permanent dipole / permanent dipole forces of attraction so more heat energy is required to overcome these resulting in a higher BPt.

Question 16

Account for the difference in boiling temperatures between the following pairs of molecules by considering both London forces and permanent dipole- permanent dipole interactions.

b) butane which boils at -0.5oC and propanone, CH3COCH3, which boils at 56oC.

Answer 16

Butane with 34 electrons is a longer non-polar molecule with London forces of attraction between molecules.

Propanone has 32 electons and is a shorter molecule so one would expect the London forces to be weaker. However it contains an electronegative d-O atom resulting in a polar d+C = d-O bond which makes to molecule polar. This results in a permanent dipole / permanent dipole attraction so more heat energy is required to overcome these resulting in a higher BPt.

Question 17

2. Why do the boiling points of the hydrogen halides, H-Cl to H-I, increase even though the strength of the permanent dipole-permanent dipole forces decrease?

Answer 17

HCl (18 electrons) has fewer electrons than HI (54 electrons) and is smaller so the instantanous dipole is smaller. Stronger London forces between HI require more heat energy to be overcome and hence a higher BPt.

Note although the electronegativity difference between H and Cl would result in a more polar molecule than H and I the permanent dipole attraction is only minor and London forces are the major factor.

Question 18

give the general rule for the increase in electro negativity

Answer 18

up and to the right of the periodic table

Question 19

Nitrogen, oxygen and fluorine are the three most electronegative elements in the periodic table. Give the formula of the hydrides formed when they are bonded to hydrogen.

Answer 19

Nitrogen…NH3…

Oxygen…H2O…..

Fluorine……HF……

Question 20

what kind of forces do you already know exist between the molecules :
NH3…
H2O…..
HF……

Answer 20

Permanent dipole-permanent dipole (as well as instantaneous dipole –induced dipole)

+ hydrogen bonds

Question 21

info card (read and recite)

Answer 21

· The O, N and F atoms have lone pairs

· The O, N and F atoms attract electron(s) from the small hydrogen atom(s) so strongly that they leave exposed protons. These protons have a high charge density and strongly attract lone pairs from neighbouring O, N and F atoms.

Question 22

what is the definition of hydrogen bonds

Answer 22

A hydrogen bond - is a particularly strong permanent dipole-permanent dipole attraction between the lone pair of electrons on a very electronegative atom (N, O or F) and a δ+ hydrogen atom directly covalently bonded to another very electronegative N, O or F atom.

Question 23

look at chem folder and look at how HF , H20 and NH3 form hydrogen bonds , see how HF and NH3 can only form 1 hydrogen bond while H20 can form two therefore is much more stronger.

Answer 23

.

Question 24

50 cm3 ethanol is pipetted into a 100 cm3 volumetric flask and the temperature recorded.

· 50 cm3 water is pipetted into the flask the meniscus level is recorded.Volume decreased to 95cm3

· The temperature is recorded. Temperature increased by 7o C

Explain the findings:
draw a diagram

Answer 24

More H-bonds are formed between ethanol and water allowing more efficient packing.

lone pair from the hydoxide group should be bonded with a hydrogen from the water molecule

Question 25

info card (read and recite )

Answer 25

https://www.worldofmolecules.com/3D/why-does-ice-float.html

Ice is less dense than liquid water and therefore floats. This is unusual as most solids are more dense than the liquid from which they are formed. The low density of ice is due to the fact that there are more extensive and permanent hydrogen bonds in ice than in water . The hydrogen bonds in ice hold the molecules apart in the crystal structure. When it melts, some of the hydrogen bonds break and the structure collapses in on itself, increasing the density. As the water continues to heat up, more and more hydrogen bonds break until they are all broken and the water boils.

Question 26

describe how a hydrogen halide looks like

Answer 26

The hydrogen halides are colourless gases at room temperature, producing steamy fumes in moist air

Question 27

Why is the B.Pt. of HF so much higher than the other hydrogen halides?

Answer 27

F is very electronegative, it has less shielding and smaller radius, and so the H-F bond is polar. H atom will be an exposed proton and form relatively strong H bonds with lone pairs of electrons from an F atom in an adjacent molecule.H-F: IIII H-F: . Strong H-bonds require more heat energy to be overcome.

Question 28

Bearing in mind that London forces are more significant than permanent dipole forces for the rest of the hydrogen halides, explain the trend in boiling points from HCl to HI

Answer 28

As HCl —–> HI, more electrons, so stronger London forces.

More heat energy is needed overcome them so the B.pt. increases

Question 29

Why is there a general increase in B.Pts. of both alkanes and alcohols as the number of carbon atoms increases?

Answer 29

An increase in C atoms is accompanied by an increase in the number of electrons and length of the molecule resulting in stronger London forces of attraction which require more heat energy to be overcome

Question 30

Why is the B. Pt. of an alcohol higher than the corresponding alkane?

Answer 30

Due to the electronegative O atom causing the attached H atom to become d+. This d+H is attracted to the lone pair of electrons on O of an adjacent molecule resulting in relatively strong H-bonding. This requires more heat energy to be overcome then the weak London forces in alkanes.

Question 31

What is meant by the specific heat capacity of a liquid?

Answer 31

The amount of energy to raise the temperature of 1g/1Kg by 1oC

Question 32

Water’s specific heat capacity is much greater than that of most other liquids. Explain why:

Answer 32

Relatively strong H-bonding in 3D which requires more heat energy to be overcome..

Question 33

the force is : metalic
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 33

between :
Metal cations and delocalised ‘sea’ of electrons

Typical strength kJ mol-1:
variable

Factors affecting the force.:

Number of delocalised electrons and radius of cation

Question 34

the force is : ionic
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 34

between :
Cations and anions

Typical strength kJ mol-1:
250

Factors affecting the force.:
Charge on ion Radius of ion (any covalent character)

Question 35

the force is : covalent (intramolecular)
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 35

between :
Atoms in a molecule or giant structure

Typical strength kJ mol-1:
200 - 500

Factors affecting the force.:
Bond enthalpy (affected by atomic radius and whether single, double or triple)

Question 36

the force is : hydration
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 36

between :
Ions and water molecules

Typical strength kJ mol-1:
100

Factors affecting the force.:
Charge on ion and ionic radius

Question 37

the force is : H-bonds
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 37

between :
d+H attached to N,O or F and lone pair on N O F

Typical strength kJ mol-1:
5 - 40

Factors affecting the force.:

Electronegativity difference between d+H and N O or F. Strongest if in a straight line with bond

Question 38

the force is : london forces
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 38

between :
All molecules and atoms

Typical strength kJ mol-1:
10

Factors affecting the force.:
Number of electrons. Shape of molecule/ radius of atom Distance apart

Question 39

the force is : Permanent.dipole
– permanent dipole
it is between which species ?
typical strength ?
Factors that affect the force ?

Answer 39

between :
Polar molecules

Typical strength kJ mol-1:
1.5

Factors affecting the force.:

Electronegativity difference of atoms in molecule. Geometry of molecule and distance apart.

Question 40

Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.

1) CH3F (-78oC) and CH3I (42- 43oC)

Answer 40

Both have London, stronger for CHI3 because more electrons and larger.

C-F electronegativity difference —> bond polar molecule not symmetrical —> molecule polar —> pd/pd forces stronger BUT proportionally a lesser effect.

Question 41

Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.

2) CH3Cl (a polar molecule) (-24.2oC)and CCl4 (a non polar molecule) (76.7oC)

Answer 41

Both have London, much stronger for CCl4 because more electrons.

C-Cl electronegativity difference —-> bond polar. CCl4 molecule is symmetrical so molecule is non-polar. CH3Cl molecule not symmetrical —> molecule polar —> pd/pd forces; they do not outweigh the stronger London in CCl4

Question 42

Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.

CH3CH2F (-37.7 oC) and CH3CH2OH (78 oC)

Answer 42

Both have London, same number of electrons.

C-F electronegativity difference —> bond polar molecule not symmetrical —-> molecule polar —> pd/pd forces stronger.

O-H electronegativity difference —> Hd+ and lone pair on O so forms H-bonds —> forces stronger so higher B.Pt.

Question 43

Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.

CH3CH2OH (78 oC) and CH3CH2NH2 (16.6 oC)

Answer 43

Both have similar London forces

Both make H-bonds but O-H electronegativity difference à Hd+ and lone pair on O so forms H-bonds. N-H electronegativity difference not as great so weaker H-bonds.

Question 44

Using your understanding of intermolecular forces, underline the compound with the highest boiling point and justify you answer.

Br2 (58.8 oC) and HBr (-66.8 oC)

Answer 44

Both have London, much stronger for Br2 because more electrons and larger.

HBr is polar so will have pd/pd as well.

Question 45

info card (read and recite)

Answer 45

When an ionic solid dissolves the ions become hydrated (electrostatic attractions form between the ions and polar water molecules) this is an exothermic process. The energy released is enough to overcome the lattice energy binding the ions together in the solid.

Sodium ions and chloride ions leaving a crystal lattice and becoming hydrated as they dissolve in water. Here there are electrostatic attractions between the ions and the polar water molecules which compensate for the attractions between oppositely charged ions in the solid.

Let us consider the old adage ‘Like dissolves like’. This means that polar molecules generally dissolve in polar solvents and non-polar molecules usually dissolve in non-polar solvents.

For a substance to dissolve the forces of attraction between the solute molecules and those between the solvent molecules must be overcome.

Question 46

Name the 3 types of force between covalent molecules that may have to be overcome:-

and rank them in order or strength

Answer 46

H-bonds. > London forces > permanent dipole.

Question 47

infocard (Read and recite )

Answer 47

If the total energy required to separate the solute particles and to separate the solvent molecules is less than the energy released when new forces between the solute and solvent are formed, then the substance will dissolve

Question 48

infocard (Read and recite )

Answer 48

Solubility can be described as: Very soluble or Soluble or Sparingly soluble or Insoluble.

The degree of solubility is largely determined by the overall energy of these processes

Polar substances contain permanent dipole / permanent dipole or H-bonds as well as

London intermolecular forces. When a polar solvent dissolves a polar solute the new intermolecular forces between solute and solvent molecules that could be formed would be

hydration enthalpy, permanent dipole/permanent dipole or H-bonds as well as

London forces. Therefore, the solute would probably dissolve because

energy required to overcome forces < energy released when new bonds are made

Non-polar substances contain London. intermolecular forces only. When a

non-polar solvent dissolves a non polar solute the new intermolecular forces between solute and solvent molecules that could be formed would be London. Therefore, the solute

would probably dissolve because energy required to overcome London forces = energy released when new London force form

Question 49

Why are polar solutes less soluble in non-polar solvents and vice versa?

Answer 49

Energy required to overcome pd/pd or H-bonds and London forces > energy released when weak London forces are made.

Question 50

Explain the relative solubility of each of the following solutes in each of the three solvents.

Iodine is most soluble in cylclohexane

Answer 50

Iodine is most soluble in cylclohexane because overcoming London between iodine molecules and cyclohexane molecules is compensated by making new London forces between the iodine and cyclohexane molecules.

In water and ethanol the new London forces do not compensate for overcoming the H-bonds in the solvent.

Question 51

Explain the relative solubility of each of the following solutes in each of the three solvents.

Sucrose is most soluble in water because

Answer 51

Sucrose is most soluble in water because overcoming the H bonds between sucrose molecules and water molecules are compensated by formation of new H-bonds between the sucrose and water molecules.

Partially soluble in ethanol because fewer H-bonds are made.

Insoluble in cyclohexane because the London forces made do not compensate….

Question 52

Explain the relative solubility of each of the following solutes in each of the three solvents.

Calcium chloride is most soluble in waterthan ethanol because

Answer 52

Calcium chloride is most soluble in water because in water the energy released as the ions are hydrated compensates for the energy needed to overcome the H-bonds in water and the lattice enthalpy between ions in the solid.

In ethanol only weak p.d. can form which partially compensates for the lattice enthalpy and in cyclohexane London forces do not compensate for overcoming the lattice.

Question 53

describe the solubility and observations of adding :

iodine and water

Answer 53

V slight colour change Insoluble

Question 54

describe the solubility and observations of adding :

iodine and ethanol

Answer 54

Deep brown colour + solid Sparingly soluble

Question 55

describe the solubility and observations of addingto I2 :

cyclohexane

Answer 55

Purple solution Very soluble

Question 56

describe the solubility and observations of adding :

sucrose and water

Answer 56

Very soluble

Question 57

describe the solubility and observations of adding :

ethanol

Answer 57

Sparingly soluble

Question 58

describe the solubility and observations of adding :

cyclohexane

Answer 58

Insoluble

Question 59

describe the solubility and observations of adding :

Calcium chloride and water

Answer 59

Thick ppt. of AgCl Solubl

Question 60

describe the solubility and observations of adding :

Calcium chloride and ethanol

Answer 60

Slight ppt. Sparingly soluble

Question 61

describe the solubility and observations of adding :

Calcium chloride and cyclohexane

Answer 61

No ppt. Insoluble