New Way Chemistry for Hong Kong A-Level Book 1 1 Intermolecular Forces 11.1Polarity of Molecules 11.2Van der Waals’ Forces 11.3Van der Waals’ Radii 11.4Molecular

New Way Chemistry for Hong Kong A-Level Book 11

Intermolecular Intermolecular ForcesForces

11.111.1 Polarity of MoleculesPolarity of Molecules

11.211.2 Van der Waals’ ForcesVan der Waals’ Forces

11.311.3 Van der Waals’ RadiiVan der Waals’ Radii

11.411.4 Molecular CrystalsMolecular Crystals

11.511.5 Hydrogen BondingHydrogen Bonding

1111


11.11.11 Polarity of Polarity of

MoleculesMolecules


Polarity of moleculesPolarity of molecules

Intermolecular forcesIntermolecular forces

Van der Waal’s forces

Van der Waal’s forces

hydrogen bonding

hydrogen bonding

(very weak when compared with covalent bond between atoms in molecule)

electrostatic attraction between dipoles, i.e. the attraction between the +ve end of one molecule and the -ve end of another molecule

11.1 Polarity of molecules (SB p.275)


3 types of dipoles3 types of dipoles

Permanent dipole

Permanent dipole

Instantaneous dipole

Instantaneous dipole

Induced dipole

Induced dipole


Polarity of moleculesPolarity of molecules


Permanent dipolePermanent dipoleA permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.

A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.



Instantaneous dipoleInstantaneous dipoleAn instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.



Induced dipoleInduced dipoleAn induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which may be a permanent or an instantaneous dipole).

An induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which may be a permanent or an instantaneous dipole).



11.11.22 Van der WaalsVan der Waals

’ Forces’ Forces


Van der Waals’ ForcesVan der Waals’ Forces

Van der Waals’ forces

Van der Waals’ forces

Dipole-Dipole

Interaction

Dipole-Dipole

Interaction

Dipole-Induced Dipole

Interaction

Dipole-Induced Dipole

Interaction

Instantaneous Dipole-Induced Dipole

Interaction

Instantaneous Dipole-Induced Dipole

Interaction

11.2 Van der Waals’ forces (SB p.276)


Dipole-dipole interactionsDipole-dipole interactions11.2 Van der Waals’ forces (SB p.277)

• Polar molecules have permanent dipole moments.

• They tend to orient themselves in such a way that the attractive forces between molecules are maximized while repulsive forces are minimized


Dipole-induced dipole interactionsDipole-induced dipole interactions11.2 Van der Waals’ forces (SB p.277)

• When a non-polar molecule approaches a polar molecule (with a permanent dipole), a dipole will be induced in the non-polar molecule

• The dipole induced will be in opposite orientation to that of the polar molecule.


Instantaneous dipole-induced Instantaneous dipole-induced dipole interactionsdipole interactions


• The instantaneous dipole will induce a dipole moment in the neighbouring atom by attracting opposite charges

• If the +ve end of the dipole is pointing towards a neighbouring atom, the induced dipole will then have its -ve end pointing towards the +ve pole of that dipole



Instantaneous dipole-induced Instantaneous dipole-induced dipole interactionsdipole interactions


Strength of van der Waals’ forcesStrength of van der Waals’ forces

Type of interaction Magnitude (kJ mol-1)

Dipole-dipole 5 - 25

Dipole-induced dipole 2 - 10

Instantaneous dipole-induced dipole

0.05 - 50



Strength of van der Waals’ forcesStrength of van der Waals’ forces11.2 Van der Waals’ forces (SB p.279)

Two factors affecting the strength of van der Waals’ forces

• Sizes of electron clouds of molecules

• Surface area of molecules


The greater the no. of e-s in a moleculeThe greater the no. of e-s in a molecule

The more weakly they are held by the nucleusThe more weakly they are held by the nucleus

The easier the instantaneous dipole can be set up (greater van der Waals’ forces)

The easier the instantaneous dipole can be set up (greater van der Waals’ forces)

Molecule Boiling point (o

C)

Helium

Neon

Argon

-269

-246

-186

Fluorine

Chlorine

Bromine

-188

-34.7

58.8

Methane

Ethane

Propane

-162

-88.6

-42.2


1. Size of electron 1. Size of electron cloudcloud


2. Surface area of 2. Surface area of moleculemolecule



The van der Waals’ forces also increase with the surface area of the molecule.The van der Waals’ forces also increase with the surface area of the molecule.

2. Surface area of 2. Surface area of moleculemolecule




Change of states and Change of states and intermolecular forcesintermolecular forces• 3 different states: solid, liquid and gas

• Molecules in different orders in the three states

highest order in the solid state

lowest order in the gas state

• Change of state is related to the strength of intermolecular forces of the molecular

substances



Pressure-temperature diagram Pressure-temperature diagram of carbon dioxideof carbon dioxide

Line TB: Melting point curve

Line TC: Boiling point curve

Critical point: Beyond this pt, liquid and vapour become indisguishable

Triple point: 3 states coexist at equilibrium



Distinguishing features of the pressure-Distinguishing features of the pressure-temperature diagram of carbon dioxidetemperature diagram of carbon dioxide

• Melting point curve has a +ve slope

melting of CO2 becomes more difficult with increase in temp.

• Triple point is at 5.1 atm and –57 oC

at 1 atm, CO2 sublimes

No liquid state of CO2 exists under normal atmospheric condition

Dry iceCheck Point 11-2Check Point 11-2


Van der WaalsVan der Waals’ Radii’ Radii

11.11.33


Van der Waals’ RadiiVan der Waals’ Radii11.3 Van der Waals’ radii (SB p.282)

• Van der Waals’ forces determine the closest distance between argon atoms


Radii of Radii of iodineiodine

11.3 Van der Waals’ radii (SB p.283)

• The covalent radius is one half of the distance between two atoms in the same molecule.

• The van der Waals’ radius is one half of the distance between two atoms in adjacent molecule.

Covalent and van der Waals’ radii

of iodine


Covalent and van der Waals’ radii oCovalent and van der Waals’ radii of some elementsf some elements

11.3 Van der Waals’ radii (SB p.283)


Structure of graphiteStructure of graphite11.3 Van der Waals’ radii (SB p.284)

Sum of covalent radii of two C atoms

Sum of van der Waals’ radii of two C atoms

Check Point 11-3Check Point 11-3


Molecular Molecular CrystalsCrystals

11.11.44


Molecular crystalsMolecular crystals

A molecular crystal is a structure which consists of individual molecules packed

together in a regular arrangement by weak intermolecular forces.

A molecular crystal is a structure which consists of individual molecules packed

together in a regular arrangement by weak intermolecular forces.

11.4 Molecular crystals (SB p.284)



IodineIodine

A unit cell of iodine crystal showing the orientation of I2 molecules



Dry iceDry ice

A unit cell of dry ice (CO2)



BuckminsterfullereneBuckminsterfullerene• New form carbon, C60


Hydrogen Hydrogen BondingBonding

11.11.55


very +vevery +veF being very

electronegativeF being very

electronegative

HF HF moleculemolecule

F atom being small enough to approach very close to the H atom in the neighbouring molecule

F atom being small enough to approach very close to the H atom in the neighbouring molecule

11.5 Hydrogen bonding (SB p.286)


Relative strength of van der Waals’ forces, Relative strength of van der Waals’ forces, hydrogen bond and covalent bondhydrogen bond and covalent bondPhenomenon Energy

involved

(kJ mol-1)

Forces overcome

Sublimation of solid helium

0.11 Van der Waals’ forces

Sublimation of

ice

46.90 Hydrogen bonds

Dissociation of hydrogen molecules

436.00 Covalent bonds



Formation of hydrogen bonds Formation of hydrogen bonds in hydrogen fluoridein hydrogen fluoride



Formation of hydrogen bonds in Formation of hydrogen bonds in waterwater



Formation of hydrogen bonds Formation of hydrogen bonds in ammoniain ammonia



Formation of hydrogen bonds in Formation of hydrogen bonds in methanolmethanol




Essential requirements for Essential requirements for the formation of hydrogen the formation of hydrogen bondbond• H atom must be directly bonded to a highly

electronegative atom (e.g. F, O and N)

• An unshared pair of electrons (lone pair electrons) is present on the electronegative

atom


Pressure-temperature diagram of Pressure-temperature diagram of waterwater


• Quite similar to that of CO2

• One exception: slope of melting point curve is negative


Extraordinary features in relation Extraordinary features in relation to hydrogen bond formationto hydrogen bond formation


• High m.p. and b.p.

• Ice melts to give liquid water with a contraction in volume


Importance of hydrogen bonding Importance of hydrogen bonding in physical phenomenain physical phenomena


1. Anomalous properties of the second period hydrides

Abnormally high b.p. of NH3, H2O and HF



Explanation:

• High electronegativities of F(4.0), N(3.0) and O(3.5)

• Formation of intermolecular hydrogen bonds

• Hydrogen bonds are much stronger than van der Waals’ forces

more energy is needed to break the hydrogen bonds

abnormally high b.p.



Enthalpy of Enthalpy of vaporizationvaporization• Energy required to vaporize 1 mole of

liquid

• Related to the strength of intermolecular forces that exist in the liquid



Enthalpy of vaporization of Group VI Enthalpy of vaporization of Group VI hydrideshydrides

• Abnormally high enthalpy of vaporization

formation of intermolecular hydrogen bonds



Boiling points and solubilities of alcoholBoiling points and solubilities of alcoholss

• B.p. of thiols are much lower than those of alcohols

formation of intermolecular hydrogen bonds between alcohol molecules



Dimerization of carboxylic acidsDimerization of carboxylic acids• When ethanoic acid is dissolved in non-polar

solvents, the molecular mass of found to be 120 (not 60)

• Formation of dimer

Example 11-5AExample 11-5A


Hydrogen bonding in water and Hydrogen bonding in water and iceice


• In water, the molecules are in constant motion. H bonds are formed and broken continually. The arrangement of molecules are thus in random.

Hydrogen bonding in water



• In ice, the molecular motion is of a minimum and the molecules are oriented in such a way that the max. no. of H bonds are formed. This creates an open structure. (density of ice < density of water)

Hydrogen bonding in

ice


Hydrogen bonding in Hydrogen bonding in proteinsproteins


• Primary structure of protein: polymer of amino acids


Hydrogen bonding in Hydrogen bonding in proteinsproteins


• H bonds formed between NH and CO groups of protein chains

• creates the secondary coiled (helix) structure of the protein chain


Hydrogen bonding in Hydrogen bonding in DNADNA• DNA (deoxyribonnuclei acid) is present in the nuclei of living cells

• carries genetic information

• consists of two macromolecular strands spiraling round each other in the form of a double helix




Hydrogen bonding and the double helix of DNA

Example 11-5BExample 11-5B

Check Point 11-5Check Point 11-5


The END


How is the enthalpy of vaporization related to intermolecular forces of a simple molecular

substance like neon?

Back

The enthalpy of vaporization of a substance is the energy needed to vaporize one mole of the substance at its boiling point. Consider a substance like neon, which consists of single atoms, Neon liquefies when the temperature is lowered to –246 oC at 1 atm. The enthalpy of vaporization of the liquid at this temperature is 1.77 kJ mol-1. Some of this energy is needed to push back the atmosphere when the vapour forms. The remaining energy must be supplied to overcome the intermolecular attractions. Because each molecule in a liquid is surrounded by several neighbouring molecules, this remaining energy is some multiple of a single molecule-molecule interaction. Typically, this multiple is about 5.

Answer




(a)Comment on the relative strength of van der Waals’ forces in solid, liquid and gaseous bromine.

(a) The relative strength of van der Waals’ forces decreases in the order:

Solid bromine > liquid bromine > gaseous bromine

The van der Waals’ forces are highly dependent on the distance between adjacent molecules. It decreases exponentially with the separation between the molecules. Going from solid to liquid and then to gaseous state, the separation between molecules increases, so the van der Waals’ forces become weaker and weaker.

Answer



(b) Plastics are substances which have very strong van der Waals’ forces. Explain why the van der Waals’ forces are so strong in plastics.

(b) A large size of a molecule of plastics indicates that it has a large electron cloud which is more easily polarized. Therefore, the molecule of plastics is more likely induced to form an instantaneous dipole. Moreover, the molecule of plastics has an extensive surface area. These make plastics have very strong van der Waals’ forces between the molecules.

Answer



(c)Arrange the following substances in an increasing order of boiling point:

(i) N2, O2, Cl2, Ne

(ii) H2, Br2, He(c) (i) Ne < N2 < O2 < Cl2

(ii) He < H2 < Br2

Answer

Back


What is the consequence of two molecules approaching each other at a distance less than the sum of their van der Waals’ radii?

The electron clouds of the two molecules will repel each other, and the distance between the two molecules will increase until the repulsion is just balanced by the attraction.

Answer

Back11.3 Van der Waals’ radii (SB p.284)


The relative molecular masses and boiling points of five compounds are given below:


Compound Relative molecular

mass

Boiling point (oC)

Ammonia (NH3) 17 -33.4Ethanol (C2H5OH) 46 78Hydrogen fluoride

(HF)20 19.5

Methanol (CH3OH) 32 66Water (H2O) 18 100


(a)Ammonia, hydrogen fluoride and water have similar relative molecular masses, yet their boiling points are different. Explain why.


(a) H2O can form 2 hydrogen bonds per molecule while NH3 and HF can only form 1 hydrogen bond per molecule. Thus, the boiling point of water is higher than those of NH3 and HF. Besides, as F is more electronegative than N, the intermolecular hydrogen bond formed between HF molecules is stronger than that between NH3 molecules.

Answer


(b)Ethanol and methanol have similar structures, yet their boiling points are different. Explain why.

Back


(b) For molecules with similar structures, their boiling points depend on their relative molecular masses. As the relative molecular mass of ethanol is greater than that of methanol, the boiling point of ethanol is higher.

Answer



Why it takes much longer time to boil an egg on a mountain peak?

Back

The boiling point of water decreases with decreasing pressure. Although water boils easily at mountain peak, the cooking of an egg takes longer time. It is because the amount of heat delivered to the egg is proportional to the temperature of water.

Answer


(a)The formation of a hydrogen bond between two molecules RAH and R’B may be represented as:

R A H · · · · · · · B R’

(i) Suggest possible elements for A and B. What are their common features?

(ii) In which of the following ranges would you expect the strength of hydrogen bonds to lie?

0.1 – 10 kJ mol-1

10 – 50 kJ mol-1

100 – 400 kJ mol-1


Answer



(a) (i) A and B can be nitrogen, oxygen or fluorine. All of them are highly electronegative atoms, thus they form highly polar

molecules, resulting in the formation of hydrogen bonds.

(ii) 10 – 50 kJ mol-1


(b) Benzoic acid has an apparent relative molecular mass of 244 in hexane, but only 122 in aqueous solution. With the aid of diagrams, explain this phenomenon.


Answer



(b) The relative molecular mass of benzoic acid (C6H5COOH) is 122. In hexane, benzoic acid molecules form dimers with hydrogen bondings between the molecules.

However, in water, the benzoic acid molecules form hydrogen bonds with the water molecules.


(c) Cyclohexane (C6H12) is insoluble in water whereas glucose (C6H12O6) is miscible with water in all proportions.


Answer



(c) Cyclohexane is non-polar, and there are only weak van der Waals’ forces holding the molecules together. Thus, cyclohexane molecules do not form hydrogen bonds with water. On the other hand, glucose can form hydrogen bonds with water molecules via its OH groups. Therefore, glucose is soluble in water but cyclohexane is not.

Cyclohexane Glucose

Back



Name the types of bonding or intermolecular forces that are broken and formed in the following processes.

• H2O(s) H2O(g)

• 2Mg(s) + O2(g) 2MgO(s)

• H2(g) + F2(g) 2HF(g)

• 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

• CH3CH2OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)Answer



Back

(a) Bond broken: hydrogen bond

(b) Bonds broken: metallic bond and covalent bond

Bond formed: ionic bond

(c) Bond broken: covalent bond

Bonds formed: covalent bond and hydrogen bond

(d) Bonds broken: covalent bond, metallic bond and hydrogen bond

Bonds formed: ionic bond and covalent bond

(e) Bonds broken: covalent bond and hydrogen bond

Bonds formed: covalent bond and hydrogen bond

Documents

New Way Chemistry for Hong Kong A-Level Book 1 1 Intermolecular Forces 11.1Polarity of Molecules 11.2Van der Waals’ Forces 11.3Van der Waals’ Radii 11.4Molecular