New Way Chemistry for Hong Kong A-Level Book 1 1 Intermolecular Forces 11.1Polarity of Molecules...
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Transcript of New Way Chemistry for Hong Kong A-Level Book 1 1 Intermolecular Forces 11.1Polarity of Molecules...
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
New Way Chemistry for Hong Kong A-Level Book 12
11.11.11 Polarity of Polarity of
MoleculesMolecules
New Way Chemistry for Hong Kong A-Level Book 13
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)
New Way Chemistry for Hong Kong A-Level Book 14
3 types of dipoles3 types of dipoles
Permanent dipole
Permanent dipole
Instantaneous dipole
Instantaneous dipole
Induced dipole
Induced dipole
11.1 Polarity of molecules (SB p.275)
Polarity of moleculesPolarity of molecules
New Way Chemistry for Hong Kong A-Level Book 15
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.
11.1 Polarity of molecules (SB p.275)
New Way Chemistry for Hong Kong A-Level Book 16
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.
11.1 Polarity of molecules (SB p.276)
New Way Chemistry for Hong Kong A-Level Book 17
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).
11.1 Polarity of molecules (SB p.276)
New Way Chemistry for Hong Kong A-Level Book 18
11.11.22 Van der WaalsVan der Waals
’ Forces’ Forces
New Way Chemistry for Hong Kong A-Level Book 19
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)
New Way Chemistry for Hong Kong A-Level Book 110
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
New Way Chemistry for Hong Kong A-Level Book 111
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.
New Way Chemistry for Hong Kong A-Level Book 112
Instantaneous dipole-induced Instantaneous dipole-induced dipole interactionsdipole interactions
11.2 Van der Waals’ forces (SB p.277)
• 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
New Way Chemistry for Hong Kong A-Level Book 113
11.2 Van der Waals’ forces (SB p.278)
Instantaneous dipole-induced Instantaneous dipole-induced dipole interactionsdipole interactions
New Way Chemistry for Hong Kong A-Level Book 114
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
11.2 Van der Waals’ forces (SB p.279)
New Way Chemistry for Hong Kong A-Level Book 115
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
New Way Chemistry for Hong Kong A-Level Book 116
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
11.2 Van der Waals’ forces (SB p.279)
1. Size of electron 1. Size of electron cloudcloud
New Way Chemistry for Hong Kong A-Level Book 117
2. Surface area of 2. Surface area of moleculemolecule
11.2 Van der Waals’ forces (SB p.280)
New Way Chemistry for Hong Kong A-Level Book 118
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
11.2 Van der Waals’ forces (SB p.280)
New Way Chemistry for Hong Kong A-Level Book 119
11.2 Van der Waals’ forces (SB p.280)
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
New Way Chemistry for Hong Kong A-Level Book 120
11.2 Van der Waals’ forces (SB p.281)
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
New Way Chemistry for Hong Kong A-Level Book 121
11.2 Van der Waals’ forces (SB p.280)
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
New Way Chemistry for Hong Kong A-Level Book 122
Van der WaalsVan der Waals’ Radii’ Radii
11.11.33
New Way Chemistry for Hong Kong A-Level Book 123
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
New Way Chemistry for Hong Kong A-Level Book 124
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
New Way Chemistry for Hong Kong A-Level Book 125
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)
New Way Chemistry for Hong Kong A-Level Book 126
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
New Way Chemistry for Hong Kong A-Level Book 127
Molecular Molecular CrystalsCrystals
11.11.44
New Way Chemistry for Hong Kong A-Level Book 128
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)
New Way Chemistry for Hong Kong A-Level Book 129
11.4 Molecular crystals (SB p.285)
IodineIodine
A unit cell of iodine crystal showing the orientation of I2 molecules
New Way Chemistry for Hong Kong A-Level Book 130
11.4 Molecular crystals (SB p.285)
Dry iceDry ice
A unit cell of dry ice (CO2)
New Way Chemistry for Hong Kong A-Level Book 131
11.4 Molecular crystals (SB p.286)
BuckminsterfullereneBuckminsterfullerene• New form carbon, C60
New Way Chemistry for Hong Kong A-Level Book 132
Hydrogen Hydrogen BondingBonding
11.11.55
New Way Chemistry for Hong Kong A-Level Book 133
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)
New Way Chemistry for Hong Kong A-Level Book 134
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
11.5 Hydrogen bonding (SB p.286)
New Way Chemistry for Hong Kong A-Level Book 135
Formation of hydrogen bonds Formation of hydrogen bonds in hydrogen fluoridein hydrogen fluoride
11.5 Hydrogen bonding (SB p.287)
New Way Chemistry for Hong Kong A-Level Book 136
Formation of hydrogen bonds in Formation of hydrogen bonds in waterwater
11.5 Hydrogen bonding (SB p.287)
New Way Chemistry for Hong Kong A-Level Book 137
Formation of hydrogen bonds Formation of hydrogen bonds in ammoniain ammonia
11.5 Hydrogen bonding (SB p.287)
New Way Chemistry for Hong Kong A-Level Book 138
Formation of hydrogen bonds in Formation of hydrogen bonds in methanolmethanol
11.5 Hydrogen bonding (SB p.287)
New Way Chemistry for Hong Kong A-Level Book 139
11.5 Hydrogen bonding (SB p.287)
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
New Way Chemistry for Hong Kong A-Level Book 140
Pressure-temperature diagram of Pressure-temperature diagram of waterwater
11.5 Hydrogen bonding (SB p.288)
• Quite similar to that of CO2
• One exception: slope of melting point curve is negative
New Way Chemistry for Hong Kong A-Level Book 141
Extraordinary features in relation Extraordinary features in relation to hydrogen bond formationto hydrogen bond formation
11.5 Hydrogen bonding (SB p.288)
• High m.p. and b.p.
• Ice melts to give liquid water with a contraction in volume
New Way Chemistry for Hong Kong A-Level Book 142
Importance of hydrogen bonding Importance of hydrogen bonding in physical phenomenain physical phenomena
11.5 Hydrogen bonding (SB p.288)
1. Anomalous properties of the second period hydrides
Abnormally high b.p. of NH3, H2O and HF
New Way Chemistry for Hong Kong A-Level Book 143
11.5 Hydrogen bonding (SB p.288)
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.
New Way Chemistry for Hong Kong A-Level Book 144
11.5 Hydrogen bonding (SB p.289)
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
New Way Chemistry for Hong Kong A-Level Book 145
11.5 Hydrogen bonding (SB p.289)
Enthalpy of vaporization of Group VI Enthalpy of vaporization of Group VI hydrideshydrides
• Abnormally high enthalpy of vaporization
formation of intermolecular hydrogen bonds
New Way Chemistry for Hong Kong A-Level Book 146
11.5 Hydrogen bonding (SB p.290)
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
New Way Chemistry for Hong Kong A-Level Book 147
11.5 Hydrogen bonding (SB p.290)
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
New Way Chemistry for Hong Kong A-Level Book 148
Hydrogen bonding in water and Hydrogen bonding in water and iceice
11.5 Hydrogen bonding (SB p.291)
• 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
New Way Chemistry for Hong Kong A-Level Book 149
11.5 Hydrogen bonding (SB p.292)
• 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
New Way Chemistry for Hong Kong A-Level Book 150
Hydrogen bonding in Hydrogen bonding in proteinsproteins
11.5 Hydrogen bonding (SB p.293)
• Primary structure of protein: polymer of amino acids
New Way Chemistry for Hong Kong A-Level Book 151
Hydrogen bonding in Hydrogen bonding in proteinsproteins
11.5 Hydrogen bonding (SB p.293)
• H bonds formed between NH and CO groups of protein chains
• creates the secondary coiled (helix) structure of the protein chain
New Way Chemistry for Hong Kong A-Level Book 152
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
11.5 Hydrogen bonding (SB p.293)
New Way Chemistry for Hong Kong A-Level Book 153
11.5 Hydrogen bonding (SB p.294)
Hydrogen bonding and the double helix of DNA
Example 11-5BExample 11-5B
Check Point 11-5Check Point 11-5
New Way Chemistry for Hong Kong A-Level Book 154
The END
New Way Chemistry for Hong Kong A-Level Book 155
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
11.2 Van der Waals’ forces (SB p.280)
New Way Chemistry for Hong Kong A-Level Book 156
11.2 Van der Waals’ forces (SB p.280)
(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
New Way Chemistry for Hong Kong A-Level Book 157
11.2 Van der Waals’ forces (SB p.280)
(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
New Way Chemistry for Hong Kong A-Level Book 158
11.2 Van der Waals’ forces (SB p.280)
(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
New Way Chemistry for Hong Kong A-Level Book 159
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)
New Way Chemistry for Hong Kong A-Level Book 160
The relative molecular masses and boiling points of five compounds are given below:
11.5 Hydrogen bonding (SB p.291)
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
New Way Chemistry for Hong Kong A-Level Book 161
(a)Ammonia, hydrogen fluoride and water have similar relative molecular masses, yet their boiling points are different. Explain why.
11.5 Hydrogen bonding (SB p.291)
(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
New Way Chemistry for Hong Kong A-Level Book 162
(b)Ethanol and methanol have similar structures, yet their boiling points are different. Explain why.
Back
11.5 Hydrogen bonding (SB p.291)
(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
New Way Chemistry for Hong Kong A-Level Book 163
11.5 Hydrogen bonding (SB p.293)
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
New Way Chemistry for Hong Kong A-Level Book 164
(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
11.5 Hydrogen bonding (SB p.296)
Answer
New Way Chemistry for Hong Kong A-Level Book 165
11.5 Hydrogen bonding (SB p.296)
(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
New Way Chemistry for Hong Kong A-Level Book 166
(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.
11.5 Hydrogen bonding (SB p.296)
Answer
New Way Chemistry for Hong Kong A-Level Book 167
11.5 Hydrogen bonding (SB p.296)
(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.
New Way Chemistry for Hong Kong A-Level Book 168
(c) Cyclohexane (C6H12) is insoluble in water whereas glucose (C6H12O6) is miscible with water in all proportions.
11.5 Hydrogen bonding (SB p.296)
Answer
New Way Chemistry for Hong Kong A-Level Book 169
11.5 Hydrogen bonding (SB p.296)
(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
New Way Chemistry for Hong Kong A-Level Book 170
11.5 Hydrogen bonding (SB p.297)
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
New Way Chemistry for Hong Kong A-Level Book 171
11.5 Hydrogen bonding (SB p.297)
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