New Way Chemistry for Hong Kong A-Level Book 1 1 Chapter 11 Intermolecular Forces 11.1Polarity of Molecules 11.2Van der Waals’ Forces 11.3Van der Waals’ Radii 11.4 Molecular Crystals 11.5 Hydrogen Bonding

New Way Chemistry for Hong Kong A-Level Book 1 2 Polarity of Molecules Intermoleculear forces Van der Waal’s forces hydrogen bonding (very weak when compare 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.257)

New Way Chemistry for Hong Kong A-Level Book types of dipoles Permanent dipole Permanent dipole Instantaneous dipole Instantaneous dipole Induced dipole Induced dipole 11.1 Polarity of Molecules (SB p.257) Polarity of Molecules

New Way Chemistry for Hong Kong A-Level Book 1 4 Permanent Dipole A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms Polarity of Molecules (SB p.257)

New Way Chemistry for Hong Kong A-Level Book 1 5 Instantaneous Dipole An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud Polarity of Molecules (SB p.258)

New Way Chemistry for Hong Kong A-Level Book 1 6 Induced 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) Polarity of Molecules (SB p.258)

New Way Chemistry for Hong Kong A-Level Book 1 7 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 Waal’s Forces (SB p.258)

New Way Chemistry for Hong Kong A-Level Book 1 8 Dipole-Dipole Interactions 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 Van der Waal’s Forces (SB p.258)

New Way Chemistry for Hong Kong A-Level Book 1 9 Dipole-Induced Dipole Interactions When a non-poar 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 orintation to that of the polar molecule Van der Waal’s Forces (SB p.259)

New Way Chemistry for Hong Kong A-Level Book 1 10 Instantaneous Dipole-Induced Dipole 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 Van der Waal’s Forces (SB p.259)

New Way Chemistry for Hong Kong A-Level Book Van der Waal’s Forces (SB p.260) Strength of Van der Waals’ Forces Type of interactionMagnitude (kJ mol -1 ) Dipole-dipole5-25 Dipole-induced dipole2-10 Instantaneous dipole-induced dipole

New Way Chemistry for Hong Kong A-Level Book 1 12 The greater the no. of e - s in a molecule The more weakly they are held by the nucleus The easier the instantaneous dipole can be set up (greater van der Waals’ forces) 11.2 Van der Waal’s Forces (SB p.260) MoleculeBoiling point ( o C) Helium Neon Argon Fluorine Chlorine Bromine Methane Ethane Propane

New Way Chemistry for Hong Kong A-Level Book 1 13 The van der Waals’ forces also increase with the surface area of the molecule Van der Waal’s Forces (SB p.261) Surface Area of Molecule

New Way Chemistry for Hong Kong A-Level Book 1 14 Van der Waals’ Radii 11.3 Van der Waal’s Radii (SB p.262)

New Way Chemistry for Hong Kong A-Level Book 1 15 Radii of iodine 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 Van der Waal’s Radii (SB p.262)

New Way Chemistry for Hong Kong A-Level Book Van der Waal’s Radii (SB p.263) Radii of some elements

New Way Chemistry for Hong Kong A-Level Book 1 17 sum of covalent radii sum of van der Waals’ radii 11.3 Van der Waal’s Radii (SB p.263) Structure of graphite

New Way Chemistry for Hong Kong A-Level Book 1 18 Molecular Crystals A molecular crystal is a structure which consists of individual molecules packed together in a regular arrangement by weak intermolecular forces Molecular Crystals (SB p.264)

New Way Chemistry for Hong Kong A-Level Book 1 19 very +ve F being very electronegative 11.5 Hydrogen Bonding (SB p.264) HF molecule F atom being small enough to approach very close to the H atom in the neighbouring molecule

New Way Chemistry for Hong Kong A-Level Book Hydrogen Bonding (SB p.265) The relative strength of van der Waals’ forces, hydreogen bond and covalent bond PhenomenonEnergy involved (kJ mol -1 ) Forces overcome Sublimation of solid helium 0.11Van der Waals’ forces Sublimation of ice 46.90Hydrogen bonds Dissociation of hydrogen molecules Cocalent bonds

New Way Chemistry for Hong Kong A-Level Book Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in hydrogen fluoride

New Way Chemistry for Hong Kong A-Level Book Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in water

New Way Chemistry for Hong Kong A-Level Book 1 23 Formation of hydrogen bonds in ammonia 11.5 Hydrogen Bonding (SB p.265)

New Way Chemistry for Hong Kong A-Level Book Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in methanol

New Way Chemistry for Hong Kong A-Level Book 1 25 Experimental Determination of the Strength of Hydrogen Bond trichloromethane Cl very +ve ?Any H-bond formed? 11.5 Hydrogen Bonding (SB p.266)

New Way Chemistry for Hong Kong A-Level Book 1 26 Experimental Determination of the Strength of Hydrogen Bond ethyl ethanoate 11.5 Hydrogen Bonding (SB p.266) ?Any H-bond formed?

New Way Chemistry for Hong Kong A-Level Book 1 27 Experimental Determination of the Strength of Hydrogen Bonds ?Any H-bond formed? (YES!) H bond formed between trichloromethane & ethyl ethanoate How strong is it? Hint: When you mix the 2 liquids together, what will happen? 11.5 Hydrogen Bonding (SB p.266)

New Way Chemistry for Hong Kong A-Level Book 1 28 Intramolecular Hydrogen bonding Butenedioic acid cis- butenedioic acidtrans- butenedioic acid m.p. = Cm.p. = C cis-trans isomers (geometric isomers) 11.5 Hydrogen Bonding (SB p.267)

New Way Chemistry for Hong Kong A-Level Book 1 29 Intramolecular Hydrogen bonding m.p. = C cis- butenedioic acidtrans- butenedioic acid m.p. = C Owing to the formation of intramolecular H bonds, cis- butenedioic acid forms less extensive intermolecular H bonds with neighbouring molecules Hydrogen Bonding (SB p.267)

New Way Chemistry for Hong Kong A-Level Book 1 30 Intramolecular Hydrogen bonding 2-nitrophenol 4-nitrophenol Can you match the two compounds with the following m.p.’s? m.p. = C m.p. = C 11.5 Hydrogen Bonding (SB p.267)

New Way Chemistry for Hong Kong A-Level Book 1 31 Intramolecular Hydrogen bonding m.p. = C m.p. = C 11.5 Hydrogen Bonding (SB p.267)

New Way Chemistry for Hong Kong A-Level Book 1 32 Anomalous Properties of the Second Period Hydrides Molecular mass  Van der Waals’ forces  (b.p.  ) There must be some type of intermolecular force (which is much stronger than van der Waals’ forces) in NH 3, H 2 O & HF (Hydrogen bonding) 11.5 Hydrogen Bonding (SB p.267)

New Way Chemistry for Hong Kong A-Level Book 1 33 Essential requirements for the formation of a hydrogen bond 1. A hydrogen atom must be directly bonded to a highly electronegative atom (F, O, N). 2. An unshared pair of electrons (lone pair electrons) on the electronegative atom Hydrogen Bonding (SB p.267)

New Way Chemistry for Hong Kong A-Level Book 1 34 Boiling Points and Solubilities of Alcohol 11.5 Hydrogen Bonding (SB p.268)

New Way Chemistry for Hong Kong A-Level Book 1 35 Organic compounds are usually insoluble in water, e.g. ethane (C 2 H 6 ) or chloroethane (C 2 H 5 Cl) But alochols of low molecular mass are soluble in water, e.g methanol (CH 3 OH) and ethanol (C 2 H 5 OH) Hydrogen Bonding (SB p.268) Boiling Points and Solubilities of Alcohol

New Way Chemistry for Hong Kong A-Level Book 1 36 Dimerization of Carboxylic Acids In vapour phase or in organic solvents, carboxylic acids (alkanoic acids) exist as dimers. A dimer of ethanoic acid 11.5 Hydrogen Bonding (SB p.268)

New Way Chemistry for Hong Kong A-Level Book 1 37 Hydrogen Bonding in Water and Ice H bonding in water 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 (SB p.269)

New Way Chemistry for Hong Kong A-Level Book 1 38 H-bonding in ice 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) 11.5 Hydrogen Bonding (SB p.269)

New Way Chemistry for Hong Kong A-Level Book 1 39 Hydrogen Bonding in Proteins The primary structure of a protein consists of a sequence of amino acids Hydrogen Bonding (SB p.270)

New Way Chemistry for Hong Kong A-Level Book Hydrogen Bonding (SB p.270) Hydrogen Bonding in Proteins H bonds formed bewteen NH and CO groups of protein chains. This creates the secondary coiled (helix) structure of the protein chain.

New Way Chemistry for Hong Kong A-Level Book 1 41 Hydrogen Bonding in DNA A model of the DNA helix DNA (deoxyribonnuclei acid) is present in the nuclei of living cells and carries genetic information. It consists of two macromolecular strands spiraling round each other in the form of a double helix Hydrogen Bonding (SB p.271)

New Way Chemistry for Hong Kong A-Level Book 1 42 H bonding in the double helix of DNA 11.5 Hydrogen Bonding (SB p.271)

New Way Chemistry for Hong Kong A-Level Book 1 43 The END