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Chapter 11. Intermolecular Forces. 11.1 Polarity of Molecules 11.2 Van der Waals’ Forces 11.3 Van der Waals’ Radii 11.4 Molecular Crystals 11.5 Hydrogen Bonding. 11.1 Polarity of Molecules (SB p.257).
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Chapter 11 Intermolecular Forces 11.1 Polarity of Molecules 11.2 Van der Waals’ Forces 11.3 Van der Waals’ Radii 11.4 Molecular Crystals 11.5 Hydrogen Bonding
11.1 Polarity of Molecules (SB p.257) electrostatic attraction between dipoles, i.e. the attraction between the +ve end of one molecule and the -ve end of another molecule Polarity of Molecules (very weak when compare with covalent bond between atoms in molecule) Intermoleculear forces Van der Waal’s forces hydrogen bonding
11.1 Polarity of Molecules (SB p.257) Polarity of Molecules 3 types of dipoles Permanent dipole Instantaneous dipole Induced dipole
11.1 Polarity of Molecules (SB p.257) Permanent Dipole 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.258) Instantaneous Dipole 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.258) 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).
11.2 Van der Waal’s Forces (SB p.258) Van der Waals’ Forces Van der Waals’ forces Dipole-Dipole Interaction Dipole-Induced Dipole Interaction Instantaneous Dipole-Induced DipoleInteraction
11.2 Van der Waal’s Forces (SB p.258) 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.
11.2 Van der Waal’s Forces (SB p.259) 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.
11.2 Van der Waal’s Forces (SB p.259) 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.
11.2 Van der Waal’s Forces (SB p.260) Strength of Van der Waals’ Forces
11.2 Van der Waal’s Forces (SB p.260) 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.261) Surface Area of Molecule The van der Waals’ forces also increase with the surface area of the molecule.
11.3 Van der Waal’s Radii (SB p.262) Van der Waals’ Radii
11.3 Van der Waal’s Radii (SB p.262) 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.
11.3 Van der Waal’s Radii (SB p.263) Radii of some elements
11.3 Van der Waal’s Radii (SB p.263) Structure of graphite sum of covalent radii sum of van der Waals’ radii
11.4 Molecular Crystals (SB p.264) Molecular Crystals A molecular crystal is a structure which consists of individual molecules packed together in a regular arrangement by weak intermolecular forces.
11.5 Hydrogen Bonding (SB p.264) F being very electronegative very +ve F atom being small enough to approach very close to the H atom in the neighbouring molecule HF molecule
11.5 Hydrogen Bonding (SB p.265) The relative strength of van der Waals’ forces, hydreogen bond and covalent bond
11.5 Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in hydrogen fluoride
11.5 Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in water
11.5 Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in ammonia
11.5 Hydrogen Bonding (SB p.265) Formation of hydrogen bonds in methanol
11.5 Hydrogen Bonding (SB p.266) very +ve Cl Cl ?Any H-bond formed? Cl Cl Experimental Determination of the Strength of Hydrogen Bond trichloromethane
11.5 Hydrogen Bonding (SB p.266) ?Any H-bond formed? Experimental Determination of the Strength of Hydrogen Bond ethyl ethanoate
11.5 Hydrogen Bonding (SB p.266) ?Any H-bond formed? Experimental Determination of the Strength of Hydrogen Bonds How strong is it? (YES!) H bond formed between trichloromethane & ethyl ethanoate Hint: When you mix the 2 liquids together, what will happen?
11.5 Hydrogen Bonding (SB p.267) Intramolecular Hydrogen bonding cis-trans isomers(geometric isomers) Butenedioic acid cis- butenedioic acid trans- butenedioic acid m.p. = 1300C m.p. = 2900C
11.5 Hydrogen Bonding (SB p.267) Intramolecular Hydrogen bonding m.p. = 2900C m.p. = 1300C cis- butenedioic acid trans- butenedioic acid Owing to the formation of intramolecular H bonds, cis-butenedioic acid forms less extensive intermolecular H bonds with neighbouring molecules.
11.5 Hydrogen Bonding (SB p.267) Intramolecular Hydrogen bonding 2-nitrophenol 4-nitrophenol Can you match the two compounds with the following m.p.’s? m.p. = 2160C m.p. = 2590C
11.5 Hydrogen Bonding (SB p.267) Intramolecular Hydrogen bonding m.p. = 2160C m.p. = 2590C
11.5 Hydrogen Bonding (SB p.267) Anomalous Properties of the Second Period Hydrides There must be some type ofintermolecular force (which is much stronger than van der Waals’ forces) in NH3, H2O & HF (Hydrogen bonding) Molecular mass Van der Waals’ forces (b.p. )
11.5 Hydrogen Bonding (SB p.267) 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.
11.5 Hydrogen Bonding (SB p.268) Boiling Points and Solubilities of Alcohol
11.5 Hydrogen Bonding (SB p.268) Boiling Points and Solubilities of Alcohol Organic compounds are usually insoluble in water,e.g. ethane (C2H6) or chloroethane (C2H5Cl) But alochols of low molecular mass are soluble in water, e.g methanol (CH3OH) and ethanol (C2H5OH).
11.5 Hydrogen Bonding (SB p.268) 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.269) Hydrogen Bonding in Water and Ice H bonding in water In water, the molecules are in constant motion. H bonds areformed and broken continually. The arrangement of molecules are thus in random.
11.5 Hydrogen Bonding (SB p.269) 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 areformed. This creates an open structure. (density of ice < density of water)
11.5 Hydrogen Bonding (SB p.270) Hydrogen Bonding in Proteins The primary structure of a protein consists of a sequence of amino acids.
11.5 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.
11.5 Hydrogen Bonding (SB p.271) Hydrogen Bonding in DNA A model of the DNA helix DNA (deoxyribonnuclei acid) is present in the nuclei ofliving cells and carries genetic information. It consists of two macromolecular strands spiraling round each other in the form of a double helix.
11.5 Hydrogen Bonding (SB p.271) H bonding in the double helix of DNA
