Bonding: Intermolecular Forces
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Transcript of Bonding: Intermolecular Forces
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Bonding:Intermolecular Forces
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Covalent BondsCovalent bonding leads to two types of molecules
Giant StructuresEg. Diamond, sandStrong covalent bonds
between atoms create a large network resulting in a high melting and boiling point
Simple Molecular Structures
Eg. Methane, hydrogen chloride
Few covalent bonds per molecule. Exist as gases or low boiling liquids therefore have low melting and boiling points.
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How we know intermolecular forces exist
Gases have large distances between molecules, short range forces between molecules are insignificant
Cool and Compress
Distances between molecules decrease and the forces between become significant. Gases liquefy (G->L)Cool and Compress
Distances are further reduced. Forces become more significant. Liquid solidifies. (L->S)
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Types of Intermolecular Forces
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Examples to explain London ForcesBoiling points of noble gases•He- 4K•Xe-165K•Higher atomic mass=higher number of electrons
Boiling points of alkanes•Methane- 111K•Hexane: 341K•As molar mass increases, london forces increase
Boiling points of Halogens•As molar mass of halogens increases from F2 to I2 the boiling points also increase
Boiling points of straight chains & branches Alkanes of the same molar massn pentane BP 309 Kneo pentane BP 283 K
Plastics and PolymersThey have very highMolar mass and very high surface area
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Examples to explain Dipole-Dipole Forces
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Hydrogen Bonding
Hδ+-Fδ---------------Hδ+-Fδ-
• Strongest of the intermolecular forces• Affects the physical properties of the
compounds
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Examples of H-Bonding
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Hydrogen Bonding in diff. compounds
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Examples of H-Bonding contd….
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Examples of H-Bonding contd….
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Trends in Boiling points of Groups 4,5,6 and 7
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Examples of H-Bonding contd….
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Anomalous behavior of Water
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Structure of ICE
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Hydrogen Bonds in Water, Ice
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Compare H2O and H2SProperties Water Hydrogen sulphide
Molar enthalpy of fusion 6.0 kJmol-1 2.4 kJmol-1
Molar enthalpy of vaporization 41.0 kJmol-1 19.0 kJmol-1
State at room temperature & Pressure
Liquid Gas
Electro negativity Very high Less
Size O atom is small S is larger than O
Hydrogen Bonding 2 H bonds per molecule of water
No H Bonding
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Effect of H bonding on Organic compounds- case 1
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Dimer of Ethanoic acid
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Effect of H bonding on Organic compounds- case 2
• Propanone is miscible in water but it has no H bonding between propanone molecules
• This is because its O atoms can bond to water molecules.
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Effect of H bonding on Organic compounds- case 3
• p- nitro phenol or 4- nitro phenol is having intermolecular H bonds , stronger forces of interaction between molecules and so its BP is 279°C
• o- nitro phenol or 2- nitro phenol has intramolecular H bonds, weaker forces of interaction between molecules and so has lower BP of 216°C
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Biological importance of H bonding
• DNA pairing occurs due to H bonds.• Secondary structure of proteins (α helix and β pleated proteins)
• α helix-is an example of INTRA MOLECULAR H Bonding
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Hydrogen bonds in DNA Base Pairs2 H bonds between Adenine and Thymine3 H bonds between Cytosine and Guanine