Intermolecular forces and redox

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  • Created on: 15-05-14 18:59
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Intermolecular forces
London forces (instantaneous and induced dipoles)
- When non-polar atoms meet, there are temporary attractions and repulsions between the nuclei and
surrounding electrons
- Temporary displacement of the electrons creates a temporary dipole which induces a temporary
dipole in neighbouring atoms/molecules
- The more electrons, the more polarisable an atom is, so the stronger the London forces
- Long thin molecules have a greater surface area for the attractions to take effect over, so have
stronger London forces than short branched molecules
Dipole-dipole interactions
- Positive dipole of a polar molecule attracts negative dipoles of others
- Stronger than London forces
Hydrogen bonding
- Affects molecules in which H is bonded to fluorine, oxygen or
nitrogen (theyre very electronegative so bond is more polar and H has stronger positive charge)
- The bond between H and the electronegative atom is polar, so H has a slight positive charge and is
attracted to the very electronegative atom in the neighbouring molecule
- Stronger than dipole-dipole interactions
- Three atoms associated in H-bond are always in a straight line
- The second electronegative atom must have an unshared pair of electrons
- Water molecules can form 2 H-bonds each as the O atom has 2 unshared electron pairs, hence the
3D structure of ice which makes it less dense than water
Effects of intermolecular forces
- Alkane bpt increases with chain length as there are more C
atoms so more electrons, so stronger London forces
- The more branched an alkane, the lower the bpt as there is a
smaller surface area for the attractions to take effect over
- Alcohols are less volatile than alkanes with similar numbers of
electrons due to H-bonds between hydroxyl groups in their
molecules
- HF has highest bpt out of all hydrogen halides due to H-bonds
between molecules. HCl has lowest bpt and it rises down the group
after that as the London forces get stronger due to more electrons
Solubility
= measure of concentration of a saturated solution of a solute at a
specified temperature (eg. Mol per 100g of water)
- Add some solid to solute to test solubility (in a test tube)- leave overnight to measure solubility-
seemingly insoluble solids like Ca(OH)2 can actually dissolve a little (seen through pH change)
- Solubility depends on: intermolecular forces between solute molecules, between solvent molecules,
and between the solute and solvent molecules
- When all 3 forces are around the same strength, the solute dissolves
- Alcohols: soluble in water as the energy released when forming H-bonds is similar to the energy lost
breaking the H-bonds between water molecules- the longer the chain the less miscible though
- Compounds that cannot form H-bonds (e.g. Halogenoalkanes): insoluble in water as there would
only be London forces between the molecules, which dont release enough energy when formed to
compensate for energy spent breaking H-bonds
- Solubility of ionic compounds: water molecules cluster round the ions and bind to them (hydration =
when water molecules bond to ions)- for NaCl for example, the huge amount of energy lost breaking
the lattice is compensated for by the energy released when the ions are hydrated
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Redox
- Oxidation number is number of atoms gained or lost by an atom when it becomes an ion
- Oxidation is loss of electrons (+ve oxidation number), reduction is gain (-ve oxidation number)
- The more electronegative atom in a covalent substance has the negative oxidation state
- In disproportionation reactions an element is both reduced and oxidised
- The most +ve oxidation number of an element corresponds to the number of outer shell electrons
- Sum of oxidation numbers in an element/neutral compound…read more

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