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CHEM1901 2012 - J- 9 June 2012
- Explain, with the aid of a diagram labelling all the key components, how sodium stearate (C 17 H 35 COONa) can stabilise long-chain non-polar hydrocarbons (“grease”) in water.
Marks 3
Sodium stearate dissolves in water to give stearate ions which act as surfactants. They have non-polar (hydrophobic) “tails” that associate with grease and charged, polar (hydrophilic) “heads” that associate with water. This leads to the formation of stable spherical micelles in which the grease molecules are contained within a monolayer of stearate anions with their heads pointing outwards.
CHEM1901 2012 - J- 9 June 2012
- Consider the complex K 4 [Mn(CN) 6 ]. Describe and contrast the origin, strength and directionality of the chemical bonds in this compound (a) between C and N; (b) between the manganese and cyanide ions; and (c) between the complex and the potassium counterions.
C–N bonds are covalent. They are relatively short, strong and highly directional. Mn–CN coordination bonds are due to the donation of the lone pair of electrons on C to the Mn2+. These bonds are weaker, longer and less directional than covalent bonds. [Mn(CN) 6 ]^4 –^ and K+^ are ionically bonded in the solid state due to coulombic attraction between the oppositely charged ions. These bonds are strong but not directional.
- From the list of molecules below, select all the polar molecules and list them from left to right in order of increasing molecular dipole moment. BF 3 , CH 3 Cl, CH 3 F, CO 2 , CF 4 , NF 3
The polar molecules are CH 3 Cl, CH 3 F and NF 3. Although the bonds in the other molecules are polar, the overall molecules are non-polar due to their symmetrical shape.
The molecular dipole moments increase in the order:
NF 3 < CH 3 Cl < CH 3 F
This increase is consistent with the difference in the electronegativity of the atoms: C < N < Cl < F.
- Describe two physical properties of liquid or solid water that distinguishes it from ‘normal’ liquids or solids.
Marks 3
The solid is less dense than the liquid. The density of the liquid can decrease on cooling. The melting and boiling points are significantly higher than would be predicted from extrapolation of the other group 16 dihydrides. It is capable of dissolving ionic solids to a larger extent than most other liquids.
- Describe one consequence of molecular shape involving non-polar molecules.^2
The shape of molecules can led to biological specificity, eg in smell and in drug action. The shape of molecules also leads to differences in intermolecular forces such as the different boiling points of straight and branched alkanes and the formation of liquid crystal phases in certain molecules.
