Pre-Exam Chemistry

• • Question: The structure of butanol is fairly similar to that of butane, yet their properties are very different. Explain why this is the case.

  The presence of an oxygen atom in butanol leads to butanol having a dipole and being a polar molecule. The dipole in one molecule of butanol will attract dipoles in neighbouring molecules, leading to the melting point and boiling point of butanol being much higher than that of butane. Butane relies upon dispersion forces. Butanol will be soluble in water due to the polar nature of its molecule.
• • Fatty acid molecules are relatively non polar. Explain why this is the case.

  Fatty acids are very long molecules. The alkane part of the chain is non polar so most of the molecule is non polar. The length of the non polar section dominates the chemical behaviour of the molecule.
• Glucose is a relatively soluble molecule. Explain why.

  A glucose molecule has 5 hydroxy groups, –O – H, as part of its structure. The hydroxyl groups causeglucose to be a quite polar molecule that will dissolve fairly easily in polar solvents like water.
• DNA is a biopolymer that exists as two separate strands. Explain what type of bonding is present in a DNA polymer.

  DNA contains many covalent bonds. The sugar molecules and base molecules are constructed with covalent bonds and are joined by covalent bonds. The separate strands of DNA are held together by hydrogen bonding between the base molecules. The nitrogen and oxygem atoms in the bases lead to significant dipoles.
• Aspirin is often sold as a sodium salt. Write an equation for the formation of the sodium salt and explain why the sodium salt might be preferred.

  Aspirin is a weak acid, represented here for simplicity as AsH. AsH(s) + NaOH(aq) -> AsNa(aq) + H2O(l) where AsNa is Na+ ions with As- ions. The sodium salt of aspirin is an ionic compound and not a covalent compound. As such, its solubility is greater. It is useful for a pharmaceutical to be soluble in water.