1. A student used the technique of titration to determine the concentration of ascorbic acid (C6H8O6) in a sample of orange juice. Excess potassium iodide, KI (aq), was added to acidified orange juice. The resulting solution was titrated with potassium iodate, KIO3 (aq), in the presence of starch as an indicator. The end-point of the titration was shown by a blue-black colour.
(a) (i) Deduce the changes in oxidation number of iodine in step 1.
IO3− to I2:
I− to I2:
(ii) Identify the oxidizing and reducing agents in step 1.
(b) The concentration of KIO3 used in the titration was 2.00 × 10−3 mol dm−3.The titration produced the following results.
(i) Calculate the percentage uncertainty associated with the mean volume of KIO3 (aq).
(ii) The colour of orange juice interfered with the blue-black colour at the equivalence point. State the name of this type of error and suggest how this can be minimized.
(iii) Determine the amount, in mol, of KIO3 (aq), in the mean volume.
(c) Determine the amount, in mol, of ascorbic acid, C6H8O6 (aq), in the sample of acidified orange juice.
(d) Calculate the mass, in g, of ascorbic acid, C6H8O6 (aq), present in the sample of acidified orange juice.
2. (a) Draw the Lewis (electron dot) structures of oxygen, O2, ozone, O3, and hydrogen peroxide, H2O2.
(b) Deduce, giving a reason, the relative lengths of the oxygen to oxygen bonds in oxygen and hydrogen peroxide.
(c) Predict, with a reason, the O−O−O bond angle in O3.
3. Propane, C3H8 (g), undergoes complete combustion to form carbon dioxide, CO2 (g), and water, H2O (g).
(a) State an equation for the complete combustion of propane, C3H8 (g).
(b) Calculate the standard enthalpy change for the reaction in part (a) using bond enthalpy values given in table 10 of the data booklet.
(c) Determine, using Hess’s law, the enthalpy change, ΔHØ, in kJ mol-1, for the complete combustion of propane by using the following data.
(d) Suggest, with a reason, why the values obtained in parts (b) and (c) are different.
4. (a) (i) Define the term electronegativity.
(ii) Suggest why the noble gases are generally not assigned electronegativity values.
(b) Explain why the melting points of the group 1 metals (Li → Cs) decrease down the group
whereas the melting points of the group 7 elements (F → I) increase down the group.