1.7  Redox

 

 

Section A

 

For each of the questions only one of the lettered responses (A - D) is correct.

 

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

 

1.      In which one of the following reactions is the acid acting as an oxidant? 

              A Cu + 2H2SO4                                   CuSO4 + 2H2O + SO2

              B   KBr + H3PO4                                    HBr + KH2PO4

              C   4HClO4 + P4O10                               2Cl2O7 + 4HPO3

              D ZnO + 2HNO3                                 Zn(NO3)2 + H2O

 

2.       Iodide ions react with iodate ions in acid solution according to the following equation:

              5I-  +  IO3- + 6H+                                  3I2 + 3H2O

Which statement about this reaction is correct?

              A The oxidation number of hydrogen changes from + 1 to 0.

       B   The oxidation number of iodine in the iodate ion changes from  -5 to 0.

       C   The iodide ion loses electrons and is reduced.

       D The iodate ion acts as an oxidising agent.

 

 

 

 

 

 

 

 

SECTION B

 

Answer all questions in the spaces provided

 

1.       (a) Bromine can be prepared by the action of concentrated sulphuric acid on a mixture of sodium bromide and manganese dioxide.  In this process, hydrogen bromide is first produced which then reacts with manganese dioxide to give bromine.

                 H2SO4 + 2NaBr                       Na2SO4 + 2HBr

                 MnO2 + 4HBr                            MnBr2 + Br2 + 2H2O

 

(i)      Write an overall equation for the reaction between sodium bromide, manganese dioxide and sulphuric acid.

 

                                                                                                                                     [2]

 

(ii)     What is the systematic name for manganese dioxide?

 

                                                                                                                                     [1]

 

 

2.    Calcium oxalate, CaC2O4, is the main component of "beer-stone" which precipitates from beers in storage tanks.

 

(a)   What is the oxidation number of carbon in calcium oxalate?

 

                                                                                                                                     [1]

 

(b)  Calcium oxalate may be converted to oxalic acid and calcium sulphate by reaction with dilute sulphuric acid:

 

CaC2O4 + H2SO4                                 H2C2O4 + CaSO4

 

          The calcium sulphate precipitated can be hydrated to make plaster of Paris, CaSO4.xH2O. Calculate the value of x in the formula if 1.45 g of plaster of Paris is heated to dryness and 1.36 g of CaSO4 remains.

 

 

 

 

 

 

 

                                                                                      X =                                 [3]

 

 

 

 

(c)  Oxalate ions, C2042-can be oxidised to carbon dioxide using manganate(VII) (permanganate) ions, MnO4-.

C2O4 2-                         2CO2 + 2e-

                        MnO4- + 8H+ + 5e-                   Mn2+ + 4H2O

Using these two half-equations, write a balanced redox equation for the reaction between oxalate ions and manganate(VII) ions.

 

                                                                                                                                     [2]

 

(d)    In the oxalate ion the two carbons are joined by a single bond and each carbon has two oxygen atoms attached.

          Suggest and explain the shape of the oxalate ion.

 

 

 

                                                                                                                                     [2]