Heep Woh College

Form 5 Chemistry Quiz 3

Chapter 21: Sulphuric acid and Sulphur dioxide

Marking Scheme

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The red vapour formed by burning sulphur in the test tube was mixed with oxygen and ignited at the jet. Gaseous product G was produced in burning.

 

(a)  Name the gaseous product G.                                                                                     [1]

 

Sulphur dioxide                                                                                                                  1

 

(b)  Write an equation for the formation of G.                                                                 [1]

 

S(s) + O₂(g) ¾® SO₂(g)                                                                                                    1

 

(c)   Gas G was then sucked into and dissolved in the test tube containing water to form solution Z. Name the solution Z.                                                                             [1]

 

Sulphurous acid / sulphur dioxide solution / aqueous sulphur dioxide             1

 

 

 

 

 

 

 

 

 

 

 

(d)  What will be observed if solution Z is added into the following solution separately? Explain your answer with the help of an ionic equation.

        (i)    Bromine water.                                                                                                           [2]

 

        Brown bromine water decolorized.                                                                      1

                (Turn brown to colourless)

Br₂ + SO₃^2- + H₂O ¾® 2Br^- + SO₄^2- + 2H⁺                                                              1

OR   Br₂ + SO₂ + 2H₂O ¾® 2Br^- + SO₄^2- + 4H⁺                                                               (1)

 

        (ii)    Acidified potassium permanganate solution.                                                  [2]

 

        Purple acidified potassium permanganate solution decolorized.              1

                (Turn purple to colourless)

2MnO₄^- + 5SO₃^2- + 6H⁺¾® 2Mn²⁺ + 5SO₄^2- + 3H₂O                                            1

OR   2MnO₄^- + 5SO₂ + 2H₂O ¾® 2Mn²⁺ + 5SO₄^2- + 4H⁺                                              (1)

 

        (iii)   Acidified potassium dichromate solution.                                                         [2]

 

        Orange acidified potassium dichromate solution turn green.                     1

                (Turn Orange to green)

                3SO₂ + Cr₂O₇^2- + 2H⁺ ¾® 3SO₄^2- + 2Cr³⁺ + H₂O                                                   1

        OR   3SO₃^2- + Cr₂O₇^2- + 8H⁺ ¾® 3SO₄^2- + 2Cr³⁺ + 4H₂O                                              (1)

 

(e)   A red flower is put into solution Z.

        (i)    What would you observe?                                                                                      [1]

 

Red colouration fade out / decolorized / turn pale red / turn white           1

 

        (ii)    Write an equation to show the reaction.                                                          [1]

 

SO₃^2- + dye ¾® SO₄^2- + (dye – O)                                                                 1

       

(iii)   What is the function of the solution Z in the reaction? Why?                       [2]

 

                Bleaching agent                                                                                                                1

                Sulphur dioxide reduce dye molecule / remove O from dye molecule  1

 

(f)    What would you observe when acidified barium nitrate solution is added in the resulting solution of (e)(i). Explain your answer with the help of an ionic equation.                                                                                                                               [2]

 

White / red precipitate formed                                                                                      1

 

Ba²⁺(aq) + SO₄^2-(aq) ¾® BaSO₄(s)                                                                               1

 

 

The End