Heep Woh College

Form 5 Chemistry Quiz 2

Chapter 20 : Chlorine and Hypochlorite

Marking Scheme

 

1.     Explain with the help of equation(s) the following statements:

(a)  When a drop of colourless sodium iodide solution is added to swimming pool water, pale yellow solution results.                                                               [2]

 

Cl₂(aq) + 2NaI(aq) ¾® 2NaCl(aq) + I₂(aq)                                                      1

                [Cl₂(aq) + 2I^-(aq) ¾® 2Cl^-(aq) + I₂(aq)]

                Cl₂(aq) oxidizes / displaces I^-(aq) to form pale yellow I₂(aq).                      1

                (Accept I^- is oxidized to I₂)

 

(b)  When concentrated nitric acid is added to a solution of household hypochlorite bleach, greenish-yellow gas bubbles produced.                     [2]

 

ClO^-(aq) + Cl^-(aq) + 2H⁺(aq) ¾® Cl₂(aq) + H₂O(l)                                         1

[2HNO₃(aq) + NaClO(aq) + NaCl(aq) ¾® 2NaNO₃(aq) + Cl₂(g) + H₂O(l)]

Cl₂(g) which is greenish-yellow is formed when hypochlorite and chloride react with acid.                                                                                                                1

 

(c)   Household hypochlorite bleach should be stored in a non-transparent container.                                                                                                                     [3]

 

2ClO^-(aq) ¾® 2Cl^-(aq) + O₂(g)                                                                            1

                [2NaClO(aq) ¾® 2NaCl(aq) + O₂(g)]

                ClO^-(aq) decomposes and loss its power under sunlight.                              1,1

 

(d)  If paraffin oil, chlorine solution and sodium bromide solution are mixed together, two immiscible layers will form finally. One of them is yellow while the other is orange.                                                                                                        [3]

 

Cl₂(aq) + 2NaBr(aq) ¾® 2NaCl(aq) + Br₂(aq)                                                 1

                Cl₂(aq) oxidizes / displaces Br^-(aq) to form yellow Br₂(aq).                           1

        Br₂ dissolves in paraffin oil and formed is orange layer.                         1

 

 

 

 

 

 

 

 

 

 

 

 

 

2.     In the electrolysis plant, there are many electrolysis cells like this in which the brine is electrolysed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

        (a)  What is brine?     Saturated sodium chloride solution                              1      [1]

 

(b)     Suggest a material for making Electrode B.      graphite / steel           1      [1]

 

        (c)   Name this electrolytic cell.      Flowing mercury cell                                1      [1]

 

(d)  Name gas X, gas Y and liquid Z.                                                                           [3]

 

X:     chlorine                                Y:    Hydrogen                     Z:     Water                     1x3

 

(e)  Write equation(s) to show the formation of gas X, gas Y and sodium hydroxide and gas Y.                                                                                                [3]

 

X:    2Cl^-(aq) ¾® Cl₂ + 2e^-                                                                                      1

 

Y and NaOH:       Na⁺ + e^- + Hg ¾® Na(Hg)                                                      1

 

                                        2Na(Hg) + 2H₂O(l) ¾® 2NaOH(aq) + H₂(g) + 2Hg(l)      1

 

(f)    Write an equation to show the formation of hypochlorite bleach.           [1]

 

        2NaOH(aq) + Cl₂(g) ¾® NaCl(aq) + NaClO(aq) + H₂O(l)                           1

        (Accept 2OH^- + Cl₂ ¾® Cl^- + ClO^- + H₂O)

 

 

The End