Past Paper Questions on Group 7

Chlorine and bromine are elements in Group 7 of the Periodic Table.

          Chlorine is used in water treatment.

          State one advantage and one disadvantage of using chlorine in water treatment.

          advantage: ...............................................................................................................

..................................................................................................................................

          disadvantage: ..........................................................................................................

..................................................................................................................................

[Total 2 marks]

Advantage
removes or kills bacteria OR kills germs OR
kills micro-organisms OR make it safe to drink
OR sterilises water OR disinfects water  

ALLOW to make water potable
IGNORE virus
IGNORE ‘purifies water’
DO NOT ALLOW ‘antiseptic’

            Disadvantage
            it is toxic OR poisonous OR could form chlorinated hydrocarbons 

ALLOW forms carcinogens OR forms toxins
IGNORE harmful
DO NOT ALLOW ‘it causes cancer’
DO NOT ALLOW “It kills you”

The electron configuration of bromine contains outermost electrons in the 4th shell.

          Using your knowledge of Group 7 elements, complete the electron configuration of bromine.

1s22s22p63s23p6 ........................................................................................................

[Total 1 mark]

3d10 4s2 4p5   

ALLOW 4s2 3d10 4p5
ALLOW subscripts or 3D10
ALLOW answers with 1s2 2s2 2p6 3s2 3p6 appearing twice

          Displacement reactions can be used to detect bromide ions in solution.

          A student has a solution that contains bromide ions. The student carries out the following experiment.

          Step 1

•        She bubbles some chlorine gas through a sample of the solution.

•        The mixture changes colour.

          Step 2

•        The student then adds an organic solvent, cyclohexane, to the mixture.

•        She shakes the contents and allows the layers to separate.

(i)      Write the ionic equation for the reaction that takes place in step 1.

.........................................................................................................................

[1]

(i)      Cl2 + 2Br– → Br2 + 2Cl– 

IGNORE state symbols
ALLOW any correct multiple including fractions

1

(ii)      What colour does the cyclohexane layer turn in step 2?

.........................................................................................................................

[1]

(ii)     Yellow / orange / red / brown 

ALLOW any combination of these, but no others

1

Chlorine reacts differently with dilute and concentrated aqueous solutions of sodium hydroxide.

•        When chlorine reacts with dilute sodium hydroxide, one of the products is sodium chlorate(I). This is the reaction that is used to manufacture bleach.

•        When chlorine is reacted with hot concentrated sodium hydroxide, a different reaction takes place. One of the products is NaClO3, used as a weedkiller.

          In each reaction, chlorine has been both oxidised and reduced.

(i)      What term is used to describe a redox reaction in which an element is both oxidised and reduced?

.........................................................................................................................

[1]

(i)      Disproportionation 

ALLOW versions which sound the same

DO NOT ALLOW disproportional OR disproportionate OR disproportion

(ii)      Write equations for these two reactions of chlorine with sodium hydroxide:

          equation for reaction with dilute sodium hydroxide,

.........................................................................................................................

          equation for reaction with hot concentrated sodium hydroxide.

.........................................................................................................................

[3]

(ii)     Cl2 + 2NaOH → NaClO + NaCl + H2O 

ALLOW multiples for either equation

          3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O

ALLOW 3Cl2 + 6NaOH → 2NaClO3 + 4NaCl + 3H2

          Cl2 and NaOH as reactants AND NaClO3 and NaCl as products 

          Rest of the equation 

3

(iii)     Chlorine forms another chlorate called sodium chlorate(VII), used in the manufacture of matches.

          Suggest the formula of sodium chlorate(VII).

.........................................................................................................................

[1]

(iii)    NaClO4 

ALLOW Na3ClO5 etc

1