6. disproportionation and water treatment

Question 1

what is disproportionation

Answer 1

when something is simultaneously oxidised and reduced

Question 2

when halogens react with cold dilute alkali solutions
e.g. X2 + 2NaOH ->
X2= 0

Answer 2

they undergo disproportionation (are oxidised and reduced)
NaXO + NaX + H2O
NaXO= +1
NaX= -1

Question 3

halogens can exist in a wide range of oxidation states

e.g. chlorine

Answer 3

chloride Cl- = -1
chlorine Cl2 = 0
chlorate (I) ClO- = +1

Question 4

how to make 
sodium chlorate (I) solution, NaClO(aq), bleach

Answer 4

mix chlorine gas with
cold dilute aqueous sodium hydroxide
2NaOH(aq) + Cl2(g) -> NaClO(aq) + NaCl(aq) + H2O(l)

Question 5

how is the formation of bleach a disproportionation reaction

Answer 5

2NaOH(aq) + Cl2(g) -> NaClO(aq) + NaCl(aq) + H2O(l)
in Cl2 oxidation number of Cl =0
in NaClO oxidation number of Cl = 1+ (OXIDISED)
in NaCl oxidation number of Cl = 1- (REDUCED)
(both oxidised and reduced)

Question 6

used of sodium chlorate (I) solution, NaClO(aq), bleach

Answer 6

water treatment

bleach paper and textiles

Question 7

what happens when you mix chlorine with water
Cl2(g) + H2O(l) ->
(*same thing happens when you mix bromine or iodine too!)

Answer 7

disproportionation. mixture of hydrochloric acid and chloric (I) acid forms
- > HCl(aq) + HClO(aq)

Question 8

what does aqueous chloric (I) acid do in water

HClO(aq) + H2O(l) ->

Answer 8

ionises to make chlorate (I) ions

ClO- + H3O+(aq)

Question 9

what do chlorate (I) ions ClO- do?

Answer 9

kill bacteria.

adding chlorine/ chlorate (I) ions to water makes it safe to drink and swim in

Question 10

benefits of chlorine in water treatment

4 things

Answer 10

1. kills disease causing micro-organisms
2. some chlorine remains in water and prevents reinfection further down supply
3. prevents growth of algae eliminating bad tastes and smells
4. removes discolouration caused by organic compounds

Question 11

risks of chlorine in water treatment

3 things

Answer 11

1. chlorine gas very harmful if breathed in (irritates respiratory system)
2. liquid chlorine on skin/eyes severe chemical burns.
   =ACCIDENTS SERIOUS OR FATAL
3. chlorine reacts with organic compounds in water to form chlorinated hydrocarbons e.g chloromethane CH3Cl - many are carcinogenic
   HOWEVER increased cancer risk is small compared to risks of untreated water e.g. cholera epidemic

Question 12

ethical implications of chlorine in water treatment

Answer 12

no choice of having water chlorinated. objections to forced mass medication

Question 13

2 alternatives to chlorine in water treatment

Answer 13

Ozone O3

Ultraviolet light

Question 14

1 + and 2 - of using alternative Ozone O3 in water treatment

Answer 14

+ strong oxidising agent = good at killing microorganisms

• expensive
• short half life in water = not permanent

Question 15

1+ and 2- of using alternative UV light in water treatment

Answer 15

+ kills microorganisms by damaging their DNA

• ineffective in cloudy water
• wont stop water being contaminated further down line