REDOX TITRATIONS

Question 1

define reduction

Answer 1

gain of electron // decr in OS

Question 2

define oxidation

Answer 2

loss of electrons // incr in OS

Question 3

define oxidising agent

Answer 3

a reagent that oxidises (removes an e- from) another species

Question 4

define reducing agent

Answer 4

a reagent that reduces (adds an e- to) another species

Question 5

what is the overall iodine thiosulfate equation

Answer 5

2 S2O3(2-) + I2 –> 2 I- + S406(2-)

Question 6

I2 : S2O3(2-)

Answer 6

1 : 2

Question 7

how to write a redox equation from 1/2 equations

Answer 7

1. balance e- so both equations are equal
2. add / cancel e-
3. cancel other species that are on both sides of the eq

Question 8

how to write a redox equation from oxidation numbers

Answer 8

1. equation
   S + HNO3 –> H2SO4 + NO2 + H2O
2. assign OS to identify atoms that have changed OS
   S: 0 –> +6 (+6 change)
   N: +5 –> +4 (-1 change)
3. balance ONLY the species that contain the elements that have changed OS
   to match +6 change of S, need total decr. of -6 from N
   .:. HNO3 + NO2 x6
   S + 6HNO3 –> H2SO4 + 6NO2 + H2O
4. balance any remaining atoms
   S + 6HNO3 –> H2SO4 + 6NO2 + 2H2O

Question 9

manganate (VII) titration procedure

Answer 9

• no indicator, reaction is self-indicating
• KMnO4- is a deep pink .:. hard to judge bottom of meniscus .:. read from the top

1. KMnO4 (pink in colour)–> burette
2. w/ pippet add known vol of solution being analysed to conical flask. add XS dilute H2SO4 (to provide H+ to reduce MnO4-)
3. in conical flask colourless –> pink (pink = end point –> XS of MnO4-)
4. repeat untill you get concordant results (±0.1cm3)

Question 10

write the manganate (VII) reduction equation

Answer 10

MnO4-(aq) + 8H+(aq) + 5e- –> Mn2+(aq) + 4H20(l)

Question 11

what does the iodine/thiosulphate titration determine

Answer 11

• ClO- content in bleach
• Cu(2+) content in Copper (II) compounds
• Cu content in copper alloys

Question 12

iodine/thiosulphate titration procedure

Answer 12

1. NaS2O3 –> burette
2. prepare sol. of oxidising agent to be analysed. add known vol to conical flask w/ pipette. then add XS potassium iodide.
   oxidising agent reacts w/ I- to form I2 (yellow-brown sol)
3. titrate w/ NaS2O3. during titration I2 reduced to I- + brown colour fades .:. hard to see endpoint.
4. when it is a straw colour, add starch indicator. –> blue/black colour. as more NaS2O3 added, sol becomes colourless. when it is totally colourless = endpoint

Question 13

ClO- : I2

Answer 13

1 : 1

Question 14

ClO- : S2O3(2-)

Answer 14

1 : 2

Question 15

ClO- + I- equation

Answer 15

ClO- + 2I- + 2H+ –> Cl- + I2 + H2O

1 : 1 ClO- : I2

Question 16

Cu(2+) + I- equation

Answer 16

Cu(2+) + 4I- –> I2 + 2CuI

Question 17

Cu(2+) : I2

Answer 17

2 : 1

Question 18

Cu(2+) : S2O3-

Answer 18

1 : 1

Question 19

how to analyse iodine/thiosulphate titration using ClO- as an example

Answer 19

1. calculate n(S2O3(2-))
2. determine n(I2) + n(ClO-) reacted from ratios
3. find n(ClO-) in initial volume (scale up)
4. find [ClO-] using n=VxC

Question 20

how to analyse % purity of Iron (as an e.g.) of MnO4- reactions

Answer 20

1. calculate n(MnO4-) reacted
2. determine n(Fe2+) + n(ClO-) reactedfrom ratios
3. scale up to fine n of initial vol
4. % purity = (mass of pure sample / mass of impure sample) x100