3.2 redox reactions

Question 1

what is the oxidation number of V in HVO4?

Answer 1

H = +1
O4 = -2 (-8)
V = +7

Question 2

what is the oxidation number of V in VO6 2-?

Answer 2

O6 = -2 (-12)
V = +10

Question 3

if oxidation is loss of e-, what side is the e- in a half equation?

Answer 3

RHS

Question 4

if reduction is gain of e-, what side is the e- in a half equation?

Answer 4

LHS

Question 5

what can the gain and loss of electrons be shown by?

Answer 5

half equations

Question 6

what are redox reactions?

Answer 6

reactions that involve the transfer of electrons
(both oxidation and reduction occurs)

Question 7

what is the half equation for the oxidation of lithium to lithium ions?

Answer 7

Li —> Li+ + e-

Question 8

write the half equation from VO3 - —> V 2+

Answer 8

oxidation numbers : +5 -2(-6) —> +2
+5 to +2 = reduction
so e- LHS

VO3 - + 3e- —> V 2+
3e- + VO3 - —> V 2+ + 3H2O
6H + + 3e- + VO3 - —> V 2+ + 3H2O

Question 9

write the half eqution from WO4 2- —> W 3+

Answer 9

oxidation numbers : +6 -2(-8) —> +3
WO4 2- + 3e- —> W 3+
3e- + WO4 2- —> W 3+ + 4H2O
8H+ + 3e- + WO4 2- —> W 3+ + 4H2O

Question 10

how can overall redox equations be made?

Answer 10

by combining the half equations
- make electron number the same
- cancel out electrons
- equal half equations to each other

e.g
Fe 2+ —> Fe 3+ + e- [x5]
MnO4 - + 8H + + 5e- —> Mn 2+ + 4H2O

= 5Fe 2+ —> 5 Fe 3+ + 5e-

MnO4 - + 8H+ + 5Fe 2+ —> Mn 2+ + 4H2O + 5Fe 3+

Question 11

what is an oxidising agent?

Answer 11

• a species which can accept electrons from another species
• it itself is reduced

Question 12

what is a reducing agent?

Answer 12

• a species which can donate electrons to another species
• is oxidised

Question 13

what are the three specific half equations you need to know?

Answer 13

1. manganate reaction
   MnO4 - (aq) + 8H+ (aq) + 5e- —> Mn 2+ (aq) + 4H2O (l)
   purple —> pink
2. dichromate reaction
   6e- + Cr2O7 2- (aq) + 14H + (aq) —> 2Cr 3+ (aq) + 7H2O (l)
   orange —> green
3. thiosulfate
   2S2O3 2- (aq) —> S4O6 2- (aq) + 2e-
   colourless —> colourless (no colour change)

Question 14

what is the manganate half equation you need to know?
what is the colour change?

Answer 14

MnO4 - (aq) + 8H + (aq) + 5e- —> Mn 2+ (aq) + 4H2O (l)

purple —> pink

Question 15

what is the dichromate half equation you need to know?
what is the colour change?

Answer 15

6e- + Cr2O7 2- (aq) + 14H + (aq) —> 2Cr 3+ (aq) + 7H2O (l)

orange —> green

Question 16

what is the thiosulfate half equation you need to know?
what is the colour change?

Answer 16

2S2O3 2- (aq) —> S4O6 2- (aq) + 2e-

colourless —> colourless

Question 17

what are disproportionation reactions?

Answer 17

redox reactions where the same element/species is both oxidised and reduced

e.g 2H2O2 —> 2H2O + O2
-1 -2 0

Question 18

an impure sample of iron of mass 2.55g was dissolved in dilute sulfuric acid and the solution made up to 250cm^3. the solution contained iron (II) ions together with the impurities. portions of 25cm^3 of this solution were titrated with potassium manganate (viii) solution of concentration 0.02 mol dm^-3. the average titre was 28.50cm^3. calculate the % purity of the sample of iron

Answer 18

MnO4 - + 8H + + 5e- —> Mn 2+ + 4H2O
Fe 2+ —> Fe 3+ + e- [x5]

MnO4 - + 8H + + 5Fe 2+ —> Mn 2+ + 4H2O + 5Fe 3+

27.20/1000
0.00205
n = 5.576x10^-5 —(x5)—> 2.788x10^-4
5/1000 dm^3
conc = 0.05576

Question 19

what is the difference between a redox titration and an acid-base titration?

Answer 19

• redox titrations follow the same procedure
• the only difference being the type of reaction that takes place. as one of the reactants in a redox titration invariably changes colour, then there is USUALLY no need for an indicator

Question 20

titre results are said to be concordant if they are within ____ of each other?

Answer 20

0.20 cm^3

(should obtain 3 concordant results)

Question 21

apart from the 3 main half equations you need to know, what are 2 other common ones?

Answer 21

Fe 2+ —> Fe 3+ + e-
pale green —> pale yellow

I2 + 2e- —> 2I-
brown —> colourless
(OR blue-black to colourless if starch indicator added)

Question 22

what is a half equation?

Answer 22

• an equation that tells you what is happening at one of the electrodes during an electrochemical reaction

Question 23

what is a redox titration?

Answer 23

• a titration of a reducing agent by an oxidising agent (or vice versa)

Question 24

what is the ionic equation for the reaction between acidified Fe2+ and potassium manganate (VII)?

Answer 24

5Fe2+ + MnO4 - + 8H+ —> 5Fe3+ + Mn2+ + 4H2O

Question 25

what is potassium manganate (VII)?

Answer 25

a strong oxidising agent

Question 26

explain what happens during the redox reaction between Cu2+ and I- ions

Answer 26

• in this reaction, I- isnt a strong enough reducing agent to completely reduce the Cu2+ ions, so they are only reduced to Cu+ ions

Question 27

give the chemical equation for the reaction between Cu2+ ions and I- ions

Answer 27

2Cu2+ + 4I- —> I2 + 2CuI

Question 28

what happens when iodine is titrated with thiosulfate ions?

Answer 28

• iodine is reduced by the thiosulfate reducing agent
• the thiosulfate ions are oxidised by the iodine oxidising agent

Question 29

what is the chemical equation for the redox reaction that takes place between iodine and thiosulfate ions?
I2 + 2e- —> 2I-
2S2O3 2- —> S4O6 2- + 2e-

Answer 29

I2 + 2S2O3 2- —> 2I- + S4O6 2-