Redox Equations and Titrations (Chapter 23.1, 23.2, 23.3)

Question 1

What is reduction?

Answer 1

Gain of electrons

Decrease in oxidation number

Question 2

What is oxidation?

Answer 2

Loss of electrons

Increase in oxidation number

Question 3

What is the oxidation number of an element?

Answer 3

0

Question 4

What is the oxidation number of combined hydrogen?

Answer 4

+1 (-1 in hydrides)

Question 5

What is the oxidation number of combined oxygen?

Answer 5

-2

Question 6

What is the oxidation number of an ion of an element?

Answer 6

The ionic charge

Question 7

What is an oxidising agent?

Answer 7

It takes electrons from the species being oxidised and contains the species being reduced

Question 8

What is a reducing agent?

Answer 8

It adds electrons to the species being reduced and contains the species that is oxidised

Question 9

What type of substance is an oxidising/reducing agent?

Answer 9

An element/ion NOT compound

Question 10

What do redox equations have to have?

Answer 10

Balanced electrons

Question 11

How do you construct redox equations using oxidation numbers?

Answer 11

1) Balance the species that have changed oxidation number only - for any decrease in oxidation number there has to be the same increase in another species (i.e. have same number of electrons)
2) Then balance any remaining atoms

Question 12

What are the likely products in aqueous redox reactions?

Answer 12

H2O, H+ and OH-

Question 13

If something is acidified, what does that mean must be on the LHS of the half equation?

Answer 13

H+

Question 14

What is the balanced equation for the reaction of chromium ions (Cr3+) and hydrogen peroxide?

Answer 14

3H2O2 + 2Cr3+ + 10OH- => 2CrO42- + 8H2O

Question 15

What is the balanced equation for the reaction of sulfur and nitric acid?

Answer 15

S + 6HNO3 => H2SO4 + 6NO2 + 2H2O

Question 16

What is the balanced equation for the reduction of acidified MnO4- ions?

Answer 16

MnO4- + 8H+ + 5e- => Mn2+ + 4H2O

Question 17

What is a manganate titration?

Answer 17

A titration involving potassium manganate (VII), KMnO4 (aq), under acidic conditions

Question 18

What happens in manganate (VII) titrations and what does this mean?

Answer 18

MnO4- ions are reduced ∴ the other chemical used must be a reducing agent that is oxidised

Question 19

Describe the procedure of a manganate titration

Answer 19

1) add a standard solution of KMnO4 to the burette
2) using a pipette, add a measured volume of the solution being analysed and an excess of dilute H2SO4 to the conical flask
3) during the titration, the KMnO4 reacts and is decolourised as it is being added ∴ the end point of the titration is judged by the first permanent pink colour, indicating when there is an excess of MnO4- ions present
4) repeat the titration until concordant titres are obtained

Question 20

Why do you add the dilute H2SO4 to the solution being analysed in a manganate titration?

Answer 20

To provide the H+ ions required for the reduction of MnO4- ions

Question 21

Why do you not need an indicator for a manganate titration?

Answer 21

The reaction is self-indicating

Question 22

How are burette readings read in manganate titrations?

Answer 22

From the top of the meniscus - bc KMnO4 is a deep purple colour and ∴ it is very difficult to see the bottom of the meniscus

Question 23

What is a titre and what does this mean for manganate titrations?

Answer 23

The difference between two readings - ∴ the titre being read from the top of the meniscus is the same as reading from the bottom of the meniscus, provided that the tip is used for both initial and final readings

Question 24

What can the manganate titration be used for?

Answer 24

The analysis of many different reducing agents e.g. Fe2+ ions and ethanedioic acid (COOH)2, provided that they can reduce MnO4- to Mn2+

Question 25

What is MnO4-/H+ and what is it used for?

Answer 25

It is an oxidising agent and is ∴ used to oxidise compounds/ions e.g. Fe2+ => Fe3+ + e-

Question 26

What is acidified dichromate (VI), Cr2O72-/H+ and what is it used for?

Answer 26

It is an oxidising agent and is ∴ used to oxidise compounds/ions e.g. (COOH)2 => 2CO2 + 2H+ + 2e-

Question 27

What happens in iodine/thiosulfate titrations?

Answer 27

Thiosulfate ions (S2O32-) are oxidised and iodine is reduced

Question 28

What is the oxidation equation for iodine/thiosulfate titrations?

Answer 28

2S2O32- => S4O62- +2e-

Question 29

What is the reduction equation for iodine/thiosulfate titrations?

Answer 29

I2 + 2e- => 2I-

Question 30

What is the overall equation for iodine/thiosulfate titrations?

Answer 30

2S2O32- + I2 => S4O62- + 2I-

Question 31

How can the concentration of iodine in a solution be determined?

Answer 31

By titration with a standard solution of sodium thiosulfate (Na2S2O3)

Question 32

What can iodine/thiosulfate titrations be used to determine?

Answer 32

1) ClO- concentration in household bleach
2) Cu2+ concentration in copper (II) compounds
3) Cu content in copper alloys

Question 33

Describe the procedure of iodine/thiosulfate titrations to obtain unknown information about many different oxidising agents

Answer 33

1) add a standard solution of sodium thiosulfate to the burette
2) prepare a solution of the oxidising agent to be analysed and using a pipette, add this to a conical flask
3) add an excess of KI - the oxidising agent reacts with I- to produce iodine, turning the solution a yellow-brown colour
4) titration this solution with sodium thiosulfate - iodine is reduced back to I-
5) when the end point is being approached and the iodine has faded enough to become a pale straw colour, a small amount of starch indicator is added, forming a deep blue-black colour to assist with the identification of the end point
6) as more sodium thiosulfate is aded, the blue-black colour fades - at the end point, the blue-black colour disappears as all the iodine will have just reacted

Question 34

Why des starch need to be added to iodine/thiosulfate titrations?

Answer 34

Bc when iodine is reduced back to I- the brown colour fades quite gradually, making it difficult to decide on an end point

Question 35

How are Cu2+ ions produced for copper (II) salts?

Answer 35

By simply dissolving the compound in water

Question 36

How do insoluble copper (II) compounds produced Cu2+ ions?

Answer 36

By reacting with acids

Question 37

How do copper alloys produce Cu2+ ions?

Answer 37

1) the alloy is reacted and dissolved in concentrated HNO3
2) neutralisation to form Cu2+ ions
Cu => Cu2+

Question 38

What are two examples of copper alloys?

Answer 38

Brass and bronze

Question 39

Describe the analysis of copper using iodine/thiosulfate titrations

Answer 39

1) Cu2+ ions react with I- ions to form a solution of iodine and white ppt of CuI (but mixture appears as a brown solution)
- 2Cu2+ + 4I- => 2CuI + I2
2) iodine is then titrated with a standard solution of sodium thiosulfate
- 2S2O32- + I2 => S4O62- + 2I-
3) 2 moles of Cu2+ produces 1 mol of iodine, which reacts with 2 mols of thiosulfate ions
4) ∴ 1 mol of Cu2+ is equivalent to 1 mol of thiosulfate

Question 40

What are examples of oxidising agents?

Answer 40

MnO4-/H+, Cr2O72-/H+, I2, Fe3+, S4O62-, CO2, ClO-, Cu2+

Question 41

What are examples of reducing agents?

Answer 41

S2O32-, I-, Mn2+, Cr3+, Fe2+, (COO2)2