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Chemistry A-Level COPY > Redox Equations and Titrations (Chapter 23.1, 23.2, 23.3) > Flashcards

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

1
Q

What is reduction?

A

Gain of electrons

Decrease in oxidation number

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2
Q

What is oxidation?

A

Loss of electrons

Increase in oxidation number

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3
Q

What is the oxidation number of an element?

A

0

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4
Q

What is the oxidation number of combined hydrogen?

A

+1 (-1 in hydrides)

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5
Q

What is the oxidation number of combined oxygen?

A

-2

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6
Q

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

A

The ionic charge

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7
Q

What is an oxidising agent?

A

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

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8
Q

What is a reducing agent?

A

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

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9
Q

What type of substance is an oxidising/reducing agent?

A

An element/ion NOT compound

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10
Q

What do redox equations have to have?

A

Balanced electrons

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11
Q

How do you construct redox equations using oxidation numbers?

A

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

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12
Q

What are the likely products in aqueous redox reactions?

A

H2O, H+ and OH-

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13
Q

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

A

H+

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14
Q

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

A

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

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15
Q

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

A

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

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16
Q

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

A

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

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17
Q

What is a manganate titration?

A

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

18
Q

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

A

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

19
Q

Describe the procedure of a manganate titration

A

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

20
Q

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

A

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

21
Q

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

A

The reaction is self-indicating

22
Q

How are burette readings read in manganate titrations?

A

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

23
Q

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

A

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

24
Q

What can the manganate titration be used for?

A

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

25
Q

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

A

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

26
Q

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

A

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

27
Q

What happens in iodine/thiosulfate titrations?

A

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

28
Q

What is the oxidation equation for iodine/thiosulfate titrations?

A

2S2O32- => S4O62- +2e-

29
Q

What is the reduction equation for iodine/thiosulfate titrations?

A

I2 + 2e- => 2I-

30
Q

What is the overall equation for iodine/thiosulfate titrations?

A

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

31
Q

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

A

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

32
Q

What can iodine/thiosulfate titrations be used to determine?

A

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

33
Q

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

A

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

34
Q

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

A

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

35
Q

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

A

By simply dissolving the compound in water

36
Q

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

A

By reacting with acids

37
Q

How do copper alloys produce Cu2+ ions?

A

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

38
Q

What are two examples of copper alloys?

A

Brass and bronze

39
Q

Describe the analysis of copper using iodine/thiosulfate titrations

A

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

40
Q

What are examples of oxidising agents?

A

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

41
Q

What are examples of reducing agents?

A

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

Chemistry A-Level COPY (27 decks)

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