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Module 5- Physical Chemistry and Transition Elements > 23 Redox and Electrode Potentials > Flashcards

23 Redox and Electrode Potentials Flashcards

1
Q

What is a reducing agent? (1 mark)

A

The species in a reaction which adds electrons to another species.

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2
Q

What is an oxidising agent? (1 mark)

A

A species in a reaction which takes away electrons from another species.

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3
Q

Does an oxidising agent get oxidised or reduced? (1 mark)

A

Reduced

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4
Q

Does a reducing agent get oxidised or reduced? (1 mark)

A

Oxidised

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5
Q

What does an increase in oxidation number indicate? (1 mark)

A

Oxidatipn

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6
Q

What does a decrease in oxidation number indicate? (1 mark)

A

Reduction

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7
Q

When writing half equation, what would you do if there was oxygen on the left but not on the right hand side of the equation? (2 marks)

A

Add water to the right hand side and then balance the equation by adding the correct number of H+ ions to the left.

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8
Q

How do you combine half equations? (2 marks)

A

Make the number of electrons the same in both.
Combine them.
Cancel out any species which are the same on both sides.

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9
Q

What colour is potassium manganate, how does this affect reading the level on the Burettes during titrations? (2 marks)

A

Dark pink/purple
Meniscus is read at the top, as it is too dark to see the bottom

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10
Q

How is the end point indicated during a potassium manganate titration? (1 mark)

A

First permanent pink colour

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11
Q

Is the thiosulfate or iodine reduced in a iodine/thiosulfate titration? (1 mark)

A

Iodine

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12
Q

Is the thiosulfate or iodine oxidised in a iodine/thiosulfate titration? (1 mark)

A

Thiosulfate

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13
Q

What is the equation for the oxidation of a thiosulfate ion? (2 marks)

A

2S2O3 2- —> S4O6 2- + 2e-

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14
Q

How is the end point detected in a iodine/thiosulfate titration? (3 marks)

A

Add the thiosulfate solution to the iodine solution until there is a colour change of orange to straw colour.
Then add a small amount of starch (this forms a black colour).
Continue adding the thiosulfate, until the solution is clear.
This is the end point.

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15
Q

Why do starch used in an iodine/thiosulfate titration? (1 mark)

A

It is hard to see the end point of the reaction, as it changes from a pale yellow to clear.

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16
Q

What oxidising agents can be used to titrate a thiosulfate solution? (3 marks)

A

I2
ClO-
Cu2+

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17
Q

How are samples of copper analysed? (3 marks)

A

The alloy is dissolved in HNO3, and then neutralised.
The Cu2+ is then reacted with I- to form I2 and CuI.
This brown mixture is titrated with Na2S2O3.

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18
Q

What colour is a mixture of I2 and CuI? (1 mark)

A

Brown

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19
Q

What is MnO4- reduced to? (1 mark)

A

Mn2+

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20
Q

What is Cr2O72- reduced to? (1 mark)

A

Cr3+

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21
Q

What does a voltaic cell do? (1 mark)

A

Converts chemical energy into electrical energy.

22
Q

How do voltaic cells convert chemical energy into electrical energy? (1 mark)

A

Redox reactions, the movement of electrons from this.

23
Q

What do half cells contain? (1 mark)

A

Chemical species present in a redox half equation.

24
Q

How many half cells make up a voltaic cell? (1 mark)

A

Two

25
Q

In a cell, why can chemicals not mix? (1 mark)

A

Electrons would flow in an uncontrolled way and heat energy would be released.

26
Q

What is a metal/metal ion half-cell? (1 mark)

A

A metal rod dipped into solution of its aqueous metal ion.

27
Q

What is the phase boundary in a metal/metal ion half-cell? (1 mark)

A

Where are the metal is in contact with its ions.

28
Q

What is set up at the phase boundary? (1 mark)

A

An equilibrium.

29
Q

What does an ion/ion half-cell contain? (1 mark)

A

Ions of the same element in different oxidation states.
A platinum electrode.

30
Q

Why is a platinum electrode used in an ion/ion half-cell? (1 mark)

A

Because there is no metal to transport electrons.

31
Q

Does the electrode with more or less reactive metal lose electrons and what is this electrode called? (2 marks)

A

The electrode with more reactive metal loses electrons, this is the negative electrode.

32
Q

Does the electrode with more or less reactive metal gain electrons and what is this electrode called? (2 marks)

A

The electrode with less reactive metal gains electrons, this is the positive electrode.

33
Q

How is a standard electrode potential found? (1 mark)

A

An electrode is connected to a standard hydrogen half-cell, under standard conditions:

34
Q

What are the standard conditions used when finding a standard electrode potential? (1 mark)

A

Concentrations of all solutions are 1moldm-3.
Temperature is 298K.
Pressure is 100kPa

35
Q

Define the term standard electrode potential. (2 marks)

A

Standard electrode potential is the e.m.f of a half-cell connected to a standard hydrogen half-cell under standard conditions.

36
Q

What does e.m.f mean? (2 marks)

A

Electromotive force: the maximum potential difference between two electrodes.

37
Q

What is the standard electrode potential of hydrogen? (1 mark)

A

0V

38
Q

If the standard electrode potential is more negative, then is it oxidised or reduced? (1 mark)

A

Oxidised

39
Q

What does a salt bridge allow? (1 mark)

A

Ions to flow

40
Q

If the standard electrode potential is more positive, then is it oxidised or reduced? (1 mark)

A

Reduced

41
Q

How do you prepare a salt bridge? (2 marks)

A

Soak a strip of filter paper in a saturated aqueous solution of potassium nitrate.

42
Q

How do you calculate the standard cell potential from standard electrode potentials? (1 mark)

A

Cell= positive electrode — negative electrode

43
Q

In terms of electrode potential, how can the reaction between two systems be predicted (Will it happen?)? (2 marks)

A

A reaction should take place if the redox system of the oxidising agent has a more positive value than the redox system of the reducing agent.

44
Q

Why is concentration a limitation of using standard electrode potentials to predict the feasibility of reactions? (2 marks)

A

Standard use 1moldm-3.
If the concentration used is different from this, the electrode potential will be different from this value.

45
Q

State 2 limitations of using standard electrode potentials to predict the feasibility of reactions? (2 marks)

A

The predictions will be wrong if :
The activation energy separating reactants and products is too high.
The conditions are not standard (conc, pressure, temp)

46
Q

Explain what the statement ‘the activation energy separating reactants and products is too high’ means when explaining the limitations of using electrode potentials to predict feasibility of reactions. (2 marks)

A

The reaction is feasible but not spontaneous.

47
Q

Are primary cells rechargeable? (1 mark)

A

No

48
Q

Are secondary cells rechargeable? (1 mark)

A

Yes

49
Q

How do fuel cells create a voltage? (1 mark)

A

Uses energy from the reaction of a fuel with oxygen.

50
Q

Fuel cells can operate continuously if supplied with…… (2 marks)

A

Oxygen and fuel

51
Q

What is the most common type of fuel cell? (1 mark)

A

Hydrogen

52
Q

Explain why hydrogen fuel cells maintain a constant voltage. (1 mark)

A

They are continuously fed with hydrogen and oxygen.

Module 5- Physical Chemistry and Transition Elements (7 decks)

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