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Chemistry > Chapter 23 - Redox and electrode potentials > Flashcards

Chapter 23 - Redox and electrode potentials 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 on an uncombined element ?

A

0

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4
Q

What is the oxidation number on a combined hydrogen ?

A

+1

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5
Q

What is the oxidation number on a combined oxygen ?

A

-2

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6
Q

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

A

Ionic charge

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7
Q

What is an oxidising agent ?

A
  • Takes electrons from the species being oxidised
  • The oxidising agent contains the species that is reduced
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8
Q

What is a reducing agent ?

A
  • Adds electrons on to the species being reduced
  • The reducing agent contains the species that is oxidised
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9
Q

What is a voltaic cell ?

A

An electrochemical cell that converts chemical energy into electrical energy

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10
Q

What do you need to make a voltaic cell ?

A
  • You need chemical reactions that transfer electrons from one species to another
    • This is because electrical energy results from the movement of electrons
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11
Q

What is a half cell ?

A

A half cell contains the chemical species present in a redox reaction

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12
Q

What does two half cells make ?

A

A voltaic cell, connecting two half cells allows for electrons to flow

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13
Q

Why must the chemicals be kept apart in the two half cells ?

A

This is because if they were allowed to mix, electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy

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14
Q

What is a metal/metal ion half cell ?

A
  • Simplest half cell
  • Consists of a metal rod dipped into a solution of its aqueous metal ion
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15
Q

What is electron transfer like in an isolated half cell ?

A

There is no net transfer of electrons either into or out of the metal

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16
Q

What is electron transfer like when two half cells are connected ?

A

The direction of electron flow depends upon the relative tendency of each electrode to release electrons

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17
Q

What is an ion/ion half cell ?

A

Contains ions of the same element in different oxidation states

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18
Q

Give an example of an ion/ion half cell

A
  • Mixture of aqueous iron(ii) and iron(iii) ions
  • Inert metal electrode
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19
Q

What metal is used in an ion/ion half cell ?

A
  • Inert metal electrode
  • Typically platinum
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20
Q

In an operating cell, which metal loses electrons ?

A
  • The electrode with the more reactive metal
  • It is oxidised
  • This is the negative electrode (anode)
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21
Q

In an operating cell, which metal gains electrons ?

A
  • The electrode with the less reactive metal
  • It is reduced
  • This is the positive electrode (cathode)
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22
Q

What does standard electrode potential measure ?

A

The tendency to be reduced and gain electrons

23
Q

What is the standard chosen half cell ?

A
  • Hydrogen gas
  • H+ ions
  • Inert platinum electrode
24
Q

What is the standard electrode potential of a hydrogen electrode ?

A

0V

25
Q

What are the standard conditions for electrode potentials ?

A
  • 298K (25C)
  • 100 kPa (1 bar)
  • 1 moldm-3
26
Q

Define standard electrode potential

A

The e.m.f of a half cell connected to a standard hydrogen half cell under standard conditions

27
Q

How can you measure a standard electrode potential ?

A

Connect the half cell to a standard hydrogen electrode

28
Q

What does the wire connecting the electrodes allow for ?

A

A controlled flow of electrons

29
Q

What connects both solutions ?

A

A salt bridge

30
Q

What does the salt bridge allow for ?

A

Allows ions to flow

31
Q

What does the salt bridge typically contain ?

A

A concentrated solution of an electrolyte that does not react with either solution

32
Q

What happens as the Ev value gets more negative ?

A

The greater the tendency to lose electrons and undergo oxidation

33
Q

What happens as the Ev value gets more positive ?

A

The greater the tendency to gain electrons and undergo reduction

34
Q

What Ev do metals tend to have ?

A

Negative Ev values and lose electrons

35
Q

What Ev do non metals tend to have ?

A

Positive Ev values and gain electrons

36
Q

How do you calculate the standard cell potential ?

A

E° cell = E° (positive electrode) - E° (negative electrode)

37
Q

What are the three main types of cells ?

A
  • Primary cells
  • Secondary cells
  • Fuel cells
38
Q

How often are primary cells recommended to be used ?

A

Single use only

39
Q

How is electrical energy produced by primary cells ?

A

Oxidation and reduction at the electrodes

40
Q

Can reactions be reversed in primary cells ?

A

No they can not

41
Q

What happens when the chemicals are used up in a primary cell ?

A
  • The voltage falls
  • The battery goes flat
  • Cell will need to be discarded or recycled
42
Q

What are modern primary cells like ?

A

Alkaline based on zinc and manganese dioxide and a potassium hydroxide alkaline electrolyte

43
Q

Are secondary cells rechargeable ?

A

Yes they are

44
Q

Can reactions be reversed in secondary cells ?

A
  • Yes they can be
  • This is done via recharging
45
Q

What happens as a result of reversible reactions in secondary cells ?

A

Chemicals in the cell are then regenerated and the cell can be used again

46
Q

Give some examples of secondary cells

A
  • Lead acid batteries in car batteries
  • Lithium ion cells in laptops, phones and cameras
47
Q

What does a fuel cell do ?

A

It uses the energy from the reaction of a fuel with oxygen to create a voltage

48
Q

How does a fuel cell work ?

A
  • The fuel and oxygen flow into the fuel cell and the products flow out
  • The electrolyte remains in the cell
49
Q

How often do fuel cells work ?

A

They operate continuously provided that the fuel and oxygen are supplied into the cell

50
Q

Do fuel cells have to be recharged ?

A

They do not have to be recharged

51
Q

What are hydrogen fuel cells said to be ?

A

Green

52
Q

Why are hydrogen fuel cells said to be green ?

A
  • This is because they do not produce carbon dioxide
  • Water is the only combustion product
53
Q

What other fuel cells are being made ?

A

Methanol fuelled cells

54
Q

What type of electrolyte can a hydrogen fuel cell have ?

A
  • Alkali
  • Acid

Chemistry (29 decks)

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