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Chemistry > 20. Electrode potentials and electrochemical cells > Flashcards

20. Electrode potentials and electrochemical cells Flashcards

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1
Q

What occurs when you place two different metals in a salt solution and connect them together

A

An electric current flows so that electrons pass from the more reactive metal to the less reactive

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2
Q

Give the equation for the half cell of zinc

A

Zn(s) —> Zn2+ (aq) + 2e-

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3
Q

What is oxidation

A

Loss of electrons

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4
Q

What is reduction

A

Gain of electrons

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5
Q

What is an oxidising agent

A

An electron acceptor

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6
Q

What is an reducing agent

A

An electron donator

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7
Q

Give an example of an ionic solution used for a salt bridge

A

KNO3

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8
Q

What is a salt bridge

A

In its simplest form, a piece of paper socked in an ionic solution which allows the movement ions between half cells

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9
Q

Why is a salt bridge used rather than a wire

A

Wire only allows the transfer of electrons whereas a salt bridge transfers ions

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10
Q

What does a more negative potential difference mean

A
  • a more negative electrode loses electrons more readily
  • it is more easily oxidised
  • it is a better reducing agent
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11
Q

What can be used to measure the potential difference of a half cell

A

SHE - standard hydrogen electrode

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12
Q

What is in a SHE electrode

A

Hydrogen gas is bubbled into a solution of H+ (aq) ions (HCl) with a Pt electrode

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13
Q

What are the standard conditions in cells

A

100kPa
1 mol dm-3
298K

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14
Q

What is the potential of a SHE

A

Defined as 0

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15
Q

What is emf

A
  • electromotive force E

- the maximum potential difference between two half cells

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16
Q

Which side does a SHE always go

A

Left

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17
Q

What does EMF=

A

EMF= E(R)- E(L)

18
Q

The species with the highest what is written next to the salt bridge

A

oxidation state

19
Q

What is an oxidation state

A

Represents how many electrons a species has lost

20
Q

Why is the reaction Cu2+ (aq) + Zn (s) —> Cu (s) + Zn2+ (aq) feasible given:
Zn2+ (aq) + 2e- Zn(s) E= -0.76V
Cu2+ (aq) + 2e- Cu(s) E= +0.34

A

Zn2+/Zn E < Cu2+/Cu therefore electrons are flowing from the more negative to the more positive (Zn giving Cu2+ electrons)
EMF= +ve therefore feasible as electrons moving from left to right (-ve to +ve)

21
Q

In a redox system that only involves metal ions but not metal e.g. Fe3+/Fe2+ what is used as an electrode

A

Platinum electrode

22
Q

What is a battery

A

A number of cells connected together

23
Q

Name 2 non-rechargeable cells

A
  • zinc/copper

- zinc/carbon

24
Q

What does zinc/carbon cell consist of

A
  • carbon electrode coated in manganese (IV) oxide
  • zinc canister
  • paste of ammonium chloride (NH4Cl)
  • water
25
Q

Why is the carbon coated in manganese (IV) oxide (zinc/carbon cell)

A

H2 gas produced in the redox reaction is oxidised to water by the manganese (IV) oxide to prevent build up of pressure

26
Q

Why is there water in the zinc/carbon cell

A

Ammonia gas dissolves in it to prevent build up of pressure

27
Q

Why are zinc/carbon cells prone to leakage

A
  • as zinc is used up the walls become thin and prone to leakage
  • NH4Cl is corrosive and acidic
  • old batteries should therefore be removed from equipment
28
Q

Name a rechargeable battery

A

Lead-acid battery

29
Q

How are rechargeable batteries recharged

A

By reversing the cell reactions by applying external voltage greater than the voltage of the cell to drive the electrons in the opposite directions

30
Q

What are lead acid batteries used in

A

Used to operate the starter motors of cars.

31
Q

What do acid lead batteries consist of

A

Six 2V cells connected in series (giving 12V)

32
Q

What do acid lead cells consist of

A
  • two plates dipped into a solution of sulfuric acid
  • positive plate is made of lead coated with lead (IV) oxide
  • the negative plate is made of lead
33
Q

Name 3 portable batteries

A

Nickel/cadmium
Lithium ion
Alkaline hydrogen-oxygen fuel cell

34
Q

Why is the nickel/cadmium battery replacing the traditional zinc/carbon

A

They are rechargeable so more cost effective

35
Q

What does the lithium ion cell consist of

A
  • positive electrode is made of lithium cobalt oxide LiCoO2
  • negative electrode is carbon
  • these are layered with a sandwich of solid electrolyte in between
36
Q

What are the reactions that take place in the lithium ion cell

A

negative electrode: Li+ +e- —> Li

positive electrode: Li+(CoO2)- —-> Li+ +CoO2 + e-

37
Q

What are the reactions that take place in an alkaline hydrogen-oxygen fuel cell

A 
negative electrode:
2H2 (g) + 4OH- (aq) ---->4H2O (l) +4e-
positive electrode:
O2 (g) + 2H2O (l) + 4e- --->4OH- (aq)
overall:
 2H2 (g) + O2 (g) ---->2H2O (l)
38
Q

What happens in an alkaline hydrogen-oxygen fuel cell

A

-hydrogen enters at the negative electrode:
2H2 (g) + 4OH- (aq) —->4H2O (l) +4e-
-this releases electrons which flows through the circuit to the other electrode
-this reaction then takes place at the positive electrode O2 (g) + 2H2O (l) + 4e- —>4OH- (aq)
-this accepts electrons from the other electrode and releases OH- ions

39
Q

What are the electrodes made of in an alkaline hydrogen-oxygen fuel cell

A

porous platinum based material

40
Q

Why are alkaline hydrogen-oxygen fuel cells used in spaceships

A

The only bi-product is water which can be used by the astronauts

41
Q

Why are alkaline hydrogen-oxygen fuel cells not as green as they appear

A

Most hydrogen is made from crude-oil, the non renewable

42
Q

Why is using an alkaline hydrogen-oxygen fuel cell in cars not safe/ good

A

It requires the storage of hydrogen gas which can explode/ is extremely flammable.
It is also hard to store hydrogen as it takes up alot of space but dangerous to pressurise.

Chemistry (27 decks)

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