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a2 chem - physical > electrode potentials > Flashcards

electrode potentials Flashcards

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1
Q

what are electrochemical cells made from?

A

two half cells

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2
Q

what is a half cell?

A

a solid metal dipped in a salt (ionic) solution of their own ions. The solid metals are connected by an external circuit

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3
Q

the half cell acts as…

A

an electrode within an electrochemical cell

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4
Q

what are the requirements for an electrode?

A
  • must be solid
  • must conduct electricity
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5
Q

what metal is used as the electrode when there is no solid metal involved?

A

platinum

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6
Q

why is platinum used?

A
  • it is chemically inert
  • it conducts electricity
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7
Q

the electrode where oxidation takes place is regarded as…

A

the negative electrode (anode)

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8
Q

the electrode where reduction takes place is regarded as…

A

the positive electrode

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9
Q

what are the two half cells connected by “internally”?

A

a salt bridge

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10
Q

what is the purpose of the salt bridge?

A

The salt bridge has mobile ions that complete the circuit. KNO3 is used as its ions are relatively inert so they don’t partake in any reaction. It’s also useful that nitrates are soluble. Salt bridge contains spectator ions which can migrate to preserve neutrality

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11
Q

within electrode potentials, where are the electrons found in the equations?

A

on the left hand side of the reversible reaction sign

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12
Q

according to IUPAC rules, where is the oxidation electrode located?

A

The oxidation electrode (i.e. the electrode with the more negative
electrode potential) is written on the LHS. The ion / metal are
written in order of being oxidised (ions are closest to the salt bridge)

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13
Q

according to IUPAC rules, where is the reduction electrode located?

A

The reduction electrode (i.e. the electrode with the less negative
electrode potential) is written on the RHS with the components in
order of being reduced.

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14
Q

according to IUPAC rules, what symbol is used to represent the salt bridge?

A

||

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15
Q

according to IUPAC rules, what symbol is used to represent a phase boundary?

A

|

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16
Q

what is the phase boundary representing?

A

a change in physical state

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17
Q

according to IUPAC rules, where should the Pt electrode be positioned (if used)

A

to the extreme left or extreme right

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18
Q

according to IUPAC rules, how do you represent components in an electrode which are in the same state?

A

separating by commas

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19
Q

what is used to measure the voltage in the electrochemical circuit?

A

a high resistance voltmeter (we assume infinite resistance)

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20
Q

what is the voltage measured called?

A

the cell potential/EMF

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21
Q

what does the EMF show?

A

the potential difference between the two half cells

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22
Q

if a metal is easily oxidised, its electrode potential will be…

A

negative

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23
Q

if it is harder to oxidise a particular metal, its electrode potential will be…

A

positive

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24
Q

what factors affect the position of equilibrium?

A

temperature, pressure, concentration

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25
Q

conditions: when we measure electrode potentials, we use…

A

standard conditions (298K, 100KPa, any solutions having a conc of 1.00 mol dm-3)

26
Q

what is used as a “reference” when measuring standard electrode potentials of half cells?

A

the standard hydrogen electrode

27
Q

what is the definition of “standard electrode potential”?

A

the electrode potential of a
standard electrode connected to a standard hydrogen electrode using a high-resistance voltmeter and a salt bridge

28
Q

what is the SEP of the SHE?

A

0.00V - it is the standard

29
Q

when the SHE is present within an electrochemical cell, it is always located…

A

on the left hand side

30
Q

the electrode on the left is always…

A

the negative electrode
- if both of the half cells have negative electrode potentials, the MORE NEGATIVE one goes on the left

31
Q

what is the electrochemical series?

A

a list of the standard electrode potentials of various elements (decreases until it reaches 0.0V and then increases in negativity)

32
Q

a strong oxidising agent will be…

A

more positive

33
Q

a strong reducing agent will be…

A

more negative

34
Q

what equation is used to measure the EMF of a cell?

A

EMF = EƟRHS - EƟLHS

35
Q

the cell potential will always be…

A

positive

36
Q

rechargability: lithium ion cells are an example of…

A

rechargeable/reversible cells

37
Q

hydrogen-oxygen cell is an example of…

A

a fuel cell

38
Q

what are fuel cells?

A

they are used to generate an electrical current and do not need to be electrically recharged

39
Q

what is the main use for lithium cells?

A

they are used within phones and laptops

40
Q

what is the conventional cell representation for a LiCoO2 and graphite electrode?

A

Li(s)|Li+(aq)||Li+(aq)|CoO2(s), Li+[CoO2]-(s), Pt(s)

41
Q

lithium cobalt: what is the reaction at the positive electrode?

A

Li+(aq) + CoO2(s) + e- → Li+[CoO2]-(s)

42
Q

lithium cobalt: what is the reaction at the negative electrode?

A

Li(s) → Li+(aq) + e-

43
Q

what is the overall discharge equation for the lithium cobalt cell?

A

Li(s) + CoO2(s) —> Li+(CoO2)-(s)

44
Q

why are lithium ion cells so light?

A

due to the low density of lithium

45
Q

what is the electrolyte within lithium ion cells?

A

a polymer (advantage = no leaks)

46
Q

what is the single cell EMF of a lithium ion cell?

A

3.5-4v

47
Q

how do fuel cells work?

A

they convert the energy from combustion of a fuel such as hydrogen

48
Q

pH: hydrogen fuel cells can be…`

A

acidic or alkaline

49
Q

in a hydrogen-oxygen half cell, hydrogen is…
and oxygen is…

A

hydrogen = oxidised
oxygen = reduced

50
Q

what is the chemical product in a hydrogen-oxygen fuel cell?

A

water

51
Q

how do the ions move in a hydrogen-oxygen fuel cell?

A

through the polymer
acidic = H+ moving from hydrogen coming in on the left to oxygen coming in on the right - water out on the right

alkaline = OH- moving from oxygen coming in on the right to hydrogen coming in on the left - water out on the left

52
Q

what type of batteries are the batteries we buy in shops today?

A

zinc-carbon batteries

53
Q

describe the structure of a zinc-carbon battery

A

carbon rod = positive electrode
zinc canister = negative electrode
manganese iv oxide - hydrogen gas to water (reduced to Mn2+)
electrolyte paste = ammonium chloride, zinc chloride and water

54
Q

what are the half equations for the zinc carbon battery?

A

2NH4+(aq) + 2e- ∏ 2NH3(g) + H2(g) E = roughly +0.7V

Zn2+(aq) + 2e- ∏ Zn(s) E = roughly -0.8

(we do not use standard electrode potential as the conditions are not standard)

55
Q

what is the IUPAC cell representation for zinc-carbon batteries?

A

Zn(s) | Zn2+ || 2NH4+(aq) | 2NH3(g) , H2(g) | C(graphite)

56
Q

car batteries are made of…

A

lead acid cells

57
Q

when do car batteries recharge?

A

when the engine is running

58
Q

what is the approximate voltage of a car battery?

A

12V

59
Q

briefly describe the structure of a car battery?

A

Cell consists of a lead plate and a lead plate coated with lead oxide dipped in 6M sulfuric acid (electrolyte)

60
Q

what are the reactions for an alkaline hydrogen-oxygen fuel cell?

A

negative: 4H2O(l) + 4e- ∏ 4OH-(aq) + 2H2(g) Eθ = -0.83V

positive: O2(g) + 2H2O(l) + 4e- ∏ 4OH-(aq) Eθ = +0.4V

a2 chem - physical (5 decks)

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