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chemistry A-level 2 > electrochemistry > Flashcards

electrochemistry Flashcards

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1
Q

what can a voltmeter measure in an electrochemical cell

A

voltmeter needs to be of infinite resistance, you can measure the “potential” of the cell

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2
Q

what will the difference in reactivity produce on the voltmeter

A

larger the difference in reactivity the larger the reading on the voltmeter

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3
Q

what is an electrochemical cell made of

A

2 half cells

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4
Q

how do we determine the potential of 1 half-cell?

A

we must compare it to a standard. (standard hydrogen electrode)

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5
Q

the standard hydrogen electrode
conditions

A
  • temp 298k
  • pressure at 100kPa
  • conc [H+] 1moldm-3
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6
Q

what is the convention rule with S.H.E

A

to draw the standard hydrogen electrode on the left hand side of the cell

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7
Q

what is the potential for the S.H.E

A

As it is the standard, it is 0.00v

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8
Q

what side is the reduction half-cell on

A

conventionally we put the reduction half-cell on the RHS of a cell, and the oxidation half-cell on the left

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9
Q

how to work out the potential of the E cell

A

E cell = Erhs - Elhs
E cell = Ered - Eox

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10
Q

what does a correct cell diagram give?

A

a correct cell diagram gives a positive value

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11
Q

what is an oxidising agent?

A

electron acceptor

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12
Q

what happens in a half-cell

A

metal is dipped into a solution containing its ions, an equilibrium is established between the metal ions and the metal atoms.

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13
Q

What is filter paper soaked in and why?

A

KCl(aq) or KNO3(aq) or KCl in dissolved agar gel set in a U tube. ​
These solutions are used as they’re soluble and neither react with either electrode.

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14
Q

how does a salt bridge work?

A

has mobile ions that complete the circuit. The salt bridge allows ions to flow through and balance the charges, completing the circuit.

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15
Q

why is platinum used as an electrode

A
  • Inert & conducts electricity​
  • Porous platinum gives a larger surface area.
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16
Q

why is a high-resistance voltmeter used to measure the potential difference?

A

maintains the conc. of ions in the solution by not allowing any current to flow yet still measuring the p.d. of electrons trying to flow.​

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17
Q

reducing agent

A

electron donor

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18
Q

oxidising agent

A

electron acceptor

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19
Q

what is the electrode potential

A

tendency to lose or gain electrons in the equilibrium state

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20
Q

Problems with predicting reactions:​

A
  • the conditions are not standard - changing the conc. or temp of the solution causes the electrode potential to change.​
  • reaction kinetics are not favourable. -ROR may be slow so the reaction might not appear to happen. ​
    -If a reaction has a high activation energy, this stops the reaction from happening
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21
Q

what is An electrochemical cell

A

controls the electron transfer to produce electrical energy. The control transfer of electrons is the basis of all cells and batteries.​

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22
Q

what are Non-rechargeable cells

A

provide electrical energy until the chemicals have reacted to such an extent that the voltage falls.​

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23
Q

what are Rechargeable cells

A

chemicals in cells react to provide electrical energy. The cell reaction can be reversed during recharging.

24
Q

non-rehargable cells
how do they work?
example?

A

use irreversible reactions
e.g. dry cell alkaline battery: useful for short-term products

25
Q

pros and cons for non-rechargeable cells
cost
+ve
-ve

A

+ve - cheaper than rechargeable
-ve - have to be replaced regularly

26
Q

pros and cons for non-rechargeable cells
lifetime
+ve
-ve

A

+ve - non-rechargeable lasts longer per charge
-ve - have to be disposed of

27
Q

pros and cons for non-rechargeable cells
power
-ve

A

non-rechargeable don’t supply much power, so not good for an item that needs lots of power
e.g. phones

28
Q

pros and cons for non-rechargeable cells
resource /waste
+ve
-ve

A

+ve - both batteries can be recycled and the metals recovered to use again or not if they are thrown away
-ve - non-rechargeable uses more resources and produces more waste

29
Q

pros and cons for non-rechargeable cells
toxicity
+ve
-ve

A

+ve - non-rechargeable less likely to contain toxic metal so are less hazardous
-ve - could leak out and pollute water

30
Q

lithium-ion cell
Positive electrode/reduction

A

Li+ + CoO2 + e- -> Li+[CoO2]- EƟ = +1.00V​
Lithium cobalt oxide​ : Li+[CoO2]-

31
Q

Lithium-ion cell
Negative Electrode​ / Oxidation

A

Li+ + e- -> Li

32
Q

lithium-ion cell
overall equation

A

Li + CoO2  Li+[CoO2]-​

33
Q

lithium-ion cell
Conventional Cell Notation

A

Li|Li+||Li+, CoO2|LiCoO2|Pt​

34
Q

how does a lithium-ion cell work

A

reactants in this cell are absorbed into graphite powder which acts as a support medium. Ions react in this support medium without the need for a solvent e.g. water. It cannot be used as it would react with the lithium

35
Q

what are modern fuel cells based on?

A

hydrogen or hydrogen-rich fuels e.g.
- methanol CH3OH
- methanoic acid HCOOH.

36
Q

how does a fuel cell work and what are the benefits?

A

uses energy from the reaction of a fuel with oxygen to create a voltage. The development of fuel cells results in the pollution of water. ​Greater Efficiency​ from 40-60%

37
Q

Storage of Hydrogen for use in fuel cells​

A
  • Gas / very low boiling point – store as a liquid under pressure. ​
  • H2 can be adsorbed onto the surface of a solid material
  • H2 can be absorbed within some materials
38
Q

Limitations of the Hydrogen fuel cell:​

A
  • Large-scale storage and transportation poses problems as storing pressurised liquid.​
  • Current adsorbers and absorbers of hydrogen have a limited lifetime.
  • Current fuel cells have a limited lifetime, need regular replacement and disposal
  • use toxic chemicals in their production.​
  • H2 is an energy carrier not an energy source. ​
  • manufactured by electrolysis of water or by reacting methane with steam. More energy may be used in making the hydrogen than saved by its use.
39
Q

Methanol for fuel cells:​

A
  • can be generated from biomass.​
  • liquid is easier to store than H2 gas.​
  • Less pollution and less CO2 / incomplete combustion produce toxic CO which must be removed by a catalytic converter.​
  • Hydrogen-rich fuel produces only small amounts of CO2 and air pollutants.
40
Q

Methanoic Acid for fuel cells​

A

HCOOH <=> HCOO- + H+​

  • liquid and is easy to transport​
  • HCOOH gives a greater cell voltage​
  • HCOOH has more political acceptance than hydrogen as a fuel.​
41
Q

what are Hydrogen rich fuels (methanol, natural gas and petrol) mixed with

A

with water and converted into hydrogen gas by an onboard “reformer”. The reformer operates at 250-300c to generate the H2 gas.

42
Q

Hydrogen-oxygen fuel cell​ with alkaline electrolyte (aka alkali fuel cell)
Oxidation/ negative terminal

A

2H2(g) + 4OH–(aq) -> 4H2O(l) + 4e–

Eϴ = -0.83V​

43
Q

Hydrogen-oxygen fuel cell​ with alkaline electrolyte (aka alkali fuel cell)
Reduction/ positive terminal

A

O2(g) + 2 H2O(aq) + 4e– -> 4 OH- (aq) Eϴ = +0.40 V​

44
Q

Hydrogen-oxygen fuel cell​ with alkaline electrolyte (aka alkali fuel cell)
Overall cell reaction: ​

A

2H2(g) + O2(g)  2H2O(l) ​

EMF: 0.40 –(-0.83) = 1.23V​

45
Q

why does the hydrogen-oxygen fuel cell work?

A
  • The electrolyte is the alkali potassium hydroxide, KOH(aq).​
  • reactants flow in and products flow out while the electrolyte remains in the cells.​
  • operate continuously as long as there is a supply of H2 and O2
46
Q

alkaline hydrogen-oxygen fuel cell
Cell notation

A

Pt|H2|OH-, H2O||O2|H2O, OH-|Pt​

47
Q

acidic hydrogen fuel cell
at the negative electrode

A

H2 -> 2H+ + 2e-

48
Q

acidic hydrogen fuel cell
at the positive electrode

A

4H+ + 4e- + O2 -> 2H2O

49
Q

acidic hydrogen fuel cell
overall equation

A

2H2 + O2 -> 2H2O
E cell = +1.23v

50
Q

acidic hydrogen fuel cell
cell notation

A

Pt|H2|H+||O2|H+, H2O|Pt

51
Q

ethanol fuel cell

A

C2H5OH + 3O2 -> 2CO2 + 3H2O

ox: C2H5OH + 3H2O -> 2CO2 + 12H+ + 12e-
red: 12H+ + 12e- + 3O2 -> 6H2O

52
Q

mole fraction

A

proportion of all the moles in a system is each gas

e.g. moles of N2/ total moles

53
Q

partial pressure

A

the contribution of the gas to the total partial pressure in a system

partial pressure of A = moel fraction of A x total pressure

54
Q

factors affecting Kp

A

temperature

55
Q

if the pressure has increased in Kp

A

the equilibrium shifts to decrease the pressure. this will favour the left-hand side, and there are fewer moles.

56
Q

what happens in T is increased

A

Equilibrium shifts to decrease temperature. This will favour the froward, endothermic reaction, so Kp will increase

chemistry A-level 2 (10 decks)

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