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Physical Chemistry > Electrochemistry > Flashcards

Electrochemistry Flashcards

1
Q

What is electrochemistry?

A

the study of chemical processes that cause electrons to move

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2
Q

How can a simple electrochemical cell be created?

A

two different metals dipped in salt solutions of their own ions joined by a salt bridge, connected to an external circuit

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3
Q

What are the two reactions that always occur in electrochemical cells?

A

oxidation and reduction

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4
Q

What does each beaker contain?

A

a half cell made up of a redox couple

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5
Q

What is a redox couple?

A

metal ions in equilibrium with its aqueous solution of ions

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6
Q

What is the salt bridge soaked in?

A

a strip of filter paper soaked in a solution of soluble ionic compound

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7
Q

What is the cell potential?

A

the voltmeter reading of potential difference between the two half-cells

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8
Q

What is the e.m.f (electromotive force)?

A

the cell potential under zero current conditions

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9
Q

Which direction do electrons flow?

A

from the more reactive metal to the less reactive metal

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10
Q

What does a positive voltmeter reading mean?

A

electrons flow left to right

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11
Q

What happens to the most reactive metal?

A

it gets oxidised and is the more negative electrode

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12
Q

How do you get the overall reaction?

A

by combining the two half equations

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13
Q

What is the electrochemical series?

A

a list of standard electrode potentials placed in order according to the value of the reduction process

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14
Q

Describe the layout of the electrochemical series

A

-most positive at the top (most strongly oxidising is positive, good at gaining electrons)
-most negative at the bottom (most strongly reducing is negative, worse at gaining electrons)

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15
Q

What is the formula for the oxidising agent accepting electrons?

A

ox + e- ⇌ red NEED STATE SYMBOLS

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16
Q

The E value of a redox couple gives a…

A

quantitative measure of equilibrium

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17
Q

What happens to the equilibrium if the E value is positive?

A

the more likely the forward reaction is to occur, correspond to reduction

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18
Q

What happens to the equilibrium if the E value is negative?

A

the more like the backward reaction is to occur, correspond to oxidation

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19
Q

Any species on the left hand side can oxidise…

A

any species on the right-hand side below it

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20
Q

Any species on the right hand side can reduce…

A

any species on the left-hand side above it

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21
Q

What are good reducing agents?

A

reactive metals

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22
Q

What are three types of groups of electrochemical cells?

A

-primary cells
-secondary cells
-fuel cells

23
Q

What is a primary cell?

A

a primary cell is irreversible and non-rechargeable

24
Q

What is a secondary cell?

A

a secondary cell is reversible and is rechargeable

25
Q

What is a fuel cell?

A

a fuel cell generates electricity from the continuous oxidation of an external source of fuel

26
Q

What is the function of a primary fuel cell?

A

to provide current to an external circuit while discharging (galvanic action)

27
Q

What is a galvanic cell?

A

a galvanic cell is one with a positive e.m.f in which the spontaneous forward cell reaction can be used to provide electric current to an external circuit

28
Q

How do secondary cells work?

A

they combine two opposing functions of all reversible electrical cells -
-they provide current to an external circuit while discharging (galvanic action) chemical into electrical energy
-they use current from an external circuit while charging (electrolytic action) electrical into chemical energy

29
Q

In order for the system to be truly reversible, what is important about the products?

A

that the products resulting from both galvanic and electrolytic action are not dispersed in the cell but remain attached to the cell electrodes

30
Q

What is the equation to find Ecell?

A

Eright - Eleft

right is the most positive, left is the most negative

31
Q

Describe the lithium rechargeable cell

A

-one electrode is LiCoO2
-one electrode is graphite
-electrolyte is a lithium salt in an organic solvent

32
Q

What are the equations for the lithium rechargeable cell?

A

positive electrode/reduction : Li+(aq) + CoO2(s) + e- –> Li+[CoO2]-(s)
negative electrode/oxidation : Li(s) –> Li+(aq) + e-

OVERALL : Li(s) + CoO2(s) –> Li+[CoO2]-(s)

33
Q

What is a benefit of lithium?

A

very light

34
Q

What are fuel cells?

A

use a supply of hydrogen or organic fuel with a supply of oxygen to provide a source of electrical power

35
Q

Why are fuel cells reliable?

A

there are no moving parts

36
Q

What is the difference between batteries and fuel cells?

A

fuel cells consume reactants that must be replenished, whereas batteries store and release energy is a closed system

37
Q

Describe the structure of a hydrogen-oxygen fuel cell

A

-hydrogen as a fuel on the anode side, oxygen as an oxidant on the cathode side
-platinum-group metal/alloy as the catalyst

38
Q

What are two properties of the platinum electrodes?

A

inert and porous - allows the passage of reactant and product gases

39
Q

What can the electrolytes be in hydrogen-oxygen fuel cells?

A

-acidic - phosphoric acid
-alkaline - hot aqueous potassium oxide

40
Q

How do acidic hydrogen-oxygen fuel cells work?

A

-hydrogen diffuses to the anode platinum catalyst where it splits up to make H+ ions and electrons
-hydrogen ions are conducted through the electrolyte to the cathode
-the electrons travel through the external circuit
-at the cathode platinum catalyst surface, oxygen molecules react with electrons and hydrogen ions to form water
-water is the only waste product

41
Q

How do alkaline hydrogen-oxygen fuel cells work?

A

-at the anode, hydrogen reacts with hydroxide ions, water and electrons are produced
-electrons enter the external circuit
-at the cathode, oxygen reacts with electrons arriving from the external circuit to make hydroxide ions

42
Q

In acidic conditions, what are the electrode reactions?

A

overall : 2H2(g) + O2(g) –> 2H2O(l)

oxidation : H2(g) –> 2H+(aq) + 2e-
reduction : O2(g) + 4H+ (aq) + 4e- –> 2H20(l)

43
Q

In alkaline conditions, what are the electrode reactions?

A

overall : 2H2(g) + O2(g) –> 2H2O(l)

oxidation : H2(g) + 2OH-(aq) –> 2H2O(l) + 2e-
reduction : O2(g) + 2H2O(l) + 4e- –> 4OH-(aq)

44
Q

Why is the e.m.f exactly the same for both acidic and alkaline reactions?

A

they have exactly the same overall reaction

45
Q

What is the cell representation for the acidic hydrogen-oxygen fuel cell?

A

Pt(s)│H2(g)│H+(aq)║O2(g)│H+(aq),H2O(g)│Pt(s)

46
Q

What is the cell representation for the alkaline hydrogen-oxygen fuel cell?

A

Pt(s)│H2(g)│OH- (aq),H2O(l)║O2(g)│H2O(l),OH-(aq)│Pt(s)

47
Q

Why do hydrogen-oxygen fuel cells only work under standard conditions limitedly?

A

-rate of flow of electrons is slow
-usual to operate cell around 200C
-however since the reaction is highly exothermic, increasing temp decreases e.m.f
-therefore an increased pressure is used to compensate

48
Q

How do we obtain hydrogen?

A

-oxygen can be cheaply obtained from the air
-most hydrogen is produced by the reaction between methane and steam
-high input of energy with a large carbon footprint

49
Q

What is the overall reaction for the production of cycle from methane and steam?

A

CH4(g) + 2H2O(g) –
> CO2(g) + 4H2(g)

50
Q

Are hydrogen-oxygen fuel cells really green?

A

water is the only product, but the hydrogen that is produced needs to be made cleanly, by the electrolysis of acidified water
-the energy for this electrolysis must come from carbon neutral sources such as solar power

51
Q

What is the overall equation for an organic fuel cell?

A

C2H6O + 3O2 –> 2CO2 + 3H2O

52
Q

What is the SHE?

A

standard hydrogen electrode - measuring standard for half-cell potentials

53
Q

What are the conditions of a cell potential 0.00V for a SHE?

A

-solutions are 1.0 moldm-3 conc
-temp of 298K
-100kPa pressure

54
Q

What does a SHE consist of?

A

hydrochloric acid, hydrogen gas, platinum electrodes

Physical Chemistry (27 decks)

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