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Chemistry > Electrochemical Cells > Flashcards

Electrochemical Cells Flashcards

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1
Q

What is the function of a cell

A

Convert chemical energy into electrical energy

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2
Q

What is a half cell

A

A metal electrode dipped into a solution of its own ions

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3
Q

What is an electrode

A

A solid (metal) conductor of electricity

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4
Q

What is an electrochemical cell

A

2 half cells connected together by a salt bridge

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5
Q

Why is a high resistance voltmeter used in a cell

A
  • ensures 0 current conditions

- allows the measure of maximum potential difference b/t 2 cells

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6
Q

How is a salt bridge made?

A

By dipping a piece of filter paper into a saturated solution of potassium nitrate (KNO3) or potassium chloride (KCL)

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7
Q

What are the functions of a salt bridge?

A

(a) to allow ions to move freely between the half cells

(b) to maintain electrical connection or to complete the circuit

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8
Q

Can a metal be used as the salt bridge? Why?

A

No
-the salt bridge must not interfere with the reaction (must not react with the solution or electrodes)

-if a metal is used, it may generate an additional metal ion potential which will interfere with the reaction

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9
Q

What is used as the electrode if a metal is not part of the reaction? Why is that used?

A

Platinum

-it is an inherent/ unreactive metal and will not interfere with the reaction

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10
Q

In a cell diagram, where does each half equation go?

A

OXIDATION ; left

REDUCTION ; right

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11
Q

What is the Standard Hydrogen Electrode (SHE)

And where is it found

A

The primary standard that is used to determine the electrode potential of half- cells

It is always the left hand electrode

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12
Q

What is the electrode potential of the SHE

A

By convention/ definition

It is 0.00V

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13
Q

What are the conditions of the SHE

A

Pressure: 100KPa/ 1 bar

Concentration of solutions: 1.0 mol dm^-3

Temperature: 298K

Current: 0 amps

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14
Q

In the SHE what is added onto the electrode

A

In the SHE a platinum electrode is used, and platinum black is added to the end of the electrode to act as a catalyst
It increases the rate at which equilibrium is attained between H2 and H+

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15
Q

What is the main disadvantage of the SHE

How is it overcome

A

It is awkward/ cumbersome to set up

A secondary standard is used to overcome this

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16
Q

What is a secondary standard

A

An electrode that has been calibrated against the SHE

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17
Q

What is the E0

A

The maximum potential difference measured relative to the SHE, under 0 current conditions

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18
Q

What is an electrochemical series

A

A list of E0 values arranged in ascending/ descending order

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19
Q

What are the uses of an electrochemical series

A

(a) used to predict which reaction will occur when 2 half cells are connected
(b) It can be used to calculate the EMF of p.d. of a cell

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20
Q

What is the EMF

A

The Electrodemotive force

Is the maximum potential difference between 2 half cells

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21
Q

What is the equation used to calculate the emf

A

E0cell

EMF) = E0(R) - E0(L

22
Q

What must the emf be for a spontaneous reaction

A

Emf/ E0 > or = 0

Must be POSITIVE

23
Q

Even if the emf is positive, a reaction may not always occur. Why?

A
  • the activation energy is too high
  • Rate is too slow
  • emf only applies to reactions under standard conditions
24
Q

How do you find the strongest oxidising agent in an electrochemical series

A
  • it is on the left hand side

- AND has the most positive emf

25
Q

How do you find the weakest oxidising agent in an electrochemical series

A
  • it is on the left hand side

- AND has the least positive emf

26
Q

How do you find the strongest reducing agent in an electrochemical series

A
  • it is on the right hand side

- AND has the least positive emf

27
Q

How do you find the weakest reducing agent in an electrochemical series

A
  • it is on the right hand side

- AND has the most positive emf

28
Q

What factors affect emf

A
  • Temperature
  • Pressure
  • [ conc ] of solutions
  • Current
29
Q

What is the effect of increasing temperature on emf

A
  • Most spontaneous reactions are exothermic
  • if temp is increased
  • equilibrium will shift to the left
  • emf will decrease
30
Q

What is the effect of increasing the concentration of one of the solutions

A

If the concentration of a solution is increased
The equilibrium will shift away from the solution
Depending on which electrode (left or right) is made more positive
The emf will increase or decrease

31
Q

What effect would current have on the emf

A

If the cell is allowed to draw current
Emf will decrease and become 0
When the reaction reaches equilibrium

32
Q

What effect would a catalyst have on the emf

A

NO EFFECT ON THE EMF VALUE

but, can increase the rate at which equilibrium is reached

33
Q

What can the electrode potential tell you about the direction of a redox reaction?

A

The more negative the electrode potential the more it oxidises;
Reaction occurs BACKWARDS

The more positive the electrode potential the more it reduces;
Reaction occurs FORWARDS

34
Q

Are primary cells rechargeable?

Why?

A

NOT rechargeable;

The reactions are not reversible

35
Q

When a cell is discharging what does that tell you about the emf

A

Emf is greater than or equal to 0

Occurs spontaneously

36
Q

State one essential property of the non reactive porous separator

A

Allows ions to flow/ move freely

37
Q

State the function of the carbon rod

A

Conduct electricity

38
Q

State one environmental advantage of rechargeable cells compared to non rechargeable ones

A
  • metal compounds can be reused
  • less mining
  • less use of landfill
  • less use of energy
39
Q

Are secondary cells rechargeable?

Why?

A

Yes;

The reactions are reversible

40
Q

When a cell is recharging what does that tell you about the emf

A

Emf < 0

Needs electrical current to occur

41
Q

What does the graph of a secondary cell look like & explain the shape

A

Voltage/ emf remains constant for some time before dropping suddenly:

Voltage drops at this point because the reactants get used up

42
Q

State the half equations & overall equation of a hydrogen- fuel cell in ACIDIC conditions

A

OXIDATION:
H2—> 2H+ + 2e-

REDUCTION:
O2 +4H+ + 4e- —> 2H2O

Overall: 2H2 + O2 —> 2H2O

43
Q

State the half equations & the overall equation of a hydrogen- fuel cell under ALKALINE conditions

A

OXIDATION:
H2 + 2OH- —> 2H2O + 2e-

REDUCTION:
O2 + 2H2O + 4e- —> 4OH-

Overall: 2H2 + O2 —> 2H2O

44
Q

What is the emf of the cell in acidic conditions? What is the emf of the cell in alkaline conditions? Why?

A

They’re the same;

The overall equations are the same

45
Q

Explain the shape of the graph of a fuel cell

A

It’s is a straight horizontal line;

Voltage is constant because fuel is consistently being supplied to the cell; the concentration remains constant

46
Q

Name some advantages of fuel cells

A
  • only waste is water

- very efficient (converts more available energy into kinetic energy)

47
Q

Name some disadvantages of fuel cells

A
  • limited life time
  • expensive
  • hydrogen is highly flammable
  • fossil fuels burnt to obtain hydrogen
48
Q

Are hydrogen fuel cells carbon neutral? Why?

A

Not entirely;
Depends on how the hydrogen obtained;
H2 obtained from methane is not carbon neutral (methane is a carbon source)

49
Q

What are the half cell equations & overall equation of ethanol fuel cells

A

OXYGEN ELECTRODE:
4e- + 4H+ + O2 —> 2H2O

ETHANOL ELECTRODE:
C2H5OH + 3H2O —> 2CO2 + 12H+ + 12e-

Overall: C2H5OH + 3O2 —> 2CO2 + 3H2O

50
Q

Name some advantages of the ethanol fuel cells

A
  • made from renewable sources in a carbon neutral way
  • abundant raw materials (to produce ethanol)
  • less explosive/ easier to store than H2

Chemistry (4 decks)

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