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Chemistry Alevel OCR YR 13 > Electrode potentials > Flashcards

Electrode potentials Flashcards

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1
Q

what is a voltaic cell?

A

type of electrochemical cell which coverts chemical energy into electrical energy

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2
Q

how can a voltaic cell be made?

A

-two different half cells are connected allowing electrons to flow.
-ion half-cell contains ions of same element in different oxidation state in solution

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3
Q

why should chemicals of the two half-cells not be mixed?

A

if mixed, electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy

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4
Q

In what direction do the electrons flow in a voltaic cell?

A

depends upon the relative tendency of each electrode to release electrons

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5
Q

if we have a half cell with 2 aqueous ions, we use what electrode?

A

Platinum electrode as it is inert and electrically conductive

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6
Q

which metals in electrodes loses and gains electrons?

A

-the electrode with a more reactive metal loses electrons and is oxidised-negative electrode
-the electrode with less reactive metal gains electrons and is reduced- positive electrode

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7
Q

Describe standard electrode potential of hydrogen

A

-standard chosen is a half-cell containing hydrogen gas and a solution containing H+(aq) ions
-platinium electrode used
-standard conditions
-electrode potential of 0V as it is the standard

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8
Q

what are the standard conditions?

A

-soutions of 1mol/dm cubed
-298K (25 degrees)
-100kPa

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9
Q

standard electrode potential definition?

A

the e.m.f of a half cell connected to a standard hydrogen half cell under standard conditions

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10
Q

the electrodes connected by wire allow the flow of ?

A

electrons

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11
Q

salt bridge allows the flow of?

A

ions

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12
Q

what does the salt bridge contain?

A

concentrated solution of an electrolyte that does not react with either solution

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13
Q

the more negative the electrode potential…

A

-the greater the tendency to lose electrons and undergo oxidation
-the less the tendency to gain electrons and undergo reduction
-the greater the reactivity of a metal in losing electrons

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14
Q

the more positive the electrode potential…

A

-the greater the tendency to gain electrons and undergo reduction
-the less the tendency to lose electrons and undergo oxidation
-the greater the reactivity of a non-metal in gaining electrons

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15
Q

Metal and non-metals tend to have what electrode potential?

A

-metals tend to have negative electrode potentials
-non-metals tend to have positive electrode potentials

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16
Q

how do you calculate standard cell potential?

A

electrode potential of positive electrode - electrode potential of negative electrode

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17
Q

how do you predict redox reactions using electrode potentials?

A

a reaction should take place between an oxidising agent on the left and a reducing agent on the right, provided that the redox system of the oxidising agent has a more positive electrode potential than the redox system of the reducing agent

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18
Q

how do you work out the overall equation?

A

-balance the electrons
-a more redox system with a more positive electrode potential will reverse the redox system of a less positive electrode potential
-combine equations
-cancel electrons

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19
Q

what are the limitations to predicting reactions using electrode potentials?

A

-very large activation energy which can result in a slow rate
-concentration of solutions should be 1mol/dm cubed
-conditions may not be standard conditions

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20
Q

what happens if the concentration of solution is higher than 1mol/dm cubed?

A

-equilibrium will shift to the right, removing electrons from the system and making the electrode potential more negative

21
Q

what happens if the concentration of solution is less than 1mol/dm cubed?

A

the equilibrium will shift to the left increasing electrons in the system and making the electrode more negative

22
Q

what are the three main types of cells?

A

primary, secondary and fuel cells

23
Q

Describe primary cells?

A
  • non-rechargeable and single use
    -when chemicals used up, voltage will fall, the battery will go flat and the cells will be discarded or recycled
    -used in low current, long storage devices e.g clocks and smoke detectors
24
Q

How is electrical energy produced in a primary cell?

A

-produced by oxidation and reduction at the electrodes
-reactions cannot be reversed

25
Q

Describe most modern primary cells?

A

based on zinc and manganese dioxide, Zn/MnO4, and a potassium hydroxide alkaline electrolyte

26
Q

What are the systems (equations) and their electrode potential in most modern primary cells? (Zn/MnO4)

A

1) ZnO(s) + H2O(l) + 2e- (reversible reaction) Zn(s) + 2OH-(aq)
electrode potential: -1.28V
2) 2MnO2(s) + H2O(l) + 2e- (reversible reaction) Mn2O3(s) + 2OH-

27
Q

What is the oxidation, reduction equations and their overall cell reaction in most modern primary cells? (Zn/MnO4)

A

oxidation: Zn(s) +2OH-(aq) -> ZnO(s) + H2O(l) + 2e-
reduction: 2MnO2(s) + H2O(l) + 2e- -> Mn2O3(s) +2OH-(aq)
cell reaction: Zn(s) + 2MnO2(s) -> ZnO(s) + Mn2O3(s) electrode potential: 1.43V

28
Q

Describe secondary cells?

A

-rechargeable, reaction producing electrical energy is reversible, chemicals in cells are regenerated

29
Q

what are common examples of secondary cells?

A
  • lead-acid batteries used in car batteries
  • nickel-radium, NiCd, cells and nickel-metal hydride, NiMH- the cylindrical batteries used in radios, torches
    -lithium-ion and lithium ion polymer cells used in laptops, cameras and cars
30
Q

describe fuel cells?

A

-fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
-fuel and oxygen flow into fuel cell and the products flow out. The electrolyte remains in the cell
-Fuel cells can operate continuously provided that the fuel and oxygen are supplied into the cell
-don’t have to be recharged

31
Q

Describe hydrogen fuel cells?

A

-water is the only combustion product
-alkali or acid electrolyte

32
Q

what are the redox systems with electrode potentials found in an alkali hydrogen fuel cell?

A

2H2O(l) +2e- (reversible reaction) H2(g) + 2OH-(aq) electrode potential: -0.83V
1/2O2(g) + H2O(l) + 2e- (reversible reaction) 2OH-(aq) electrode potential: 0.40V

33
Q

What is the oxidation, reduction and overall equation in an alkali hydrogen fuel cell?

A

oxidation: H2(g) + 2OH-(aq) -> 2H2O(l) +2e-
reduction: 1/2O2(g) +H2O(l) + 2e- -> 2OH-(aq)
overall: H2(g) + 1/2O2(g) -> H2O(l)
electrode potential: 1.23V

34
Q

What is the oxidation, reduction and overall equation in an acid hydrogen fuel cell?

A

oxidation: H2(g) -> 2H+(aq) + 2e-
reduction: 1/2O2(g) + 2H+(aq) + 2e- -> H2O(l)
overall: H2(g) + 1/2O2(g) -> H2O(l)
electrode potential: 1.23V

35
Q

what are the redox systems with electrode potentials found in an acid hydrogen fuel cell?

A

2H+(aq) + 2e- (reversible reaction) H2(g)
electrode potential: 0.00V
1/2O2(g) + 2H+(aq) + 2e- (reversible reaction) H2O(l)
electrode potential: 1.23V

36
Q

Describe a lead-acid battery

A

negative electrode: lead (Pb)
positive electrode: lead with lead oxide coating (PbO2)
electrolyte: dilute sulfuric acid
-rechargeable battery used in cars

37
Q

what are the common rechargeable cells?

A

lead-acid batteries, lithium ion cells

38
Q

How is a lead-acid battery used in cars

A

-When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead(IV) oxide
-Lead-acid batteries are designed to produce a high current for a short period of time, hence their use in powering a starter motor in car engines

39
Q

What are the disadvantages of lead-acid batteries

A

-They are very heavy
-They contain toxic materials: lead and lead(IV) oxide
-The sulfuric acid electrolyte is very corrosive
-challenges of disposal

40
Q

Describe lithium ion cells

A

-powers mobile devices
-At the negative electrode (graphite, carbon), Lithium is discharging
Li -> Li+ + e-
-Graphite acts as a support medium for the Li+ ions
-Li+ ions move to the positive electrode, Cobalt (IV) oxide, across the electrolyte
-during charging Li+ ions move to positive electrode, during discharging Li+ ions move to negative electrode

41
Q

Advantages of lithium ion cells

A

-Lithium is the lightest metal.
-Lithium is one of the strongest reducing agents (meaning that it can create a relatively high voltage).
-Lithium ion batteries can be constructed without any moisture whatsoever (saving weight and avoiding water-related damage over time).

42
Q

disadvantages of lithium ion battery?

A

-cost
-cell temperature is monitored to prevent temperature extremes

43
Q

Describe fuel cells

A

generate electricity from electrochemical reaction in which oxygen (from air) and a fuel (e.g hydrogen) combine to form water. Electrolyte can be acidic or alkali.

44
Q

what is the overall equation in a hydrogen fuel cell?

A

2H2 + O2 -> 2H2O

45
Q

Disadvantages of hydrogen fuel cells?

A

-producing hydrogen gas from fossil fuels releases greenhouse gases. Therefore hydrogen should be produced by electrolysis.
-hydrogen is highly flammable
-hydrogen is difficult to store

46
Q

Describe what happens in a hydrogen fuel cell with acidic electrolyte?

A

-hydrogen gas is oxidised at anode
2H2 -> 4H+ + 4e-
-electrons travel via external circuit
-H+ ions travel through electrolyte and membrane
-oxygen enters and is reduced at the cathode.
O2 + 4H+ + 4e- -> 2H2O
- A catalyst speeds up the reaction with oxygen

47
Q

Describe what happens in a hydrogen fuel cell with alkaline electrolyte?

A

-At the cathode, platinum (catalyst) causes hydrogen to split into H+ ions and electrons
-electrons travel via external circuit
-In electrolyte, H+ ions and OH- ions react to form water
-reaction at the negative electrode: 2H2 + OH- -> 4H2O + 4e-
-oxygen enters the cell
-reaction at the positive electrode: O2 + 2H2O + 4e- -> 4OH-

48
Q

what are the limitations for using electrode potentials to predict reactions?

A

-activation energy may be too high to overcome
-rate of reaction is too slow

Chemistry Alevel OCR YR 13 (13 decks)

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