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Electrode Potentials & Electrochemical Cells PHYSICAL Flashcards

1
Q

How do electrochemical cells generate electricity?

A

Spontaneous redox reactions

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2
Q

Why is a cell built?

A

Separate two redox processes
Stops direct transfer of electrons

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3
Q

What creates the equilibrium?

A

Metal rod dipped in solution of own ions

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4
Q

What is each part of the cell called?

A

Half cell

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5
Q

What does the position of equilibrium determine?

A

Potential difference between metal and metal solution

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6
Q

What is meant by electrode potential?

A

Potential difference set up between rod and solution

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7
Q

What does the potential difference between rod and solution depend on?

A

Nature of ions in solution
Concentration of ions in solution
Type of electrode
Temperature

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8
Q

What symbol is used to represent electrode potential?

A

E

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9
Q

What is meant by electrode potential?

A

Value showing how easily substance is reduced

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10
Q

How are electrode potentials shown?

A

Reversible half equations

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11
Q

Where are the electrons in an electrode potential half equation?

A

Left-hand side
Show reduction

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12
Q

Why do different species have different electrode potentials?

A

Different positions of equilibrium

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13
Q

What happens to the more positive electrode potential?

A

More likely to undergo reduction
Equilibrium position lies more to right

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14
Q

What is a solid vertical line used to show in electrochemical cell?

A

Phase boundary

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15
Q

What is a double vertical line used to show in electrochemical cell?

A

Salt bridge

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16
Q

What is the purpose of a salt bridge?

A

To complete the circuit with its mobile ions

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17
Q

Which electrode does oxidation take place at?

A

Negative electrode

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18
Q

Which electrode does reduction take place at?

A

Positive electrode

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19
Q

What does the position of equilibrium and therefore electrode potential in an electrochemical cell depend on?

A

Temperature
Pressure of gases
Concentration of reagents

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20
Q

What is needed to compare electrode potentials?

A

Standard conditions
All measured against standard

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21
Q

What standard conditions are used for electrochemical cells?

A

Ion concentration of 1.00 mol dm⁻³
Temperature of 298 K
Pressure of 100 kPa

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22
Q

Why are measurements for electrode potentials made using high resitance voltmeter?

A

No current flow
Maximum potential difference is achieved

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23
Q

What are electrode potentials measured against?

A

Standard hydrogen electrode

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24
Q

What value is given to the standard hydrogen electrode?

A

0.00 V

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25
What symbol is used to represent standard electrode potential?
Eꝋ
26
What is the equation for the standard hydrogen electrode?
2H⁺ (aq) + 2e⁻ ⇌ H₂ (g)
27
What is meant by standard electrode potential?
The potential difference produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions
28
What is needed for the standard hydrogen electrode to work?
Inert platinum electrode
29
What three types of half-cells can be connected to a standard hydrogen electrode?
A metal/metal ion half-cell A non-metal/non-metal ion half-cell An ion/ion half-cell (ions in different oxidation states)
30
How is standard cell potential calculated?
E(cell)ꝋ = E(right)ꝋ - E(left)ꝋ
31
When is platinum added to an electrode?
Non-metal Gas or aqueous solution
32
Why is platinum used for non-metal half cells?
Inert so doesn't take part in reaction Redox equilibrium is established on platinum surface
33
How do you describe which half cell is being oxidised?
Eꝋ (-x/x) more positive than Eꝋ (+y/y) so -x can oxidise y to +y
34
How can the direction of electron flow in an electrochemical cell be determined?
Compare Eꝋ values of two half-cells
35
Which pole readily accepts electrons?
Positive pole
36
Which pole readily loses electrons?
Negative pole
37
Which direction do electrons flow?
From negative to positive pole
38
What does the Eꝋ value of a species indicate?
How easily oxidised/reduced
39
What does a more positive Eꝋ value mean?
Easier to reduce species on left of half-equation Proceeds in forward direction
40
What does a less positive Eꝋ value mean?
Easier to oxidise species on right of half-equation Proceeds in backward direction
41
Does a positive Eꝋ value mean reaction is feasible or not?
Feasible
42
Where do electrons move in electrochemical cell?
Wires of external circuit Not through electrolyte solution
43
What are the three different types of cells that can be used as a commerical source of electrical energy?
Non-rechargeable Rechargeable Fuel
44
What four factors are used to determine which cell is used in a commercial application?
Voltage required Current required Size of cell Cost
45
What does the term battery refer to?
A collection of cells
46
What is the Daniell cell?
Early electrochemical cell with a simple metal-metal ion system
47
Describe the Daniell cell.
Zinc rod in solution of zinc sulfate Copper cylinder filled with copper sulfate solution Porous pot separate copper sulfate and zinc sulfate
48
What is the positive electrode in the Daniell cell?
Copper
49
What is the negative electrode in the Daniell cell?
Zinc
50
What is the drawback of Daniell cell?
Impractical for portable devices because of hazardous liquids in cell
51
What is a zinc-carbon cell?
Most common non-rechargeable cell
52
Describe a zinc-carbon cell.
Zinc casing Ammonium chloride acts as electrolyte Carbon rod acts as electron carrier
53
What is the positive electrode in the zinc-carbon cell?
Ammonium chloride
54
What is the negative electrode in the zinc-carbon cell?
Zinc
55
What is the electrolyte in an electrochemical cell?
Liquid that conducts electricity to complete circuit
56
What is the electron carrier in an electrochemical cell?
Pulls electrons from compounds and carry to other locations within cell
57
What is the drawback of zinc-carbon cell?
Zinc casing wears away and corrosive contents can leak out
58
What are the advantages of zinc-carbon cell?
Provides small current Relatively cheap
59
How can cells be rechargeable?
Use chemical reactions which can be reversed by applying voltage greater than cell voltage so electrons push in opposite direction
60
What are three common rechargeable cells?
Lead-acid batteries NiCad cells Lithium cells
61
What are lead-acid batteries?
Six cells joined in series
62
What is the negative electrode in the lead-acid batteries?
Lead
63
What is the positive electrode in the lead-acid batteries?
Lead oxide
64
What is the electrolyte in the lead-acid batteries?
Sulfuric acid
65
What combined voltage is given by the six cells of a commercial car battery?
12V
66
What happens in a car battery when the car is in motion?
Generator pushes electrons that reverse the reaction and regenerate lead and lead oxide
67
What are the disadvantages of lead-acid batteries?
Heavy Contain toxic materials Corrosive electrolyte
68
What is the positive electrode in a NiCad cell?
Cadmium
69
What is the negative electrode in a NiCad cell?
Nickel hydroxide-oxide system
70
What is the drawback of NiCad cells?
Cadmium is toxic so disposal environmental issue
71
How are NiCad cells commercially viable?
Expensive but can be recharged hundreds of times
72
What are lithium ion cells most commonly used for?
Laptops Mobile devices
73
Why is lithium used in cells?
Low density High electrode potential
74
What is the positive electrode of a lithium cell?
Lithium cobalt oxide
75
What is the negative electrode of a lithium cell?
Carbon
76
What is the electrolyte of a lithium cell?
Porous polymer membrane
77
What is the advantage of lithium cell's polymer electrolyte?
Cannot leak
78
How are lithium cells structured?
Sandwich of layers each electrode
79
What happens when lithium cell is charged and discharged?
Lithium ions flow between negative and positive through electrolyte
80
What is the overall equation for lithium cell?
Li (s) + CoO2 (s) → Li + (CoO2) – (s)
81
What is meant by memory effect in NiCad cells?
Gradually lose charge after repeated charge cycles when cell is not fully discharged
82
Do lithium cells show memory effect?
No
83
What are the problems with lithium cells?
Global shortage of lithium means demand exceeds supply If cells aren't recycled, lithium lost Reports of lithium cell fires
84
What is a fuel cell?
Electrochemical cell where fuel donates electrons at one electrode and oxygen gains electrons at other
85
Where are fuel cells common?
Automotive industry Replace petrol/diesel engines
86
What happens when fuel enters fuel cell?
Becomes oxidised which creates potential difference
87
What is released by hydrogen-oxygen fuel cell?
Energy and water
88
State the four features of a hydrogen-oxygen fuel cell.
Reaction chamber Outlet Electrolyte Semi-permeable membrane
89
What is the electrolyte used in hydrogen-oxygen fuel cell?
Aqueous sodium hydroxide
90
What is the reaction chamber in hydrogen-oxygen fuel cell?
Separate inlets for hydrogen and oxygen gas
91
What is the outlet for in hydrogen-oxygen fuel cell?
Release water product
92
What is the semi-permeable membrane for in hydrogen-oxygen fuel cell?
Separates hydrogen and oxygen gases
93
What are the four benefits of hydrogen-oxygen fuel cells?
Environmental advantage as water only product All bond energy converted to electrical energy instead of heat and light No harmful oxides of nitrogen produced Use on spacecraft, water can be drank
94
How is all the bond energy converted to electrical energy in an hydrogen-oxygen cell?
Reaction takes place at room temperature without combustion
95
State the five disadvantages of hydrogen-oxygen fuel cells.
Safety hazards with production and storage of hydrogen Expense of thick walled cylinders and pipes needed for storage By-product of crude oil industry so relies on finite resource Too expensive to be widespread Hydrogen has high energy density so larger containers needed
96
Why is there hazards with production and storage of hydrogen?
Hydrogen is highly flammable
97
Whats the big difference between fuel cells and other cells?
Fuel cell operates continuously in supply of oxygen and hydrogen

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