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Chemistry Physical 2 > Chapter 20 (Electrode Potential and Electrochemical Cells) > Flashcards

Chapter 20 (Electrode Potential and Electrochemical Cells) Flashcards

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1
Q

What is oxidation?

A

Loss of electrons/increase in oxidation number

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2
Q

What is reduction?

A

Gain of electrons/decrease in oxidation number

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3
Q

What does an oxidising agent do?

A

Accepts electrons

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4
Q

What does a reducing agent do?

A

Donates electrons

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5
Q

What are the steps for writing a redox reaction?

A

1) Write more negative reaction as oxidation
2) Write more positive reaction as a reduction
3) Balance the number of electrons being transferred between the two reactions
4) Combine the two half equations

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6
Q

How is an electrode potential between a metal and solution formed?

A

When a metal in placed in a solution of its ions, some of the metal atoms form ions and go into the solution, resulting in a slight build up of electrons on the metal. This build up of charge produces an electrode chemical potential between the metal and its solution.

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7
Q

What is a half reaction?

A

The equilibrium between the metal and its ions.

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8
Q

How is an electrochemical cell formed?

A

Two half reactions are connected using a salt bridge.

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9
Q

What factor increases a potential difference?

A

A greater tendency of a metal to produce ions

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10
Q

What are the standard conditions for electrode potentials?

A

Concentration = 1 mol dm-3 solution of ions
Temperature = 298 K
Pressure(if gas involved) = 100kpa
Zero current

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11
Q

Why is a high resistance voltage meter used?

A

The high resistance stops the current from flowing in the circuit which stops reactions from occurring. This allows it to measure the maximum possible potential difference.

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12
Q

Where must the S.H.E be when measuring standard electrode potential?

A

On the left`

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13
Q

How is standard electrode potential measured?

A

The difference between the standard electrode potential between a given electrode under standard conditions and the standard hydrogen electrode.

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14
Q

What does a vertical bar mean for cell notation?

A

Phase boundary

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15
Q

What does a double vertical bar mean for cell notation?

A

Salt bridge

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16
Q

What is used to calculate standard cell potentials?

A

Standard electrode potentials of half cells

17
Q

What type of cell is a lithium ion cell?

A

Rechargeable cell

18
Q

How do lithium ion cells work?

A

Lithium ions move between the graphite electrode (-) and the CoO2 electrode (+) depending on whether it charging or recharging.

During discharge - > +
During recharge + > -

19
Q

Why are fuel cells reliable?

A

There are no moving parts

20
Q

Why can fuel cells operate virtually continuously?

A

The reactants and products (O2 and H2) are pumped in and out but the electrolyte remains in the cell.

21
Q

How is E° cell calculated?

A

E° cell = E° forward/right - E° backwards/left

22
Q

What conducts charge in electrochemical cells?

A

Free moving ions

23
Q

How are salt bridges made?

A

A piece of filter paper soaked in salt solution , usually potassium nitrate.

The salt should be unreactive with the electrodes and electrode solutions.

24
Q

Why is a salt bridge used instead of a wire?

A

A wire would set up its own electrode solution.

25
Q

What is used when a system does not include a solid metal and why?

A

A platinum electrode is used because it provides a conducting surface for electron transfer, is unreactive and can conduct electricity.

26
Q

What is the effect of changing concentration on Eº?

A

Increasing concentration of one of the reactants increases Eº and decreasing concentration decreases Eº

27
Q

What is the effect of increasing temperature on Eº?

A

Most cells are exothermic so increasing the temperature would decrease Eº because equilibrium reaction would shift backward.

28
Q

Why does a fuel cell maintain a constant voltage?

A

Continuous supply of O2 and H2 so constant concentration of reactants.

29
Q

What are the half and overall equations for a hydrogen cell in alkaline conditions and the Eº?

A

4e- + 4H2O > 2H2 + 4 OH- Eº = -0.83 v
4e- + 2H2O + O2 > 4OH- Eº = + 0.4 v

Overall: 2H2 + O2 > 2H2O Eº = + 1.23 v

30
Q

What is the reaction at the positive electrode of a lithium-ion cell and Eº?

A

Li+ + CoO2 + e- > Li+[CoO2]-

Eº = + 0.6 v

31
Q

What is the reaction at the negative electrode of a lithium-ion cell and Eº?

A

Li > Li+ + e-

Eº = -3.0 v

32
Q

What is the overall discharge equation for lithium-ion cells and Eº?

A

Li + CoO2 > LiCoO2

Eº = + 3.6 v

33
Q

What are the advantages of fuel cells over conventional petrol vehicles?

A
  • Less pollution and less CO2 emitted as fuel cells only emit water
  • Greater efficiency (more thermal energy transferred to kinetic energy)
34
Q

What are the limitations of using fuel cells over petrol vehicles?

A
  • Expensive
  • Storing and transporting hydrogen is difficult as has to be a pressurised liquid and is flammable
  • Limited lifetime (regular replacement and disposal) and high production costs
  • Use of toxic chemicals in production
35
Q

Write the conventional representation for an alkaline hydrogen-oxygen fuel cell

A

Pt|H2(g)|OH–(aq),H2O(l)||O2(g)|H2O(l),OH–(aq)|Pt

36
Q

Why is the EMF of a hydrogen-oxygen fuel cell the same in both acid and alkaline conditions?

A

Same overall reaction

37
Q

Why might a porous platinum electrode be used rather than a platinum rod

A

Increase surface area so faster reaction

38
Q

Suggest why the recharging of a lithium cell may lead to release of carbon dioxide into the atmosphere.

A

Electricity for recharging the cell may come from power stations burning (fossil) fuel.

39
Q

Describe a standard hydrogen electrode (S.H.E)

A

Hydrogen gas with a solution on 1 mol dm-3 of HCl/H+ at 298K and 100kPa with a platinum electrode,

Chemistry Physical 2 (5 decks)

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