Electrode Potential Flashcards

1
Q

What happens when a rod of a metal is dipped into a solution of its own ions ?

A

An equilibrium is set up between the solid metal and the aqueous metal ions

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2
Q

Write a half-equation for zinc (s) to zinc (2+)

A

Zn (s) = Zn2+(aq) + 2e-

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3
Q

Write a half-equation for copper (II) to copper (lI).

A

Cu2+(aq) = Cu3+(aq) + e-

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4
Q

What is the simplest salt bridge made out of ?

A

Filter paper soaked in a saturated solution of KNO3 (potassium nitrate)

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5
Q

What is the purpose of a salt bridge ?

A
  • complete the circuit
  • allows the ions to move to balance the charge
  • do not react with electrodes
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6
Q

What is electrode potential?

A

Measure of how easily a metal is oxidised

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7
Q

What electrode potential does a metal that is easily oxidised have ?

A

Negative

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8
Q

What electrode potential does a metal that is hard to oxidise have ?

A

Positive

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9
Q

Why is there an equilibrium within a cell ?

A

There is a tendency for metal ions to form + ions and go into solution, but there is also the tendency for for metal ions to gain electrons in solution .

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10
Q

What happens if the equilibrium shifts to the left ?

A

The metals acquires a negative charge due to the build up of electrons on the metal

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11
Q

What happens when the equilibrium shifts to the right ?

A

Positive charge will build up on the metal as electrons have been used up to form metal from the metal ions

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12
Q

What happens at the left hand electrode ?

A
  • oxidation
  • is the half cell with the most negative Eº value
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13
Q

What happens at the right hand electrode ?

A
  • reduction
  • electrode with the most positive Eº value
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14
Q

What conditions is the standard hydrogen electrode used in ?

A
  • 298K
  • 100kPa
  • [H+] 1.00 mol dm-3
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15
Q

What is the standard hydrogen electrode used for ?

A
  • Comparing other cells against it because the Eº of SHE is 0
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16
Q

Why might you use other standard electrodes ?

A

They are cheaper/easier/quicker to use and can provide just as good a reference.
Platinum is expensive

17
Q

If an E° value is more negative, what does it mean in terms of oxidising/reducing power?

A

Better reducing agent (easier to oxidise)

18
Q

If an E° value is more positive, what does it mean in terms of oxidising/reducing power?

A

Better oxidising agent (easier to reduce)

19
Q

What factors will change E° values?

A

Concentration of ions
Temperature

20
Q

What happens if you reduce the concentration of the ions in the left hand half cell?

A

Equilibrium moves to the left to oppose the change of removing ions; this releases more electrons, the E° of the left hand cell becomes more negative, so the e.m.f. Of the cell increases.

21
Q

How do you calculate the emf of a cell from E° values?

A

E°cell = E° right - Eº left

22
Q

When would you use a Platinum electrode?

A

When both the oxidised and reduced forms of the metal are in aqueous solution

23
Q

Why is Platinum chosen?

A

Inert so does not take part in the electrochemistry
Good conductor to complete circuit

24
Q

How would you predict if a reaction would occur?

A
  • Take the 2 half equations.
  • Find the species that is being reduced (this is effectively the right hand electrode)
  • Calculate its E° value minus the E° value of the species that is being oxidised (effectively the left hand cell).
  • If E° overall > 0, reaction will occur.
25
How does Concentration and Pressure Effect the EMF?
- Increasing the concentration of the solutions used in the electrochemical cell makes the cell EMF more positive as fewer electrons are produced in the reaction. - Increasing the pressure of the cell will make the cell EMF more negative as more electrons are produced.
26
What are the two reactions that take place in zinc/carbon cells (disposable batteries)?
Zn oxidised to Zn2+ NH4+ reduced to NH3 at carbon electrode
27
What are the reactions that occur in a lead/acid battery (car batteries)?
Pb + SO42- → PbSO4 (S) + 2e- PbO2 + 4H+ + SO42- + 2e- → PbSO4 + 2H20
28
How are cells recharged (if they are rechargeable)?
Reactions are reversible and are reversed by running a higher voltage through the cell than the cell's E°
29
What is a fuel cell ?
A cell that is used to generate an electrical current and does not require charging as it has a constant supply of the chemicals it needs
30
What are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?
2H2 + 40H-→ 4H20 + 4e- O2 + 2H20 + 4e- → 40H-
31
Why is it better to use a fuel cell than to burn H, in air, even though the same overall reaction occurs?
In combustion, sulfur containing compounds (SO2, SO3) and nitrogen containing compounds (NO2, NOx) are produced due to the high temperatures and the S and N in air. These are bad for the environment. This does not occur in a fuel cell; the only product is water. More efficient
32
Disadvantages of fuel cells?
Hydrogen is a flammable gas with a low b.p. → hard and dangerous to store and transport → expensive to buy Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
33
Why might the e.m.f. Of a cell change after a period of time?
Concentrations of the ions change - the reagents are used up
34
How can the e.m.f. Of a cell be kept constant?
Reagents are supplied constantly, so the concentrations of the ions are constant; E° remains constant