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AQA A Level Chemistry > Electrochemical Cells > Flashcards

Electrochemical Cells Flashcards

1
Q

How do you make a half cell and therefore a electrochemical cell

A

When a metal is dipped into a solution of its ions, you get a half-cell
The combination of two half cells gives an electrochemical cell

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2
Q

What is the standard hydrogen electrode (SHE)

A

The reference electrode, which is assigned an electrode potential of 0.00V under standard conditions

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3
Q

How can the standard electrode potentials of other systems be found

A

By combining them with a standard hydrogen electrode and measuring the EMF of the cell formed

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4
Q

How is the equation for each half cell written

A

As a reduction process e.g.
M2+ + 2e- → M

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5
Q

What are the standard conditions referred to be standard electrode potential

A

298K
100kP
1.00 mol dm^-3 solution of ions

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6
Q

How do you display the conventional representation of cells

A

Draw the salt bridge.
The half cell with the more positive E is always reduction and so placed on RHS
The standard hydrogen electrode is ALWAYS on the left hand side
Write the ions/atoms beside the salt bridge with the higher oxidation atoms closer to salt bridge.
Separate different phases with a boundary e.g. separate solids and gases with I
If one side does not have a solid, add a Pt(s) catalyst to the end of that side

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7
Q

How do you calculate the EMF of a cell

A

E of RHS - E of LHS

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8
Q

How can you determine whether a reaction is thermodynamically feasible based on the E value of the cell

A

If the E value of the cell is positive, the reaction is feasible

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9
Q

What should always be used for the solution

A

A nitrate as all nitrates EXCEPT CuSO4 and ZnSO4 are soluble

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10
Q

Why is a salt bridge used

A

It completes the circuit and allows the movement of ions
Keeps the two solutions electrically neutral as it provides a reservoir of positive and negative ions

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11
Q

Why is a high resistance voltmeter used

A

A high resistance voltmeter is used since it draws no current from the cell, and the so the cell has its maximum potential difference

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12
Q

Why are two half-cells required to make the complete cell

A

Both oxidation and reduction must take place separately, so that electrons are transferred via the external circuit

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13
Q

Why does a voltage from when a zinc electrode in zinc solution is connected to a copper electrode in copper solution

A

When connected together the zinc half-cell has more of a tendency to
oxidise to the Zn2+ ion and release electrons than the copper half-cell.
(Zn Zn2+ + 2e-)

More electrons will therefore build up on the zinc electrode than the
copper electrode.

A potential difference is created between the two electrodes.

The zinc strip is the negative terminal and the copper strip is the
positive terminal.

This potential difference is measured with a high resistance voltmeter

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14
Q

Identify the substances and conditions used in the standard chlorine half cell

A

A platinum electrode
1.00 mol dm^-3 solution of Cl- ions
Chlorine gas at 100kPa pressure
Temperature at 298K

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15
Q

How do you know which cell is being reduced and oxidised

A

The cell on the right is the cathode unless told otherwise
CRAO
Cathode is reduction
Anode is oxidation

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16
Q

How is a salt bridge made

A

From a piece of filter paper soaked in a salt solution, usually potassium nitrate

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17
Q

What property should the salt used to make the salt bridge have

A

Unreactive with the electrodes and electrode solutions.

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18
Q

Why is a wire not used as a salt bridge

A

The metal wire would set up its own electrode system with the solutions

19
Q

What happens if current is allowed to flow

A

If the voltmeter is removed and replaced with a bulb or if the circuit short circuited, a current flows.
The reactions occur separately at each electrode.
Voltage falls to 0 as the reactants are used up.

20
Q

What are the components of the standard hydrogen electrode

A

A platinum electrode immersed in 1.00 mol dm^-3 dilute HCl
Hydrogen gas at 100kPa bubbled over the platinum electrode
298K temperature

21
Q

What would a diagram of the standard hydrogen electrode look like

A

A platinum electrode immersed in HCl and a test tube upside down over the electrode with a gap to let in hydrogen gas

22
Q

What is the purpose of a platinum electrode

A

It is an inert/unreactive electrode
Allows electron transfer
Conducts electricity
Allows hydrogen gas to be adsorbed onto its surface

23
Q

Why are platinum electrodes made by coating porous material with platinum rather than using platinum rods

A

To increase the surface area and speed up the rate of reaction

24
Q

What problems are encountered with the standard hydrogen electrode

A

It is difficult to maintain the pressure of hydrogen at 100kPa
It is a very slow electrode reaction

25
Q

Why is a reference electrode necessary to measure the standard electrode potential

A

A reference electrode is needed to measure the potential difference.
You cannot measure the electrode potential of a single electrode as electron transfer to another electrode is necessary to complete the circuit

26
Q

Why is a saturated solution used to make the salt bridge

A

So no ions enter the salt bridge but ions can come out of the salt bridge.

27
Q

Why is a salt bridge used and not a metal piece of wire

A

A salt bridge is used to allow the movement of ions
The metal wire would set up its own potential difference and interfere with cell reaction

28
Q

What are the standard electrode potentials of oxidising and reducing agents

A

Oxidising agents have a HIGH POSITIVE potential
Reducing agents have a HIGH NEGATIVE potential

29
Q

Consider the reaction :
Ag+ + e- → Ag
E = +0.80V

What effect would decreasing the concentration of silver ion have on the value of the electrode potential

A

The position of equilibrium shifts to the left, producing more Ag+ ions and producing more electrons. This makes the electrode potential less positive.

30
Q

Consider the reaction :
Ag+ + e- → Ag
E = +0.80V
State and explain the effect on the cell emf on using a smaller silver rod

A

There is no effect on the cell EMF since the equilibrium is unaffected by the amount of solid. It is only affected by the conc of Ag+ ions

31
Q

Why can the Zn2+/Zn half cell reduce the Co2+ ions to cobalt metal

A

The E of the Zn2+/Zn half cell is more negative than that of the Co2+/Co half cell. Therefore Zn releases electrons to Co2+ and reduces it to Co.

32
Q

Why might a reaction not take place even though the E of the cell is positive

A
  1. Reaction has a high activation energy
  2. Actual conditions different to the standard conditions used to measure E values and this will affect value of the electrode potential.
33
Q

V3+ + e- = V2+ and E = -0.26V

(SO4)2- + 4H+ + 2e- = SO2 + 2H2O and E = +0.17V

Explain why SO2 will not reduce V3+ ions to V2+ ions

A

SO2 will not reduce V3+ ions to V2+ ions because E of SO4 2-/ SO2 half cell is more positive than that of V3+/V2+ half cell. Therefore SO2 will not release electrons to the V3+ ions and wont’t reduce them.

34
Q

Why might a battery be used instead of a cell

A

A battery is 2 or more cells linked together in series. A cell may not have a high enough voltage.

35
Q

What are electrochemical cells used as

A

A commercial source of electrical energy

36
Q

What are the different types of cells

A

Non-rechargeable (irreversible)
Rechargeable
Fuel cells

37
Q

What are the electrode reactions in a lithium cell

A

Positive electrode:

Li+ + CoO2 + e- = Li+[CoO2]-

Negative electrode:

Li = Li+ + e-

38
Q

How does a fuel cell generate a voltage

A

It uses the energy released from the reaction of a fuel with oxygen to produce a voltage

39
Q

Uses of Lithium ion cells

A

To power cameras and mobile phones

40
Q

Why would water not be a good solvent in lithium ion cells

A

It would react with the lithium metal

41
Q

Why do fuel cells not need to be charged

A

They maintain a constant voltage over time because they are continuously fed with fresh O2 and H2 so maintaining constant conc of reactants.

42
Q

What are the reactions in a hydrogen fuel cell under alkaline conditions

A

4e- + 4H2O = 2H2 + 4OH-

4e- + 2H2O + O2 = 4OH-

43
Q

What are the limitations of hydrogen fuel cells

A

Expensive
Difficult to store and transport hydrogen
Hydrogen is explosive
Limited lifetime
The hydrogen made from water by electrolysis uses electricity generated in power plants that burn fossil fuels
Use of toxic chemicals in their production

44
Q

Advantages of fuel cells

A

less pollution and CO2 produced as the only by product is water
Greater efficiency

AQA A Level Chemistry (30 decks)

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