Electrochemical Cells (Physical) (complete)

Question 1

Define half-cell/electrode (metal).

Name and describe the three general types of electrode.

Answer 1

• A metal dipping into a solution of its own ions.

1- Metal electrodes - Consist of metal dipping into a solution of its own ions.
2- Gas electrode - A gas and a solution of it’’s own ions, am inert metal (usually platinum) is the actual electrode to allow the flow of electrons.
3- Redox electrodes - for two different ions of the same element, where the two types of ions are in solution, and an inert metal (usually platinum) is used as the electrode to allow the flow of electrons.

Question 2

Describe how you would measure the potential of an electrode.

Answer 2

• Connect your half cell to another half cell of a known potential, (e.g: SHE etc…)
• Measure the potential difference between the two half-cells, and calculate the potential of the unknown electrode using the known value.

Question 3

Describe how you would set up an electrochemical cell.

Answer 3

• Set up two beakers with the respective ions solutions for the metals being used in each.
• Take your strips of metal, and sand both off to remove any outer oxidised layer, then clean using forceps and acetone to remove any grease.
• Place both strips of metal into their respective beakers and solutions, and attach a wire to the top of each strip of metal using a crocodile clip.
• Plug the other ends of the wires into a voltmeter.
• Take a strip of filter paper, and submerge it into saturated potassium nitrate solution to create a salt bridge.
• Place the salt bridge between the two beakers, so either end of the salt bridge is being dipped into the solutions in either beaker.

Question 4

Name the four standard conditions for an electrochemical cell.

Answer 4

• Cell concentration = 1.0 moldm-3 of the ions involved in the half-equation.
• Cell temperature = 298K
• Cell pressure = 100kPa (only affects half-cells with gases)
• High resistance voltmeter - prevents current being drawn from the cell.

Question 5

What is an SHE?
Why/How is it used?

Answer 5

• Standard Hydrogen Electrode.
• Used as the primary standard as the electrode potential is a known value, to calculate other electrode potential values.

Question 6

How would you write a conventional representation of a cell?

Answer 6

Reduced | Oxidised || Oxidised | Reduced

(write no electrons or balancing numbers, however add states)
( || = salt bridge). ( | = phase boundary (only write between changes of state in the half cell, if same state, comma).)
(If no solid metal electrode in a half cell, must add platinum metal as the solid metal)
(SHE always on the right, but otherwise the more positive metals go on the right).

Question 7

How do you calculate the electrode potential (emf) against the SHE?
How would you use other secondary standards?

Answer 7

emf = Ecell = E(right) - E(left)
(the SHE is always the left hand electrode.)

• If the SHE is too difficult to use (due to it involving a gas), a different standard which is easier to use can be used, which has been calibrated against the SHE.)

Question 8

What is the list of electrochemical series?
How would you write the short hand technique for the equations?
(e.g: write V3+ + 2e- —> V2+ in shorthand technique)

Answer 8

• A list of electrode potentials in order of decreasing (or increasing) potential.
• write the only two particles involved in reduction order.
• ((E V3+/V2+ )- only V particles involved, in reduction order.)

Question 9

Name and describe the 2 types of non-rechargeable cells.

Answer 9

• Zinc-carbon cells - standard, cheap, non-rechargeable cell with a fairly short life.
• Alkaline cells - higher cost cell but has a slightly longer life. (more expensive due to brass rod instead of carbon rod.

Question 10

Name and describe the 3 types of rechargeable cells.

Answer 10

• Lithium ion cells - used in phones, tablets etc… recharged by plugging in to power source to reverse the reaction.
• Lead-acid cells - used in car engines (six of the cells together), recharged by running the engine for long period of time).
• Nickel-cadmium - standard rechargeable batteries, recharged by a machine.

Question 11

Describe fuel cells. (Name the most common type.)

Answer 11

• They have a continuous supply of the chemicals into the cell so neither run out of chemicals or need re-charging.
• The most common type of fuel cell is the hydrogen-oxygen fuel cell.

Question 12

Describe the benefits of using a hydrogen fuel cell.
Give the overall equation for a hydrogen fuel cell

Answer 12

• High efficiency
• Doesn’t need recharging
• Only give off water as a waste product

2H2 + O2 —> 2H20

Question 13

Give the benefits and risks of using cells (in general) and non-rechargeable cells.

Answer 13

Cells:
- Benefits - portable source of electrical energy
- Risks - waste issues

Non-rechargeable cells:
- Benefits - cheap
- Risks - waste issues

Question 14

Give the benefits and risks of using re-chargeable cells and hydrogen fuel cells.

Answer 14

Re-chargeable cells:
- Benefits - less waste, cheaper in long run, lower environmental impact.
- Risks - some waste issues at end of useful life

Hydrogen fuel cells:
- Benefits - only waste product is water, don’t need recharging, very efficient.
- Risks - need constant supply of fuels, hydrogen is flammable and explosive, hydrogen usually made using fossil fuels, high cost of fuel cells, produce water which is greenhouse gas.