Electrochemistry

Question 1

Explain the function of a salt bridge (2 marks)

Answer 1

The ions in the ionic substance in the salt bridge move through the salt bridge

to maintain charge balance and complete the circuit

Question 2

Which side is the anode and which side is the cathode. Which one is positive and which one is negative?

Answer 2

Anode = left hand side = negative electrode

Cathode = right hand side = positive electrode

Question 3

What factors effect the E.M.F of the cell?

Answer 3

• the material of electrodes
• electrolyte used in the cell
• concentration of the electrolyte
• temperature
• partial pressure (for gaseous electroylyte)
  La chatlier principle is applied for conc, temperature and pressure

It is independent of shape of electrodes, distance between electrodes and the amount of electrolyte.

Question 4

How is current made in an electrochemical cell

Answer 4

• There are two processes happening in a cell: oxidation and reduction.
  
  • One metal is oxidised and gives up electrons.
  • In the other side of the cell, the other metal accepts these electrons.
• The electrons travel from one side of the cell to the other via the wire, which is the electrical current.

Question 5

How can you tell whether a reaction is feasible or not?

Answer 5

• If the standard cell potential is positive, the reaction is feasible.
• If the standard cell potential is negative, the reaction is NOT feasible.

Question 6

What is electromotive force (e.m.f)?

Answer 6

Difference between the two electrodes

Question 7

What is the Standard Hydrogen Electrode?

Answer 7

• reference point for all potential measurements at all temperatures
• Hydrogen gas is bubbled into a solution containing hydrogen ions with a platinum electrode

Question 8

What are the standard conditions for Standard Hydrogen Electrode (SHE) (4)

Answer 8

• 100kPa
• 298K (room temperature)
• concentration 1 moldm-3
• zero current

Question 9

Why is platinum electrode typically used for SHE

Answer 9

because it is inert

Question 10

What are fuel cells?

Answer 10

Fuel must constantly be supplied (for example hydrogen) to react with oxygen and produce electrical energy.

The cell e.m.f. is constant because the supply of reactants is constantly fed in.

Question 11

What are the major advantages of using a fuel cell?

Answer 11

converts more of the available energy from the combustion of hydrogen into kinetic energy of the car

Wastes less heat energy than an internal combustion engine

constant supply of energy

doesn’t need recharging

water is its only waste product

Question 12

Draw a diagram to show how the e.m.f of a hydrogen-oxygen fuel cell changes with time

Answer 12

Question 13

Suggest why the use of a hydrogen-oxygen fuel cell might not be carbon neutral

Answer 13

CO2 i released because fossil fuels are burned to produce electricity to generate hydrogen

CO2 is released when methane reacts with steam to produce hydrogen

Question 14

How does the alkaline hydrogen-oxygen fuel cell work?

Answer 14

1. The hydrogen gas is delivered to the negative electrode (made of platinum) where it reacts with the hydroxide ions. The half equation for this reaction is: $2H_2(g)+4OH^- = 4H_2O +4e^-$
2. The electrons flow through the platinum electrode and this flow of electrons is used to power something
3. Oxygen is delivered to the positive electrode and reacts with water: $O_2(g) +2H_2O +4e^- → 4OH^-(aq)$
4. The hydroxide ions made from oxygen travel to the negative electrode (made of platinum).The hydroxide ions travel through thesemi permeable membrane, carried by the electrolyse (KOH solution)to the negative electrode where they react with the hydrogen atoms and oxidation occurs. The cycle repeats itself.

Question 15

What happens if 2 gases are want to measured.

Answer 15

They are bubbled through the solution so it it in contact with the electrode

Question 16

What happens if 2 gases are want to measured?

Answer 16

They are bubbled through the solution so it it in contact with the electrode

Question 17

Why are electrodes rubbed with sandpaper before the experiment?

Answer 17

to remove the oxide layer on the electrode.

Question 18

Why are electrodes degreased before the experiment

Answer 18

Grease could prevent the cell working as efficiently

Question 19

Draw electrochemical cell for hydrogen and copper

Answer 19

Question 20

What side of the electrochemical series are reducing agents and why?

Answer 20

On the right hand side has from the right to the left of the equation the species is losing electrons. For example Li is the reducing agent because if you read the equation backwards its losing electrons (which would be gained and hence reducing something else)

Question 21

What are the simplified electrode reactions in a lithium cobalt cell?

Answer 21

Question 22

What are primary, secondary and fuel cells?

Answer 22

Primary cells = irreversible and not intended to be recharged by an electric current

Secondary cells = reversible and is specifically designed to be recharged by an electric current

Fuel cell = generates electricity from the continuous oxidation of an external source of fuel (technically a type of primary cell as they are charged by oxidation not electrical current)

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Question 23

What is a salt bridge?

Answer 23

A simple salt bridge consists of a strip of filter paper soaked in saturated aqueous KCI or KNO3. (or KC or KNOs in agar jelly)

• the solution must not react/interfere with the solution in either half cell.