Electrochemistry

Question 1

What is electrolysis?

Answer 1

The decomposition of a compound into its elements by an electric current.

Question 2

Which ions appear at the anode?

Answer 2

Negative ions (anions)

Question 3

Which ion appears at the cathode?

Answer 3

Positive ions (cations)

Question 4

Principles to use to find the mass of substance deposited at electrode.

Answer 4

• Equation Q = I×t

* Unit of the faraday is Cmol¹(minus)

Question 5

What is 1 Faraday?

Answer 5

The quantity of electric charge carried by one mole of electrons or singly charged ions. ( e.g Pb² + 2e –> Pb , this shows that two moles of electrons are required per mole of Pb formed , so we multiply 96500 by 2)

Question 6

Equation for Avogadro constant.

Answer 6

L = charge on one mole of electrons/ charge on one electron.

Question 7

What is the standard Electrode potential for a half cell?

Answer 7

The voltage measured under standard conditions, with a standard hydrogen electrode as the second half cell.

Question 8

The more positive the value of E.p is…….

Answer 8

The easier it is to reduce the ions on the left , so the metal on the right is relatively unreactive and it is a relatively poor reducing agent.

Question 9

The more negative the value of E.p is…….

Answer 9

The more the metal is a reducing agent, it is relatively reactive.

Question 10

What is made when two half cells are connected together?

Answer 10

An electrochemical cell.

Question 11

What is the use of the salt bridge?

Answer 11

To complete the electrical circuit allowing the movement of ions between the two half-cells so that ionic balance is maintained. However it doesn’t allow the movement of electrons.

Question 12

What are the standard conditions when comparing electrode potentials.

Answer 12

• Conc. of ions at mol/dm³
• Temp. of 25⁰C
• Gases at pressure of 1atm.
• The value of the E.p of the half cell is measured relative to the standard hydrogen electrode.

Question 13

Define standard E.p for a half cell.

Answer 13

The voltage measured under standard conditions with a standard hydrogen electrode as the other half cell.

Question 14

At which terminals of the cell do reduction and oxidation occur?

Answer 14

Reduction at the positive.

Oxidation at the negative.

Question 15

Which element is at the positive terminal?

Answer 15

The element that is easier to reduce.

Question 16

For electrodes of non-metals……..

Answer 16

Electrical contact with the solution is made by using platinum wire or foil (for gases, a gas chamber is used and for liquids it is kept at the bottom of the beaker with its ions above it)

Question 17

To calculate the standard cell potential (difference between E⁰ values or cell voltage)

Answer 17

Always subtract the less positive E⁰ value from the more positive E⁰ value.

Question 18

How to determine electron flow.

Answer 18

From negative terminal to positive or from the harder to reduce(-) to the easier to reduce(+).

Question 19

What word is used if a reaction is likely to occur?

Answer 19

Feasible.

Question 20

If the forward reaction is feasible then…..

Answer 20

The backward reaction isn’t feasible.

Question 21

When is the reaction not feasible?

Answer 21

If the difference in their values of E⁰ is less than 0.3. (Under nonstandard conditions. May not occur)

OR

If the sum of the sum of the voltages of the half cells is negative.

OR

If reactants shown are not the strongest oxidising and reducing agents.

Question 22

What happens in the half cell equation as the values of E⁰ get more negative?

Answer 22

• Species on the left of the equation become weaker oxidising agents.
• Species on the right become stronger reducing agents.

Question 23

How does value of E⁰ vary with ion Conc.

Answer 23

If Conc. of ion on the left is greater than 1mol/dm then E⁰ becomes more + and if Conc. is less than 1dm/mol then E⁰ becomes more -.( opposite happens for the species on the right)

Question 24

State Nerst equation.

Answer 24

E = E⁰ + RT/zF x ln ox form/red form.

• where E is electrode potential under nonstandard conditions.
• z is number of electrons transferred.
• F is value of Faraday constant in C/mol.
• E⁰ is standard electrode potential.

Question 25

For the Nernst equation.. If the temp is 25⁰C ,then the equation is…

Answer 25

E = E⁰ + 0.059/z log₁₀ (oxidised form)

• Full equation must be used if temp isn’t 25C

Question 26

Why don’t feasibility predictions always work?

Answer 26

Because some reactions may be feasible but they proceed slowly that they don’t seem to be taking place e.g Zn and water.

Question 27

Examples of rechargeable cells . (Secondary cells)

Answer 27

Pb-acid cells ,Nickel-cadmium cells and Aluminum-air batteries.

Question 28

Advantages and Disadvantages of Nickel-cadmium cells.

Answer 28

Adv - Smaller and of lower mass than Pb-acid accumulator and doesn’t run down quickly.

Dadv- Gives a lower voltage than Pb-acid cells.

Question 29

Advantages and Disadvantages of Aluminum-air cells.

Answer 29

Adv- light weight and produce higher voltage than Pb-acid cells.

Dadv- expensive and aren’t true secondary cells as Al electrode has to be replaced from time to time.

Question 30

What are solid state cells?

Answer 30

They are primary cells that have been developed with improved voltage and reduced size. They have medical uses and are used in watches.
Also called button cells- Li or Zn as - electrode and IMnO₄ or Ag₂O as + electrode.

Question 31

Advantages of solid-state cells.

Answer 31

• Give high voltage.
• Give constant voltage over a long period of time.
• Don’t contain liquid or paste so they don’t leak.

Question 32

What are fuel cells?

Answer 32

They are electrolytic cells in which fuel gives off electrons at one electrode (anode from H), then oxygen gains electrons at the other electrode.

Question 33

Advantages of Fuel cells.

Answer 33

• No pollutant produced (water only product).
• Produce more energy per gram of fuel.
• Very efficient.
• No moving part in which energy is lost as heat.

Question 34

Disadvantages of fuel cells.

Answer 34

• High pressure tanks needed to store sufficient amount of fuel (H) and refueling has to be done regularly.
• High cost (materials to make electrode and membrane is expensive)
• Manu. of fuel cells involves prod. of toxic by products.
• The H can only be manu. cheaply using fossil fuels .
• They don’t work well at low temp. as temp below 0 the water freezes.

Question 35

The more positive E⁰ the …..( in terms of discharge)

Answer 35

Easier it is for the cation to be discharged at the cathode.

Question 36

The more concentrated ion is discharged if…..

Answer 36

The difference between their E⁰ values is less than 0.3V.