Electrode potentials and electrochemical cells

Question 1

What is a half cell

Answer 1

A rod of metal dipped in a solution of its own ions where an equilibrium is established

Question 2

What is a salt bridge and what is its purpose

Answer 2

• A salt bridge completes the electrical circuit.
• it is a piece of filter paper soaked in a solution of a salt (this is usually saturated potassium nitrate)

Question 3

Why is a salt bridge used instead of a metal wire to complete the circuit in an electrochemical cell

Answer 3

To avoid further metal ion potentials in the circuit.

Question 4

What does having the more negative e.m.f make a chemical

Answer 4

• A better reducing agent
• This means it will be oxidised in an electrochemical cell.

Question 5

What two things happen when two solutions with different emf values are connected in an electrochemical cell. Compound X has a more negative e.m.f value than compound Y

Answer 5

• Compound X is a better reducing agent so is oxidised.
• This oxidation occurs at the negative anode.
• Compound Y is reduced at the positive cathode.

Question 6

What happens to the concentration of metal ions in the half cell where the metal is oxidised

Answer 6

The solid metal dissolves to form positive cations and this increases the concentration of positive metal cations.

Question 7

What two types of change should
you mention if asked changes in an electrochemical cell

Answer 7

• How the concentrations of the various metal ions in the solution change.
• If the metal being reduced is deposited on the rod.
• The direction of electron flow.

Question 8

What do we use the hydrogen electrode for and why

Answer 8

• Comparing the tendency of different metals to release electrons.
• It is a standard electrode and we define its e.m.f value as zero.

Question 9

Describe the reagents and conditions for the standard hydrogen electrode

Answer 9

• Hydrogen gas is bubbled into a solution of aqueous H+ ions.
• Platinum metal makes the electrical contact as hydrogen does not conduct.
• The electrode is used under standard conditions.
• This is a 1.00 moldm^-3 solution of H+ ions.
• A temperature of 298K
• A pressure of 100kPa

Question 10

How does the standard hydrogen electrode act as a control

Answer 10

The potential of the standard hydrogen electrode is defined as zero so if it is connected to another electrode, the measured voltage is the electrode potential of that cell.

Question 11

What is the electromotive force E (emf) of a cell

Answer 11

The measured voltage (against a standard hydrogen electrode)

Question 12

What does the emf value being positive tell us

Answer 12

That the reaction is feasible

Question 13

What are the conventions for representing an electrochemical cell with a phase diagram

Answer 13

• A vertical solid line represents a phase boundary
• A double vertical line shows a salt bridge
• The species with the highest oxidation state is written next to the salt bridge.
• When you have the emf value for the cell, write it to the right of the cell diagram as you would enthalpy change of a reaction.

Question 14

What is the equation for emf

Answer 14

Emf= Electrode potential (R)- Electrode potential (L)

Question 15

What is a Daniell cell

Answer 15

• A zinc/copper cell
• Zn—> Zn2+ + 2e-
• Cu 2+ 2e- —> Cu (s)

Question 16

Describe the uses of a Daniell cell, it’s advantages and it’s disadvantages

Answer 16

• The Daniell cell provides an emf of 1.1V
• It was used to provide the energy for old fashioned telegraphs but is not practical for portable devices.
• This is because of the solutions of liquids it contains.

Question 17

What is a Lechanché cell

Answer 17

• The cell is the basis of most ordinary disposable batteries and the electrolysed is a paste rather than a liquid.
• In it’s most common commercial form, Zn is oxidised and ammonium ions are reduced.

Question 18

What is a problem of many commercial batteries

Answer 18

• As the cell discharges, the zinc is used up and the walls of the zinc canister become thin and prone to leakage.
• Electrolytes such as ammonium chloride are acidic and corrosive.

Question 19

How are rechargeable batteries ‘recharged’

Answer 19

An external voltage is applied which drives the electrons in the opposite direction to the feasible reaction and hence resets the cell.

Question 20

What do the rechargeable lead-acid batteries used in cars consist of

Answer 20

• Six 2V cells connected in series.
• Each cell consists of two plates dipped in a solution of sulfuric acid.
• The positive plate is made of lead coated with lead IV oxide PbO2 and the negative plate is made of lead.
• On discharging, the battery drives electrons from the lead plate to the lead IV oxide coated one.

Question 21

What are the advantages and disadvantages of a nickel-cadmium battery

Answer 21

• They are more expensive to buy
• They can be recharged up to 500 times, reducing the effective cost significantly.

Question 22

When drawing cell representations which side of the cell has the species being oxidised and which has the one being reduced

Answer 22

• The right hand side has the compound being reduced
• The left hand side has the compound being oxidised.

Question 23

At which electrode does reduction occur

Answer 23

The positive electrode which is the cathode

Question 24

At which electrode does oxidation occur

Answer 24

At the negative electrode which is the anode

Question 25

What does the left hand side of a fuel cell diagram show us

Answer 25

• The species being oxidised
• The anode (negative electrode)

Question 26

What does the right hand side of a diagram of a fuel cell show us

Answer 26

• The compound being reduced
• The cathode (positive electrode)

Question 27

What is different about the structure of a lithium ion cell

Answer 27

• The electrolyte is a solid polymer rather than a liquid or paste so it cannot leak
• Thus also means it’s charge can be topped up at any point without the memory effect of some other rechargeable batteries which can only be recharged efficiently when they have been fully discharged.
• The cell can be bent or folded without leaking.

Question 28

Describe how the electrolyte and electrodes are arranged in a lithium ion cell

Answer 28

• The positive electrode is made up of lithium cobalt oxide and the negative electrode is made up of carbon.
• On charging, electrons are forced through the external circuit from positive to negative electrode and at the same time lithium ions move through the electrolyte towards the positive electrode to maintain the balance of charge.

Question 29

What is the overall reaction that takes place in a hydrogen-oxygen fuel cell

Answer 29

2H2 (g) + O2 (g) —> 2H2O (l)

Question 30

What are the two half equations for what occurs in a hydrogen-oxygen fuel cell

Answer 30

1) 2H2 (g) + 4OH- —> 4 H20 (l) + 4e-
2) O2 (g) + 2H2O + 4e- —> 4OH- (aq)

Question 31

What are the advantages of hydrogen fuel cells

Answer 31

• Hydrogen fuel cells do not need recharging
• Hydrogen fuel cells produce a voltage as long as a fuel is supplied
• The only by-product is water which is safe and is not a greenhouse gas.

Question 32

What are the disadvantages of hydrogen fuel cells

Answer 32

• They are very expensive
• The fuel for hydrogen fuel cells is not readily available.
• hydrogen gas is hard to store safely as it is incredible flammable
• Hydrogen fuel cells use fuels which are gained from burning hydrocarbons (methane +steam) which produces carbon dioxide which is a greenhouse gas.

Question 33

What are the advantage of rechargeable cells and batteries

Answer 33

They are cheap

Question 34

What are the disadvantages of rechargeable cells and batteries

Answer 34

• They need recharging which takes time
• They contain harmful toxic chemicals which can be dangerous and pollute the environment is not recycled (and they require a separate recycling process).

Question 35

State the meaning of the term electrochemical series

Answer 35

List of electrode potentials in numerical order