Electrochemical Cells

Question 1

In cells what is used to produce a current in an external circuit?

Answer 1

A redox reaction. Cells use seperated metal/metal-ion solutions which are connected by an electrolyte

Question 2

How is a battery made?

Answer 2

Cells are connected together to produce a large enough potential difference and current for the task required

Question 3

What happens when a metal solution is dipped in a solution of one of it’s salts?

Answer 3

Some of the metal atoms give up electrons and dissolve to form metal ions, leaving behind electrons on the metal strip. However the metal ions in the solution may reconbine with the electrons to reform metal atoms. An equilibrium is set up where the rate of formation of metal ions equals the rate of formation of metal atoms

Question 4

The reactions of a strip of metal in a solution of its salt is a dynamic equilibrium. How is a potential difference set up at equilibrium?

Answer 4

The negatively charged electrons that remain on the metal strip set up a potential difference between the metal and solution. The greater the tendency of the metal to produce ions, the greater is the potential difference at equilibrium

Question 5

In terms of equilibrium what does it mean if some metals give up electrons and dissolve in a solution of their ions more readily?

Answer 5

That the equilibrium lies to the right in the half equation

Question 6

What are the half reactions for metal/metal ion systems described by?

Answer 6

Half equations

Question 7

By the IUPAC convention what are stabdard half reactions written as?

Answer 7

Reduction reactions

Question 8

How is the half-reaction for the zinc/zinc-ion system written?

Answer 8

Zn2+ (aq) + 2e- Zn(s)

Question 9

Where do half reactions in a metal/metal-ion system produce a potential?

Answer 9

At the electrode

Question 10

How is an electrochemical cell of Zn/Cu set up?

Answer 10

The two electrodes are immersed in solutions of their own salts (CuSO4 and ZnSO4). The two solutions are connected via a salt bridge and the two metal electrodes are joined via an electric circuit

Question 11

When do electrons start to follow in a cell?

Answer 11

When two half-cells are connected to complete a circuit

Question 12

Why does the zinc electrode have a more negative electrode potential than the copper electrode?

Answer 12

Because the zinc metal has a greater build up of negative charge at its surface than copper. Therefore this makes copper the positive electrode

Question 13

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

Answer 13

The difference in electrode potential. It is a measure of the force that moves electrons around the circuit. The bigger the difference in electrode potential and the more cells connected in series, the greater the e.m.f

Question 14

How is the overall equation for the zinc/copper cell found?

Answer 14

By combining the two half equations. Electeons flow around the circuit from the more negative electrode to the less negative electrode. So the overall equation is obtained by subtracting the more negative (zinc) half-equation from the less negative (copper) one.

Cu2+(aq) + 2e- Cu(s)
-(Zn2+ (aq) + 2e- Zn(s))

= Cu2+ (aq) + 2e- -Zn2+ - 2e- Cu(s) -Zn(s)

Which can be rearranged to

Zn(s) + Cu2+(aq) Cu(s) + Zn2+(aq)

Question 15

Why is there a salt bridge between the two solutions in a cell?

Answer 15

Because the two half-reactions need to be kept seperate, yet they still need to be connected by a conductor. The salt bridge provides an ionic connection between the two ionic solutions. The ions are free to move in the bridge so charge is transferred through the bridge solution and this keeps each compartment of the cell electrically neutral.

Question 16

What does a salt bridge typically contain?

Answer 16

A solution of a salt such as potassium chloride or potassium nitrate. A salt bridge solution can be set in agar jelly and held in a glass tube with a porous plug at each end. In commercial cells, the bridge jelly is held in an absorbent material

Question 17

How do you assemble an electrochemical cell consisting of two different metals, A and B?

Answer 17

1) clean a piece of A, e.g. copper, and a piece of B, e.g. zinc usint emergy paper or fine grade sandpaper and then degrease the pieces using some cotton wool and propanone
2) connect A to an electrical lead and place in a 100cm3 beaker with about 50cm3 of a 1 mol dm -3 solution of salt A. Make sure the connection is not in or touching the solution
3) connect B to another electrical lead and place it into a 100cm3 beaker with about 50cm3 of a 1 mol dm -3 solution of salt of B. Make sure the connection is not in or touching the solution.
4) join the two beakers with a salt bridge. This can be made by lightly pushing a cotton wool plug into one end of a plastic tube, filling the tube with a suitable conducting solution, and finall6 plunging the other end of the tube with cotton wool
5) connect the two electrical leade to a voltmeter and read off the voltage

Question 18

Why can cell potentials only be compared if they are measured under standard conditions and against a standard half-reaction?

Answer 18

Because the two half reactions that make up a cell are in equilibrium

Question 19

What are some factors that affect the cell potential?

Answer 19

• the concentration of ions in each half reaction
• temperature
• pressure if gasses form part of the cell
• cell current

Question 20

To produce standardised values how are cell potentials measured and what are these values called?

Answer 20

Under standard conditions using a high-resistance voltmeter (100kPa, 298K, solution of metal ion at 1.00 mol dm-3 concentration). The values are measured in volts and are called standard cell (electrode) potentials symbol E°

Question 21

What are all potentials measured relative to?

Answer 21

The standard hydrogen electrode (SHE), operating under standard conditions. This is assigned an electrode potential E° = 0

Question 22

What does the SHE cell consist of and what is the half reaction for this cell?

Answer 22

A platinum electrode with hydrogen gas bubling over its surface. The electrode is dipped in a solution containing hydrogen ions at a concentration of 1 mol dm-3. The purposes of the platinum electrode are to provide a nonreacting (inert) metal electrical contact and to act as a sink or sourve for electrodes, thereby allowing hydrogen molecules to reach equilibrium with hydrogen ions

2H+ (aq) + 2e- H2 (g)

Question 23

How are electrodes written and give an example for zinc and copper

Answer 23

In a coventional form where a single vertical bar | represents a boundary between two different phases. Two electrodes in a cell can be represented by putting the notations for the two electrodes together and joining them with a salt bridge denotated by a double vertical var ||. The oxidation reaction (more reactive metal) is on the left and the reduction on the right. If a gas is present then the platinum is included in the notation.

Zn(s) | Zn2+ (aq) || Cu2+ (aq) | Cu(s)

Question 24

What is the definition for the standard electrode potential E°?

Answer 24

It is the difference in potential between a given electrode under standard conditions and the standard hydrogen electrode

Question 25

What does the series of standard electrode potentials form part of?

Answer 25

The electrochemical series

Question 26

What are the summary points of electrode potentials?

Answer 26

• the strongest reducing agents lose electrons easily and have more negative potentials
• the strongest oxidising agents accept electrons easily and have more positive potentials
• half reactions with more negative potentials correspond to electron loss (oxidation) and go readily from right to left
• half reactions with more positive potentials correspond to electron gain (reduction) reactions and go from left to right

Question 27

What are standard electrode potentials used to calculate?

Answer 27

Standard cell potentials (e.m.f) across electrochemical cells

Question 28

How can we determine standard cell potentials?

Answer 28

Using the overall equation for the cell reaction written as a spontaneous change

Question 29

How can you determine the standard cell potential with zinc and copper?

Answer 29

Zinc is more reactive than copper and its electrode potential is more negative so the cell is written:
Zn (s) | Zn2+ (aq) || Cu2+ | Cu (s)

The cell potential is calculated by subtracting the left-hand electrode potential from the right hand one so that

E°cell = E°right - E°left

For the cell reaction to be spontaneous E°cell must be positive. For the zinc copper cell E° eill be positive if the standard electrode potential of the Zn2+ | Zn electrode is subtracted from the standard electrode potential of the Cu2+ | Cu electrode:

E°cell = +0.34 V - (-0.76V) 
E°cell = +1.10 V

Question 30

How can the direction of redox reaction be predicted?

Answer 30

By using standard electrode potentials

Question 31

What happens when two half equations are put together?

Answer 31

• the more positive system will gain electrons (be reduced)

* the more negative system will lose electrons (be oxidised)

Question 32

Whatbare the steps for writing a redox equation?

Answer 32

1) write the more negative half-reaction as an oxidation
2) write the more positive half reaction as an oxidation
3) balance the number of electrons being transferred in each equation if necessary
4) combine the two half-reactions

Question 33

What can be used to predict the direction of chemical reactions?

Answer 33

Tbe electrochemical series and the standard electrode potentials of the corresponding half reactions

Question 34

Whatbare the rules for determining the direction of reactions and cell potential?

Answer 34

1) draw horizontal lined against the two half equations you are interested in
2) mark with a minus sign the one that is more negative
3) mark with a plus sign the one that is more positive
4) mark the direction of electron flow (from the minus to plus sign
5) the electron flow will produce a reduction reaction in the half reaction marked with the plus sign, and an oxidation reaction in the half-reaction marked with the minus sign

Question 35

What is a lithium ion cell?

Answer 35

A type of rechargeable cell where lithium ions move from the negative electrode (carbon, graphite X) to the positive electrode (CoO2), during discharge and from the positive electrode to the negative one during charging.

Question 36

What is special about the electrodes which lithium ion batteries use?

Answer 36

The Li+ ions are allowed to flow and to enter and exit the solid structure of the electrodes by a process called intercalation

Question 37

What are the half reactions of the lithium ion cell?

Answer 37

Positive electrode: Li+ + CoO2 + e- LiCoO2
E°= 0.36 V

Negative electrode: Li+ + (graphite C) + e- Li(in graphite C) E° = -3.04 v

Question 38

How does a fuel cell produce electricity?

Answer 38

By using a fuel (on the positive electrode side) and an oxidant (on the negative electrode side), which react in the presence of an electrolyte.

Question 39

Why can fuel cells operate virtually continuously?

Answer 39

Because the reactants flow in and the products flow out, while the electrolyte remains in the cell, so as long as the necessary flow is maintained the fuel cell can operate virtually continuously

Question 40

What are the typical fuels used in a fuel cell?

Answer 40

Methanil, CH3OH and hydrogen H2

Question 41

Why does a hydrogen fuel cell have no carbon emissions?

Answer 41

Because the product of the redox reaction between H2 and O2 is water

Question 42

Why does a methanol fuel cell produce carbon emissions?

Answer 42

The products of the redox reaction between methanol and oxygen are water and carbon dioxide

Question 43

What are the advantages and disadvantages of using hydrogen over methanol in a fuel cell?

Answer 43

Hydrogen is harder to store than methanol

Hydrogen doesn’t produce carbon emissions whereas methanol does

Question 44

What is the catalyst usually used in a fuel cell?

Answer 44

A platinum group metal or alloy

Question 45

Why doess rhe efficiency of hydrogen ion fuel cells need to be as high as posible?

Answer 45

To match the output from silver | zinc or Li | SO2 batteries. Their e.m.f is only 0.40V

Question 46

What is one way of increasing the current from each cell?

Answer 46

Increasing the cell surface area

Question 47

What is one of the major advantages of fuel cells?

Answer 47

The lightness of the components

Question 48

What are the electrochemical reactions in a fuel cell?

Answer 48

• on the positive side of the fuel cell, hydrogen diffuses to the catalyst, where is later dissociates into hydrogen ions and electrons. These hydrogen ions often react with oxidants. The hydrogen ions are conducted through the membrane to the negative electrode but the electrons travel in an external circuit
• on the negative electrode catalyst, oxygen molecules react with electrons (which have travelled through the external circuit) and hydrogen ions to form water. Any excess gases are recycled so water is the only waste product

Question 49

Why does the reaction in a fuel cell use up twice as much oxygen as hydrogen?

Answer 49

To maintain the balance of electrons in the reduction and oxidation process

Question 50

What are the two half reactions at the positive and negative electrodes of a fuel cell?

Answer 50

H2 -> 2H+ (aq) + 2e- E° = 0.00 V

O2 (aq) + 2H2O + 4e- -> 4OH- (aq), E°=+0.40V

Question 51

In a combined system e.g. when solar cells provide electricity what can the reactuond in a fuel cell be used to do?

Answer 51

Electrolyse water for reuse as a fuel

2H2O -> 2H2 + O2
H2O H+ + OH-

Question 52

What can we write as the equations overall in a fuel cell?

Answer 52

2H+ + 2e- -> H2 2OH- -> 2OH• + 2e-
2h+ + 2e- -> H2 2OH• -> H2O + O•
O• + O• -> O2