Electrochemistry

Question 1

What is an electrode and how does it work?

4

Answer 1

• Metal dipped in solution of its metal ions
• equilibrium is set up
• metal has a tendency to form positive ions and go into solution
• metal ions have tendency to gain electrons and form metal

Question 2

What does it mean if the electrode potential equilibrium is left?
Is the electrode potential positive or negative?
Redox?
4

Answer 2

• Metal has a negative charge
• build up of electrons on metal
• so negative electrode potential
• anode is oxidised

Question 3

What does it mean if the electrode potential equilibrium is right?
Is the electrode potential positive or negative?
Redox?
4

Answer 3

• metal has a positive charge
• electrons used up to form metal
• so has positive electrode potential
• cathode is reduced

Question 4

What are the standard conditions for half cells?
Conc
Temp
Pressure

Answer 4

1.0 moldm-3 of ions involved in the half equation

298K

100kPa (only affects half-cells with gases

Question 5

What is an electode?

two words

Answer 5

Half cell

Question 6

What is a metal electrode?

1

Answer 6

-Metal surrounded by a solution of its ions

Question 7

What is a gas electrode?

2

Answer 7

• gas with a solution of its ions

- inert metal (which is the actual electrode) for flow of electrons

Question 8

What is a redox electrode?

2

Answer 8

• two different ions of the same element

- two types present with inert metal electrode

Question 9

What is the purpose of the salt bridge?

2

Answer 9

• allow ions in ionic solution to flow through

- maintains charge balance/completes circuit

Question 10

How does salt bridge maintain equilibrium of positive ions?

1

Answer 10

there is a build up of positive charge at the anode due to the loss of electrons so negative salt bridge ions are attracted to it which maintains flow of ions

Question 11

How does salt bridge maintain equilibrium of negative ions?

1

Answer 11

ions are being removed at the cathode as they are attracted to electrons so form solid metal, positive salt bridge ions are attracted so maintains flow of ions

Question 12

Which electrode is traditionally more positive?

Left or right?

Answer 12

Right, cathode

Question 13

How do you measure the electrode potential of a half cell?

3

Answer 13

• Connect with a half-cell of known potential
• measure potential difference using a high resistance voltmeter
• this is so potential can be measured under zero-current conditions as no current can be drawn from the cell

Question 14

What is the SHE?
use?
What is looks like?
3

Answer 14

Standard hydrogen electrode

used as primary standard so all potentials are compared to it so it has 0 volts

Pt(s) | H2(g) | H+(aq)

Question 15

Why are standard conditions needed?

1

Answer 15

Position of the redox equilibrium will change with conditions
(remember equilibria)

Question 16

What is the set up of an electrochemical cell?

3/4

Answer 16

• Two half cells
• the two metals joined with a wire (e- flow)
• two solutions joined with salt bridge (flow of ions)
• voltmeter to measure potential difference (emf)

Question 17

Which electrode is oxidised and which is reduced?

anode vs cathode

Answer 17

Anode = oxidised
Cathode = reduced

Question 18

What is emf?
2
including equation

Answer 18

Electromotive force

E* Cell = E* right - E* left

(E* = electrode potential under standard conditions)

Question 19

What is the conventional representation of cells?

1

Answer 19

R | O || O | R

Question 20

What are the | lines and why are they important?

3

Answer 20

• the are phase boundaries
• use them whenever there is a change in state
• if none then use comma

Question 21

What does || represent?

Answer 21

Salt bridge

Question 22

What does ROOR stand for?

2

Answer 22

Reduced, oxidised, oxidised, reduced

-species with highest oxidation state near salt bridge

Question 23

Which electrode is the standard electrode?

right or left?

Answer 23

Left

Question 24

Why are standards other than the SHE used sometimes?
What is a secondary standard?
3

Answer 24

• SHE is difficult to use as it involves and gas and is flammable so other standards are used
• they are calibrated against the SHE and are known as
• secondary standards

Question 25

What happens at the anode? Redox 3

Answer 25

- The metal loses electrons so is oxidised to its metal ions - these electrons make the electrode negative - these electrons travel through the wire to the other electrode, adding to ions to produce metal ions

Question 26

What happens at the cathode? | redox

Answer 26

- The metal ions gain electrons and so are reduced to metal atoms - as electrons are used up this makes the electrode positive

Question 27

What is the electrochemical series? | 2

Answer 27

List of half cell potentials in order of decreasing/increasing potential compared against SHE

Question 28

What does it mean if an electrode is negative in the electrochemical series?

Answer 28

It is worse than Hydrogen at gaining electrons / less reductive / more oxidative

Question 29

What does it mean if an electrode is positive in the electrochemical series?

Answer 29

It is better than Hydrogen at gaining electrons / more reductive / less oxidative

Question 30

What are the half equations in the electrochemical series written as? Why 2

Answer 30

Reduction half equations measure of how easily it reduces

Question 31

What are three types of commercial cells?

Answer 31

- Non-rechargeable - rechargeable - fuel cells

Question 32

What happens in non-rechargeable cells? | 2

Answer 32

- The chemicals are used up over time and the emf drops | - once one or more of the chemicals have been used up the cell is flat and emf is 0 volts

Question 33

What happens in rechargeable cells? | 2

Answer 33

- the reactions are reversible | - they are reversed by applying an external current and regenerating the chemicals

Question 34

What happens in fuel cells? | 2

Answer 34

- have a continuous supply of the chemicals so don't run out of chemicals and don't need recharging - do need constant supply of chemicals

Question 35

What is an example of a rechargeable cell? what is it used for?

Answer 35

Lithium ion | phones, tablets, cameras, laptops etc

Question 36

What is the equation at the positive electrode of a lithium ion cell? This is also what happens during discharge

Answer 36

Li+ + CoO2 + e– → Li+[CoO2]–

Question 37

What is the equation at the negative electrode of a lithium ion cell? This is also what happens during re-charge

Answer 37

Li → Li+ + e–

Question 38

What is an example of a fuel cell?

Answer 38

Alkaline Hydrogen Fuel Cell

Question 39

What happens at the anode and cathode of an alkaline hydrogen fuel cell?

Answer 39

``` Anode = H2 in cathode = O2 in and H2O out ```

Question 40

How/where do the ions travel in an alkaline hydrogen fuel cell? 2

Answer 40

H+ ions move between electrodes through the electrolyte electrons move between electrodes through a wire

Question 41

What reaction happens at the anode of an alkaline hydrogen fuel cell? equation

Answer 41

H2 + 2OH- --> @H2O + 2e-

Question 42

What reaction happens at the cathode of an alkaline hydrogen fuel cell? equation

Answer 42

O2 + 2H2O + 4e- ---> 4OH-

Question 43

What is the overall equation for an alkaline hydrogen fuel cell?

Answer 43

2H2 + O2 --> 2H2O

Question 44

What the benefits of using non-rechargeable cells? | 1

Answer 44

Cheap

Question 45

What the risks of using non-rechargeable cells? 1 plus explanation

Answer 45

Waste issues | -they leak and corrode and chemicals sink into the soil contaminating ground water

Question 46

What the benefits of using rechargeable cells? | 3

Answer 46

less waste cheaper in the long run lower environmental impact

Question 47

What the risks of using rechargeable cells? 1 plus explanation

Answer 47

some waste issues at end of useful life | -they leak and corrode and chemicals sink into the soil contaminating ground water

Question 48

What the benefits of using hydrogen fuel cells? | 3

Answer 48

only waste product is water do not need recharging very efficient- less energy wasted and it converts more of the available energy from hydrogen into kinetic energy than other cells

Question 49

What the risks of using hydrogen fuel cells? | 4

Answer 49

need constant supply of fuels hydrogen is flammable and explosive hydrogen usually made using fossil fuels high cost of fuel cells