3.1.11: Electrode potentials Flashcards
(30 cards)
1
Q
What are electrode potentials?
A
- made from 2 different metals dipped in salt solutions of their own ions connected by a wire (external circuit)
- redox process (2 reactions occur)
- electrons flow through wire from more reactive to less reactive metal
- half cell/ electrode- metal dipping into solution of its ions
2
Q
What is a metal electrode?
A
a metal surrounded by a solution of its ions
3
Q
What is a gas electrode?
A
- for gas and a solution of its ions
- inert metal (e.g. Pt) used as actual electrode to measure flow of electrodes
4
Q
What is a redox electrode?
A
- 2 different ions of the same element
- where the 2 types of ions present in solution 2/ an inert metal electrode to allow flow of electrodes
5
Q
How do you measure potential of an electrode?
SHE
A
- comparison to SHE (standard hydrogen electrode)
- connect half-cell to another half-cell of known potential and measure p.d. beteen 2 half cells
- 2 half cells produces electrochemical cell
- SHE- potential of 0V (primary standard which all other potentials compared too)
6
Q
How do you set up an e-cell?
A
- two half cells joined, giving a complete circuit
- two metals joined by wire- electrons flow through
- two solutions joined by salt bridge- where ions flow through
- voltmeter inclused to measure EMF
- salt bridge- paper soacked in uncreactive ions (e.g. KNO3) or tube w/ions in agar gel
7
Q
What are standard conditions of an electrode?
A
- concentration- 1.0 moldm-3
- temperature- 298K
- pressure- 100 kPa
8
Q
What is the SHE?
A
- the standard in which all electrodes are measured by
- 2H+(aq) + 2e- ⇌ H2 (g)
- E = +0.00V
- Pt (s) | H2 (g) |H+ (aq) ||
9
Q
How do you write a cell notation?
A
- left side- oxidised, (-) EMF
- right side- reduced, (+) EMF
- R O || O R
- highest oxidation state in centre
- solid electrodes at ends, if no solid, Pt used
- ||- salt bridge, |- phase boundary
10
Q
How do you calculate cell EMF?
A
- E= E right- E left
- E= more positive- more negative
11
Q
Why is there sometimes a second standard?
A
- used as standard electrode
- calibrated against primary standard
- as SHE uses flammable gas
12
Q
How does REDOX reactions occur in an electrode?
A
- on left side, metal atoms lose e-
- travel through wire to other electrode on right side
- adding to ions producing metal atoms (reduction)
OILRIG
13
Q
What is the electrochemical series?
A
a list of electrode potentials in order of decreasing (or increasing) potential
14
Q
What does a very (+) potential mean?
A
- best oxidising agent
- worst reducing agent
15
Q
What does a very (-) potential mean?
A
- worst ocidising reagent
- best reducing agent
16
Q
What are uses of the electrochemical series?
A
- predict direction of redox reactions
- more postive gains electrons (remains forward reaction)
- more negative goes in backwards reaction
17
Q
Why are electrochemical cells used as commercial cells?
A
- can be used as commercial source of electrical energy (portable)
- e.g. non-rechargeable cells, rechargeable cells, fuel cells
18
Q
What is a battery?
A
more than one cell joined together
19
Q
How do non-rechargeable cells work?
A
- chemicals used over time, EMF drops, when one or more chemicals used, cell goes flat (EMF = 0V)
- can’t be recharged, have to be disposed of after single use
- standard, chap, non-rechargeable cell, but has fairly short life
20
Q
What is the structure of an alkaline non-rechargeable cell?
A
- KOH
- Zn
- MnO2
- graphite
21
Q
How do rechargeable cells work?
A
- reversible reaction used to recharge
- done by applying external current, regenerating chemicals
- e.g. lithium ion, lead acid, nickel cadmium
22
Q
What is the use of a lithium ion rechargeable cell?
A
- used in phones, tablets, cameras, laptops etc.
- (+) electrode- Li+ + CoO2 +e- ⇌ LiCoO2 | E= +0.60V
- (-) electrode- Li+ + e- ⇌ Li | E= -3.00V
23
Q
What is the use of a lead acid rechargeable cell?
A
made of 6 cells, used in cars
24
Q
How do fuel cells work?
A
- used to generate electric current, doesn’t need recharge
- have continuous supply of chemicals into cell
- efficient as only gives water as waste
25
How do hydrogen- oxygen fuel cells work?
- H2 and O2 in on opposite sides
- OH- ions move through/ between electrodes through **electrolyte**
- H2O out
- e- moves between electrodes through wire
26
What is the difference between acidic and alkaline H fuel cells?
**alkaline:**
- (-) electrode- 2H2 + 4OH- -> 4H2O + 4e- | **E= -0.83 V**
- (+) electrode- O2 + 2H2O + 4e- -> 4OH- | **E= +0.40V**
- overall equation- 2H2 + O2 -> 2H2O
- **EMF- +1.23V**
**acidic:**
- (-) electrode- 2H2 -> 4H+ + 4e- | **E= 0.00V**
- (+) electrode- O2 + 4H+ + 4e- -> 2H2O | **E= 1.23V**
- overall equation- 2H2 + O2 -> 2H2O
- EMF- +1.23V
27
What are benefits and risks of cells?
**benefits:**
- portable chemical energy source
**risks:**
- waste issues
28
What are benefits and risks of non-rechargeable cells?
**benefits:**
- standard
- cheap
**risks:**
- waste issues
29
What are benefits and risks of rechargeable cells?
**benefits:**
- less waste
- cheaper overall
- lower environmental impact
**risks:**
- some waste issues (at end of useful life)
30
What are benefits and risks of hydrogen fuel cells?
**benefits:**
- only waste product is water
- don't need recharging
- very efficient
**risks:**
- need constant fuel supply
- H flammable
- H expensive
- H usually made of fossil fuels
- high cost of fuel cells
