Electrochemical Cells Flashcards
What is the function of a cell
Convert chemical energy into electrical energy
What is a half cell
A metal electrode dipped into a solution of its own ions
What is an electrode
A solid (metal) conductor of electricity
What is an electrochemical cell
2 half cells connected together by a salt bridge
Why is a high resistance voltmeter used in a cell
- ensures 0 current conditions
- allows the measure of maximum potential difference b/t 2 cells
How is a salt bridge made?
By dipping a piece of filter paper into a saturated solution of potassium nitrate (KNO3) or potassium chloride (KCL)
What are the functions of a salt bridge?
(a) to allow ions to move freely between the half cells
(b) to maintain electrical connection or to complete the circuit
Can a metal be used as the salt bridge? Why?
No
-the salt bridge must not interfere with the reaction (must not react with the solution or electrodes)
-if a metal is used, it may generate an additional metal ion potential which will interfere with the reaction
What is used as the electrode if a metal is not part of the reaction? Why is that used?
Platinum
-it is an inherent/ unreactive metal and will not interfere with the reaction
In a cell diagram, where does each half equation go?
OXIDATION ; left
REDUCTION ; right
What is the Standard Hydrogen Electrode (SHE)
And where is it found
The primary standard that is used to determine the electrode potential of half- cells
It is always the left hand electrode
What is the electrode potential of the SHE
By convention/ definition
It is 0.00V
What are the conditions of the SHE
Pressure: 100KPa/ 1 bar
Concentration of solutions: 1.0 mol dm^-3
Temperature: 298K
Current: 0 amps
In the SHE what is added onto the electrode
In the SHE a platinum electrode is used, and platinum black is added to the end of the electrode to act as a catalyst
It increases the rate at which equilibrium is attained between H2 and H+
What is the main disadvantage of the SHE
How is it overcome
It is awkward/ cumbersome to set up
A secondary standard is used to overcome this
What is a secondary standard
An electrode that has been calibrated against the SHE
What is the E0
The maximum potential difference measured relative to the SHE, under 0 current conditions
What is an electrochemical series
A list of E0 values arranged in ascending/ descending order
What are the uses of an electrochemical series
(a) used to predict which reaction will occur when 2 half cells are connected
(b) It can be used to calculate the EMF of p.d. of a cell
What is the EMF
The Electrodemotive force
Is the maximum potential difference between 2 half cells
What is the equation used to calculate the emf
E0cell
EMF) = E0(R) - E0(L
What must the emf be for a spontaneous reaction
Emf/ E0 > or = 0
Must be POSITIVE
Even if the emf is positive, a reaction may not always occur. Why?
- the activation energy is too high
- Rate is too slow
- emf only applies to reactions under standard conditions
How do you find the strongest oxidising agent in an electrochemical series
- it is on the left hand side
- AND has the most positive emf
How do you find the weakest oxidising agent in an electrochemical series
- it is on the left hand side
- AND has the least positive emf
How do you find the strongest reducing agent in an electrochemical series
- it is on the right hand side
- AND has the least positive emf
How do you find the weakest reducing agent in an electrochemical series
- it is on the right hand side
- AND has the most positive emf
What factors affect emf
- Temperature
- Pressure
- [ conc ] of solutions
- Current
What is the effect of increasing temperature on emf
- Most spontaneous reactions are exothermic
- if temp is increased
- equilibrium will shift to the left
- emf will decrease
What is the effect of increasing the concentration of one of the solutions
If the concentration of a solution is increased
The equilibrium will shift away from the solution
Depending on which electrode (left or right) is made more positive
The emf will increase or decrease
What effect would current have on the emf
If the cell is allowed to draw current
Emf will decrease and become 0
When the reaction reaches equilibrium
What effect would a catalyst have on the emf
NO EFFECT ON THE EMF VALUE
but, can increase the rate at which equilibrium is reached
What can the electrode potential tell you about the direction of a redox reaction?
The more negative the electrode potential the more it oxidises;
Reaction occurs BACKWARDS
The more positive the electrode potential the more it reduces;
Reaction occurs FORWARDS
Are primary cells rechargeable?
Why?
NOT rechargeable;
The reactions are not reversible
When a cell is discharging what does that tell you about the emf
Emf is greater than or equal to 0
Occurs spontaneously
State one essential property of the non reactive porous separator
Allows ions to flow/ move freely
State the function of the carbon rod
Conduct electricity
State one environmental advantage of rechargeable cells compared to non rechargeable ones
- metal compounds can be reused
- less mining
- less use of landfill
- less use of energy
Are secondary cells rechargeable?
Why?
Yes;
The reactions are reversible
When a cell is recharging what does that tell you about the emf
Emf < 0
Needs electrical current to occur
What does the graph of a secondary cell look like & explain the shape
Voltage/ emf remains constant for some time before dropping suddenly:
Voltage drops at this point because the reactants get used up
State the half equations & overall equation of a hydrogen- fuel cell in ACIDIC conditions
OXIDATION:
H2—> 2H+ + 2e-
REDUCTION:
O2 +4H+ + 4e- —> 2H2O
Overall: 2H2 + O2 —> 2H2O
State the half equations & the overall equation of a hydrogen- fuel cell under ALKALINE conditions
OXIDATION:
H2 + 2OH- —> 2H2O + 2e-
REDUCTION:
O2 + 2H2O + 4e- —> 4OH-
Overall: 2H2 + O2 —> 2H2O
What is the emf of the cell in acidic conditions? What is the emf of the cell in alkaline conditions? Why?
They’re the same;
The overall equations are the same
Explain the shape of the graph of a fuel cell
It’s is a straight horizontal line;
Voltage is constant because fuel is consistently being supplied to the cell; the concentration remains constant
Name some advantages of fuel cells
- only waste is water
- very efficient (converts more available energy into kinetic energy)
Name some disadvantages of fuel cells
- limited life time
- expensive
- hydrogen is highly flammable
- fossil fuels burnt to obtain hydrogen
Are hydrogen fuel cells carbon neutral? Why?
Not entirely;
Depends on how the hydrogen obtained;
H2 obtained from methane is not carbon neutral (methane is a carbon source)
What are the half cell equations & overall equation of ethanol fuel cells
OXYGEN ELECTRODE:
4e- + 4H+ + O2 —> 2H2O
ETHANOL ELECTRODE:
C2H5OH + 3H2O —> 2CO2 + 12H+ + 12e-
Overall: C2H5OH + 3O2 —> 2CO2 + 3H2O
Name some advantages of the ethanol fuel cells
- made from renewable sources in a carbon neutral way
- abundant raw materials (to produce ethanol)
- less explosive/ easier to store than H2
