1.11 Electrochemical Cells Flashcards
What is an electrochemical cell?
A metal is placed into a solution containing its ions
An equilibrium is established
One of these is called a half cell
What are two half cells called?
A Daniell cell
Two half cells combined by a salt bridge
With a voltmeter in circuit
An electric current flows so the electrons pass from the more reactive metal to the less reactive metal = equilibrium’s can be different
What is a salt bridge?
A piece of filter soaked in a solution of potassium nitrate KNO3
Containing electrolyte which doesn’t react
It completes the circuit so ions can move in and out of it
It is unreactive with half cells
What is significant about the voltmeter?
A high resistance voltmeter will not draw current from the circuit
What is the standard hydrogen electrode?
It is used to compare how easily metals lose electrons
It is the standard that we measure the emf values against
In the standard hydrogen electrode what is the general set up?
H2 gas is bubbled in
H+ are in aqueous solution provided by and acid eg H2SO4 or HCl
Platinum electrode
It goes on the LEFT ALWAYS!
What does the standard hydrogen electrode include?
A platinum electrode
Which is platinum foil covered in porous platinum to increase surface area
It is unreactive and a good conductor of electricity
What are the conditions for the standard hydrogen electrode?
100kPa
298K
1.0 moldm-3
How do we determine if something is a strong/weak reducing/oxidising agent?
More negative E° = stronger reducing agent
More positive E° = stronger oxidising agent
What is the standard electrode potential? E°
The potential difference between any electrode under standard conditions and the standard hydrogen electrode
How do you workout the overall E° cell?
E° right hand side - E° left hand side
Or
E° comparatively more positive - E° comparatively more negative
What does an overall more positive E° value mean?
The reaction will happen spontaneously
In a Daniell Cell set up whig side does each half cell go on?
The more negative E° value goes on the left hand side
What is included in cell representation?
|| = salt bridge (goes between the two half cells)
, = separates different substances in the same phase
= phase boundary (only between different states of matter)
How do you know where the half cells go in cell representation?
The form of the substance with the highest oxidation state is put next to the salt bridge
Eg 0, 2+, 2+, 0
What can we deduce from cell representation?
Zn(s)| Zn2+(aq) || Cu2+(aq) | Cu(s)
Reactant, product, reactant, product
And so we can easily form half equations in the order we read them
Zn -> Zn2+ + 2e-
In cell representation what happens if there is no solid surface?
No solid surface to conduct electricity
A platinum electrode is used like in the SHE (standard hydrogen electrode)
Eg
Pt(s) | Fe2+(aq), Fe3+(aq) || Cl2(g) | Cl-(aq) | Pt(s)
Only put on an end of a half cell with no (s)
What happens to the E° if concentration is changed?
Increase concentration = E° increases
Decrease concentration = E° decreases
How can temperature change an E° cell?
Most cells are exothermic in the spontaneous direction
So
Increase in temperature moves the equilibrium to the left hand side
= decrease E° cell
How can activation energy affect an E° cell?
If the reaction in the cell has a high activation energy
There’s a chance the reaction won’t happen regardless of a positive E° cell value
What are the types of chemical cells?
Primary cells - irreversible and must not be recharged
Secondary cells - reversible and can be recharged by an electrical current
Fuel cells - generated electricity by the continual oxidation of an external source of fuel
Give an example of a primary cell?
Zinc-Carbon dry cell
Positive electrode: Graphite rod(carbon)
Negative electrode: Zinc container
Surrounded by a paste of manganese dioxide and ammonium chloride (MnO2 + NH4Cl)
What is the zinc-carbon dry cell overall equation?
Zn2+ + 2NH4+ ⇌ 2NH3 + H2 + Zn
Why is Zinc-carbon dry cell a primary cell?
You don’t recharge it as the zinc container will wear away
Therefore not safe
What batteries are most common?
Alkaline batteries
They contain a paste of KOH
And use Zinc as a reducing agent
Zn + 2MnO2 + 2H2O ⇌ 2MnO(OH) + 2OH- + Zn+
What do secondary cells do?
They provide current to an external circuit while discharging
They use current from mains electricity while charging
What are some examples of secondary cells?
Lead-acid battery
Nickel-cadmium cell
Lithium ion ***
What is the overall equation for a lead-acid battery?
Discharging:
PbO2 + Pb + 2H2SO4 ⇌ 2PbSO4 + 2H2O
Charging is the other way around
What are nickel-cadmium cells used for? Fact?
Electric shavers
Drills
They can be recharged up to 500 times
What are the discharging and charging equations of the lithium ion? What is used?
Discharging
Li + CoO2 ⇌ LiCoO2
Charging
LiCoO2 ⇌ Li + CoO2
Reagents are absorbed onto powdered graphite
What is a fuel cell?
An electrochemical cell that maintains a constant voltage due to constant concentration of reactants (continuous flow of fuel)
What does an alkaline (hydrogen-oxygen) fuel cell include?
Porous platinum electrodes with a semi-permeable membrane
NaOH or KOH electrolyte
What is significant about the rate of fuel cells?
Under standard conditions the rate is too slow for a decent current
Higher temperatures are used - to increase rate
But it is exothermic so the E° cell falls
Can be counteracted with high pressure
What are some factors of acidic hydrogen oxygen fuel cells?
They are more efficient than combustion of H2 - as it releases electrical rather than heat energy
H2 comes from electrolysis of water - via steam reformation of methane
Hydrogen under high pressure = explosive
What are the benefits and risks of cells, non-rechargeable cells and rechargeable cells?
Cells - portable source of electrical energy
BUT there are waste issues
Non-rechargeable cells - cheap
BUT there are waste issues
Rechargeable cells - less waste & cheaper in the long run
BUT some waste issue (at end of life)
What are some benefits and risks of using hydrogen fuel cells?
Benefits:
Only waste product is water
Don’t need recharging
Very efficient
Risks: Needs a constant supply Hydrogen = flammable & explosive Hydrogen is often made using fossil fuels High cost of fuel cells
