Physical 11: Electrochemistry Flashcards
What is a half cell?
Also known as an electrode
An equilibrium set up when a metal is dipped in a solution of its own ions
How do you make an electrical cell?
By joining two half cells together
What does the potential difference of an electrochemical cell tell you?
The direction of electron flow in the cell
If the potential difference is positive, it means that the reaction is spontaneous in the direction written in the notation
If the potential difference is negative, the reaction is not spontaneous in the direction written (under standard conditions)
How do you measure potential difference of an electrochemical cell?
Using a voltmeter to connect the two electrodes
What is a salt bridge?
Usually a piece of filter paper soaked in a salt solution
Solution is usually saturated potassium nitrate, potassium chloride, sodium chloride, sodium nitrate, or ammonium nitrate
Why is a salt bridge used instead of a wire?
Because a wire would only transfer electrons, but the salt bridge transfers ions
Describe the standard hydrogen electrode.
Beaker of 1moldm-3 HCl (aq) to supply H+ ions
Hydrogen gas (100kPa) bubbled through the aqueous solution
Platinum wire coated with finely divided platinum to increase surface area for reaction allows electrical contact to be made
What is platinum black?
The platinum wire coated in finely divided platinum used to make electrical contact in a standard hydrogen electrode
What is the standard hydrogen electrode used for?
The potential of the standard hydrogen electrode is zero, so it can be used to compare the emf of other electrodes
What is the symbol for emf in standard conditions?
E°
What is emf?
Electromotive force
The potential difference between a half cell and the standard hydrogen electrode
What are the standard conditions of the hydrogen electrode?
100kPa hydrogen gas
1moldm-3 HCl solution
298K
What is the electrochemical series?
A list of reduction reactions (electrode potentials)
Arranged so that the most negative electrode potentials are at the top and the most positive are at the bottom
Where are the best oxidising agents in the electrochemical series?
Top left
Because they have very positive potentials and so are the best at attracting electrons
Where are the best reducing agents in the electrochemical series?
Bottom right
Because they have very negative potentials and so are the worst at attracting electrons (repel them)
What are the three general rules for IUPAC cell representation?
1) A vertical solid line represents a phase boundary (i.e. between a solid and solution)
2) A double vertical line represents the salt bridge
3) The species with the highest oxidation state is written next to the salt bridge (R O || O R)
Where are the anode and cathode written in conventional cell representation?
Anode is on the left
Cathode is on the right
What is used to make electrical contact if there are no solid species in the half cell?
An inert platinum electrode
Give the cell representation for an aluminium (Al(s)/Al3+(aq)) and copper (Cu(s)/Cu2+(aq)) cell, given that the aluminium cell is the anode.
How do electrons move between two connected electrodes?
From the more negative to the more positive electrode
How do you predict the direction of the half cell reactions?
The more negative of the two will go in the reverse direction
How do you calculate emf?
Cathode potential - Anode potential
When is a cell’s reaction feasible?
If the forward reaction occurs spontaneously under standard conditions
When the electrons move from the anode to the cathode
The emf should be positive
Name three non-rechargeable cells.
Leclanché cells (Zinc/carbon)
Daniell cells (Zinc/copper)
Zinc chloride
Describe the Daniell cell.
Non-rechargeable zinc/copper cell
Provides an emf of 1.10V
Developed to provide electricity for old-fashioned telegraphs
Uses electrolytes of copper and zinc sulfate
Why was the Daniell cell not practical for portable devices?
Because it contained copper sulfate and zinc sulfate as its electrolytes, both of which are corrosive so could’ve been harmful if they leaked
Describe the Leclanché cell.
Non-rechargeable zinc/carbon cell
The carbon electrode acts like the inert platinum electrode
Basis of most ordinary disposable batteries
Zinc canister
Electrolyte is a paste of ammonium chloride and water (electrolyte)
Carbon rod is surrounded by magnesium(IV) oxide and powdered carbon
Describe the carbon electrode of the Leclanché cell.
Hydrogen gas is oxidised to water by magnesium(IV) oxide while the ammonia dissolves the water of the paste
Why do Leclanché cells need to be replaced?
As the cell discharges, the zinc is used up, and the walls of the zinc canister become thin and prone to leakage
Ammonium chloride is acidic so can be corrosive if it leaks
Describe the zinc chloride cell.
Similar to the Leclanché cell but zinc chloride is used as the electrolyte instead of water
Why are zinc chloride cells better than Leclanché (zinc/carbon) cells?
Better at supplying high currents
Powdered zinc is used, whose surface area is greater so allows the battery to supply higher currents at a time
Cell is enclosed in a steel container which prevents leakages but makes them more expensive
What are lead-acid batteries used for?
Operating the starter motors of cars
What is the main advantage of lead-acid batteries?
They are rechargeable
When you reverse the direction of the cell by applying an external voltage greater than the voltage of the cell to drive the electrons in the opposite direction
Describe the structure of a lead-acid battery.
Six 2V cells connected in series to give 12V overall
Each cell consists of two plates dipped into a sulfuric acid solution:
The positive plate is made of lead coated in lead(IV) dioxide
The negative plate is made of lead
What is the overall reaction as a lead-acid battery cell discharges?
What is the advantage of nickel/cadmium batteries?
Rechargeable
They are used to replace traditional zinc-carbon batteries
More expensive but can be recharged about 500 times, reducing the effective cost
What is the overall equation for the reaction in a nickel/cadmium cell?
What are lithium ion cells used for?
Laptops, tablets, smartphones, and other mobile gadgets
Why are lithium ion cells light?
Because lithium is the least dense metal
What is the electrolyte in a lithium ion cell?
A solid polymer rather than a liquid or paste
This is beneficial because it will not leak, and the charge can be topped up at any time without the memory effect of some rechargeable batteries
The cell can even be bent or folded without leaking
What is the memory effect that some rechargeable cells have?
They can only be efficiently recharged if they have been completely discharged
What are the electrodes in a lithium ion cell?
Positive electrode: Lithium cobalt oxide (LiCoO2)
Negative electrode: Carbon
Arranged in layers with a sandwich of solid electrolyte in between
Describe how the electrons move when a lithium ion cell is being charged.
The electrons are forced through the external circuit from positive to negative electrode
At the same time, the lithium ions move through the electrolyte towards the positive electrode to maintain the charge balance
Reversed on discharging
Describe an alkaline hydrogen-oxygen fuel cell.
Two electrodes of porous, platinum-based material
Separated by a semi-permeable membrane
Electrolyte is sodium hydroxide
Hydrogen enters at the negative electrode
What is the overall reaction of an alkaline hydrogen-oxygen fuel cell?
2H2(g) + O2(g) –> 2H2O (l)
E = +1.23V
Describe how the reaction of an alkaline hydrogen-oxygen fuel cell occurs.
1) Hydrogen enters at the negative electrode:
2H2(g) + 4OH-(aq) –> 4H2O(l) + 4e-
2) This releases electrons which flow through the circuit to the positive electrode, where oxygen enters:
O2(g) + 2H2O(l) +4e- –> 4OH-(aq)
3) This accepts electrons from the negative electrode and releases hydroxide ions which travel through the semi-permeable membrane to the electrode
What is the advantage of using an alkaline hydrogen-oxygen fuel cell over simply burning hydrogen and oxygen?
Takes place at a lower temperature
No nitrogen oxides are produced
What are alkaline hydrogen-oxygen fuel cells used for?
Generating electricity on spacecraft because the only byproduct is pure water, which can be drank by astronauts
Important use in generating electrical energy as it produces no carbon dioxides
What are the disadvantages of alkaline hydrogen-oxygen fuel cells?
Most hydrogen is made from crude oil, which is non-renewable
Could be made by electrolysis of water, but most electricity is made by burning fossil fuels which emits CO2
Hydrogen-powered vehicles will need an infrastructure of hydrogen-filling stations - raises issues of transporting and storing a highly flammable gas
E.g. Hindenburg disaster
How can you store hydrogen safely?
Absorb it into solid compounds (metal hydrides)
It is absorbed under pressure and released by gentle heating
