Electrode potentials snd electrochemical cells 3.1.11 Flashcards
what is a half cell
One half of an electrochemical cell . they can be constructed of a metal dipped in its ions , or a platinum electrode with 2 aqueous ions
what is an inert but electrically conductive electrode commonly used
platinum
how is an electrochemical cell created
by joining 2 different half cells together
what are electro cells made up of
2 half cells joined by a wire voltmeter and a salt bridge
what does a voltmeter do
•Measures voltage between two half cells . Known as the emf
• electrons flow from a more reactive metal to a less reactive one
what is a salt bridge made out of
filter paper with potassium nitrate solution
what does a salt bridge do
The ions in the ionic substance in the salt bridge move through the salt bridge .
To complete the circuit
what does the electrode potential value measure
tells us how easily the half cell gives up electrons ( oxidised )
how can you tell which half cell is being reduced or oxidised
Look at the their electrode potential value from the data book
what if the electrode potential value is negative
it undergoes oxidation
what if the electrode potential value is positive
undergoes reduction
what is the standard hydrogen electrode
Is used to as a reference to measure standard electrode potentials
what’s the electrode potential value for the standard hydrogen electrode
0.00V
what are the general conditions and set ip for the standard hydrogen electrode
- H2(g) in at 298K and 100KPa
- 1 moldm-^3 of Cu2+ ions ( solution)
- 1 moldm-3 of H+ ions (solution)
- Platinum electrode
what is the electrochemical series
A list of electrode potentials in numerical order
What order can the electrochemical series be in
either ascending or descending order
what are oxidising agents
increasing tendency to gain electrons
which half cell reactions would be stronger oxidising agents
The ones with a more positive electrode potential
if the electrochemical series was in descending order which parts of the half cell reactions would be an oxidising agent or a reducing agent
-agents on the left hand side of the equation are more easily reduce so they are more powerful oxidising agents
- agents in the right hand side of the equation are more easily oxidised so they are more powerful reducing agents
what is a reducing agent
They have an increasing tendency to lose electrons
what is the equation to work out the cell potential
E cell = E reduced ( most positive) - E oxidised ( most negative )
what are cell notations
Used to simplify how we draw the set up of cell . They are the standard way of representing cells in chemistry
what’s the general way of writing cell notation
Reduced form | oxidised form | | oxidised form | reduced form
( the most negative half cell potential goes to the left of the double line )
what does the solid line in a cell notation represent
Show a physical change in state e.g solid to gas
what do double solid lines show in a cell notation
a salt bridge
what if there are 2 aqueous ions in a half cell how would you represent that in a cell notation
Separate ions with a comma
what can standard electrode potentials be used for
to predict if a stated reaction is likely to proceed under standard conditions
what do we do with an oxidation equation
have to reverse it
what are the steps to predict feasibility in reactions
1) identify which is being oxidised
2) take the oxidised equation and reverse it. Write two equations next to each other
3) combine the 2 equations to obtain a feasible reaction
4) compare this equation to the reaction stated in the question
5) confirm this by calculated the E cell . All feasible reactions will have a positive E cell value
how can you tell if a reaction is feasible
work out E cell.
All feasible reactions will have a positive E cell value
what are batteries
electrochemical cells that come in 2 main forms . rechargeable and non- rechargeable
what are some features of non rechargeable batteries
• tend to be cheaper than rechargeable batteries.
however rechargeable batteries are reversible and can last longer so cheaper in the long term
what are lithium ion batteries
an example of a rechargeable battery.
what are lithium ion batteries commonly used in
wireless power tools
tablets
mobile phones
electric cars
what are the components of a mobile phone that use a lithium ion cell
Electrode A - lithium cobalt oxide (LiCoO2)
Electrode B - graphite (C)
Electrolyte - Lithium salt dissolved in an organic solvent
how do rechargeable batteries work
Plugging them in to supply a current
This current forces electrons to flow in the opposite way
all we do is reverse the overall discharge equation to show a battery recharging it
what is a fuel cell
electricity generated by a continuous external supply of chemicals rather than a “ready store” like in batteries
what’s an example of a fuel cell
an alkaline hydrogen - oxygen fuel cell
what are the stages that happens in an alkaline hydrogen-oxygen fuel cell
1) Hydrogen feed
Hydrogen is fed here. It reacts with OH- ions in solution. The reaction :
2H2(g) + 4OH-(aq) => 4H2O(l) + 4e-
2) Flow of electrons
Electrons produced in reaction 1 travel through a platinum electrode. Platinum is a good conductor of electricity but inert.
3) Component
The flow of electrons is used to power something
4) Oxygen Feed
Oxygen is fed in here. It reacts with water and the 4 electrons made from step 1 to make OH- ions.
The reaction : O2(g) + 2H2O (l) + 4e- => 4OH- (aq)
5) Negative electrode (cathode)
Electrons flow to the negative electrode which is made from platinum
6) Electrolyte
The electrolyte is made from KOH solution . It carries the OH- ions from the cathode to the anode
7) Positive electrode (anode)
Electrons flow from the positive electrode which is made from platinum
8) Water emitted
The product of the reaction in step 1 is released into the surroundings
9) Movement of OH- ions
OH- ions produced from reaction 4 are carried towards the anode via the electrolyte
what are Ion exchange membranes in an alkaline hydrogen-oxygen fuel cell
These lines on the platinum electrodes and these allow OH- pins to pass through but NOT hydrogen and oxygen gas
what are the reactions taken place in an alkaline hydrogen - oxygen fuel cell
Half equations :
2H2 (g) + 4OH- (aq) => 4H2O(l) + 4e-
O2 (g) + 2H2O(l) + 4e- => 4OH-(aq)
Overall equation :
2H2(g) + O2(g) => 2H2O
What are advantages of fuel cells
• more efficient
• more energy is converted into kinetic energy
• fuel cells don’t need to be recharged ~ you just need a ready supply of oxygen and hydrogen
• the only waste product is water and no co2 emitted directly from the cell unlike a combustion engine
what are some disadvantages of fuel cells
• hydrogen is highly flammable and must be stored and transported correctly
• It is expensive to transport and store hydrogen. Storage of hydrogen is in pressurised containers
• energy is required to make the hydrogen and oxygen in the first place.
Fossil fuels are generally used to pass water through an electrolysis process. the use of fossil fuels contributes to CO2 emissions
EQ : An Emf Value for a commercial lithium-iodine cell is 2.80V
Suggest why this value is different from the value calculated in part (b) (1)
Non - standard conditions
Equation for the reaction that occurs at the positive lithium cobalt oxide electrode
Li^+ + CoO2 + e^- => LiCoO2
