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
Used to generate an electric current and do not need to be electrically charged
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 :
negative electrode
H2 + 2OH- => 2H2O + 2e-
positive electrode
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] -
why is a wire not used to connect up the circuit
Because the metal wire would set up its own electrode system with the solutions
What should the salt consist of
should be unreactive with the electrode and electrode solution . e.g potassium chloride would not be suitable for copper systems as chloride ions can form complexes with copper ions
what’s the effect of concentration of cell emf
Increasing conc of reactants would increase EMF and decreasing them would cause EMF to decrease
what’s the hydrogen electrode equilibrium for standard hydrogen electrode
H2 (g) (reverse sign ) 2H+ (aq) + 2e-
whys pt the catalyst in a standard hydrogen cell
it’s porous and can absorb hydrogen gas
when is a pt electrode used in the cell diagram
if a system does not include a metal that can act as an electrode
- it conducts surface for electron transfer
whys a pt electrode used
because it’s unreactive and can conduct electricity
How can hydrogen be stored in fuel cells
- as a liquid under pressure
- adsorbed on the surface of a solid material
- absorbed within a solid material
what’s the equation for the negative electrode of a lithium cell
Li => Li+ + e-
EQ : State the substances and conditions needed in a standard hydrogen electrode (3)
- H2 and 100KPa
- 1 moldm-3 and HCl / HNO3 / H+
- Pt electrode and temperature of 298k
EQ : Both electrodes contain a strip of copper metal in a solution of aq Cu2+ ions . State why the left hand electrode does not have an electrode potential of +0.34 (1)
Conc is not 1.0moldm-3
EQ : There are 2 ways to use hydrogen as a fuel for cars . One way is in a fuel cell to power an electric motor , the other is as a fuel in an internal combustion engine. Suggest the major advantage of using the fuel cell (1)
Converted more of the available energy from combustion of hydrogen into kinetic energy of the car / internal combustion engine wastes more energy
What are advantages of ethanol fuel cells compared to hydrogen fuel cells
- ethanol can be made from renewable sources in a carbon neutral way
- raw materials to produce ethanol by fermentation are abundant
- ethanol is less explosive and easier to store than hydrogen
- new petrol stations would not be required as ethanol is a liquid fuel
What’s the equation for an ethanol fuel cell that occurs at the oxygen electrode
4 e- + 4H+ + O2 => 2H2O
what’s the equation for ethanol fuel cell that occurs at ethanol electrode
C2H5OH + 3H2O => 2CO2 + 12H+ + 12 e-
what’s the overall equation in an ethanol fuel cell
C2H5OH + 3O2 => 2CO2 + 3H2O
EQ : Explain why an aq electrolyte is not used for a lithium cell ( using the table )
Li = -3.04
H2O = -0.83
(2)
lithium would react with the electrolyte / water
E - for Li + < H2O
what must be done to maintain the EMF of a fuel cell when in use
Constantly add reactants
whats an essential property of the non reactive porous separator in a non rechargeable cell
allows ions to pass
what’s the function of the carbon rod in a non rechargeable cell
allows electrons to flow
EQ : the zinc electrode acts as a container for the cell and is protected from external damage . Suggest why a cell often leaks after being used up for a long time (1)
Zinc has reacted or “used up”
what do you do with the equation if the question acts for the equation when the cell is RECHARGED
reverse the equation
EQ : state and explain the effect if any of increasing pressure of oxygen on the emf of this cell
Increases
Overall equation moves to the right
what would a graph to show how the emf of a hydrogen oxygen fuel cell changes with time (1)
Straight line going horizontally in the middle
EQ : suggest why the use of a hydrogen oxygen fuel cell might not be carbon neutral (1)
CO2 is released because fossil fuels are burned to produce electricity to generate hydrogen
EQ : using the half equations of a hydrogen oxygen fuel cell , explain how an electric current can be generated (2)
Hydrogen produced electrons
Oxygen accepts electrons
EQ : explain why a fuel cell does not need to be recharged (1)
Reactants are continuously supplied
