5.2.3 Redox, Electrode Potential and Fuel Cells
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what is a redox reaction
where one or more elements change oxidation state
what is a disproportionation reaction
when one species is simultaneously oxidised and reduced
what is an oxidising agent
causes a species to lose electrons
what is a reducing agent
causes a species to gain electrons
describe the thiosulfate redox titration and give the equation and colour changes
A starch indicator is added is added near the end point and iodine fades to a pale yellow and this causes the colour change to be from blue/black to colourless 2S2O3^2- + I2 –> 2I- + S4O6^2-
describe the manganate redox titration and give the equation and colour changes
If the manganate is in the burette then the end point of the titration will be the first permanent pink colour so colour change is colourless to purple MnO4- + 8H+ + 5Fe^2+ –> Mn^2+ + 4H2O + 5Fe^3+
what do insufficient volumes of sulfuric acid or weak acids cause to happen in the manganate redox titration
the solution is not acidic enough so MnO2 to be produced instead of Mn^2+ which is brown and masks the colour change causing greater volumes of manganate to be used in the titration
Why can conc HCl not be not be used in the manganate redox titration
the Cl- ions would be oxidised to Cl2
describe an electrochemical cell
a cell has two half cells which must be connected by a salt bridge the cells will produce a small volatge if connected in a circuit
why does a voltage form
• the LHS anode has more of a tendency to oxidise and release electrons that build up on the anode • a potential difference is created between the two electrodes • this is measured with a high resistance meter and has the symbol E^o
what are the standard conditions in an electrochemical cell when measuring potential difference
• 298k • 100kPa • 1.00 moldm^-3 conc • no current flowing
what metal is used if there is no solid metal and why
Pt because it isn’t very reactive
why is a high resistance voltmeter used
it needs to stop the current flowing in the circuit so maximum potential difference can be calculated
what is the function of the salt bridge and how does it work
• connects the circuit with free moving ions that conduct the charge • the salt is KNO3 as this is unreactive with the electrodes eg KCl would form complexes with some ions • a wire would set up its own electrode system
what happens at each side of the cell
LHS/anode- oxidation, negative RHS/cathode- reduction, positive
what is standard cell potential
potential of a cell composed of two electrodes under standard conditions
what voltage does a standard hydrogen electrode have
0.00 volts
why are standard conditions needed in a standard hydrogen electrode
position of redox equilibrium will change with conditions SHE is difficult to use so a secondary standard is used where other electrodes are calibrated against SHE
what is standard electrode potential
potential difference measured when an electrode system is connected to the hydrogen electrode system and standard conditions apply
what happens to potential if pressure of H2 increases
equilibrium shifts to the left top oppose potential is more negative
what happens to potential if surface area of electrode increases
faster rate of electron transfer but has no effect on potential
what happens to potential if temp increases
if forward reaction is exothermic equilibrium shifts to the left to oppose and potential is negative
how is EMF calculated and what makes a reaction feasible
EMF= Ered-Eox (RHS-LHS) if EMF is positive it is feasible
what happens to EMF is the voltmeter is replaced with a bulb
EMF falls to zero and reactant concentrations drop