Redox And Electrodes Flashcards
What can be used to carry out titrations?
KMnO4 in acidic conditions MnO4- -> Mn2+
NaS2O3 for determination of I2
What happens in a titration using KMnO4 in acidic conditions?
Decolourised when added - end point judged as it pink - excess of MnO4-
Used to analyse reducing agents: Fe2+ and ethandioic acid
MnO4- + 8H+ +5e- -> Mn2+ + 4H2O
Fe2+ -> Fe3+ + e-
How can the concentration of I2 be determined?
By titration with standard solution of sodium thiosulfate. Also to find the conc of ClO-, Cu2+, Cu
-> yellow brown colour then starch indicator is added to form a pale yellow.
How can the copper content of salt/alloys be determined?
Sodium thiosulfate. For Cu2+ salts, dissolved in water and insoluble compounds reacted with acids to form Cu2+ aq
Alloys are reacted and dissolved in conc HNO3 then neutralised. White precipitate of copper iodide = brown. Titrated with Na2S2O3.
Define a half cell.
Contains chemical species present in redox half equation.
Define an electrode.
Combination of a metal dipping into a solution containing its ions. A potential is set up (electrode potential).
The desire of the species to lose or gain electrons.
What does the direction of electron flow depend upon?
The relative tendency of each electrode to release electrons when the 2 half cells are connected.
What are the different types of half cells?
Metals in contact with solution of their ions
Gases in contact with solution of their ions
SHE = standard hydrogen electrode. An inert platinum electrode is used.
Standard conditions always.
Define standard electrode potential.
The tendency to be reduced and gain electrons.
More positive = more willing.
Why is a salt bridge needed?
Connects free moving ions conducting charge. KNO3 does not interfere with solutions.
What are features of the negative electrode?
In an operating cell, electrode with more reactive metal loses electrons and is oxidised. The more negative the electrode potential, increased reactivity of a metal in losing electrons.
What predictions can be made?
The most negative system has the greatest tendency to be oxidised and lose electrons. So the more negative half cell is always ox (backwards).
More negative electrode potential, equilibrium further to the left and easier to lose electrons.
What are limitations of half cells?
Predictions for feasibility lie with a very high activation energy so there is a slow rate. Potential can indicate feasibility but not rate.
Based on standard potentials, so invalid if not.
What are features of primary cells?
Non rechargeable, single use, electrical energy from redox at electrodes. Irreversible.
Low current, long storage devices eg. Clocks, smoke detectors.
Alkaline based on Zn/MnO2 and KOH
What are features of secondary cells?
Rechargeable as chemicals regenerated - reversible.
Lead acid batteries - car batteries.
Nickel cadmium batteries - radios, torches.
Lithium ion batteries - laptops, phones.
