5.2.3 Redox And Electrode Potential Flashcards
What is an oxidising agent
Accepts electrons from a species being oxidised, is reduced itself
What is a reducing agent
Donates electrons to another species, is itself oxidised
2 common redox titrations
Potassium manganate (VII) under acidic conditions (KMnO4 (aq) )
Sodium thiosulfate for determination of iodine (Na2S2O3 (aq) )
How to carry out a Manganate (VII) titration
- Add a standard solution of potassium manganate (VII) to the Burnett
- Use a pipette to add measured volume of solution being analysed
- add excess of dilute sulfuric acid as H+ is needed to reduce MnO4-
- add until end point
-continue until you obtain concordant results
How to determine end point in a manganate titration
Colourless to pink (purple manganate solution)
Oxidising agent in manganate titration
MnO4-
Examples of reducing agents for manganate titrations
Fe2+
Ethanedioic acid (COOH2)
How to carry out an iodine/thiosulfate titration
- Add standard solution of Na2S2O3 to Burette
- prepare solution of oxidising agent, add this to conical flask using pipette, add excess potassium iodide, iodine produced
- titrate with sodium thiosulfate
- record titre at end point
- repeat for concordant results
How to determine end point of a iodine thiosulfate titration
Solution is yellow-brown and fades, end point is difficult to determine on it’s own
When end point is being approached (pale straw colour), add starch indicator
Blue-black colour forms, fades and colour disappears at the end point (all I2 reacted)
Oxidising agent for iodine-thiosulfate titration
Chlorate (I) ions, ClO-
Copper (II) ions, Cu2+
Reducing agent for iodine-thiosulfate titration
S2O32- (aq)
Standard electrode potential definition
E.m.f. Of a half-cell connected to a standard hydrogen half-cell under standard conditions (298K, 1moldm-3, 100kPa)
What is the negative electrode
Electrode with more reactive metal which loses electrons and is oxidised
So: more negative cell potential
What is the positive electrode
Less reactive metal which gains electrons and is reduced
What is a standard hydrogen electrode
Half cell with hydrogen gas and solution containing H+ (aq)
Inert platinum electrode used to allow electrons in/out
Standard hydrogen electrode voltage
0
Features of measuring electrode potentials
- Electrodes connected by wire to allow flow of electrons from negative to positive electrode
- solutions connected with a salt bridge, allows flow of ions, salt bridge is filter paper soaked in KNO3 (aq)
What does a negative emf indicate
Greater tendency to lose electrons and be oxidised
Greater reactivity of a metal in losing electrons
Stronger reducing agent
What does a positive emf indicate
Greater tendency to gain electrons and undergo reduction
Greater reactivity of a non-metal to gain electrons
Stronger oxidising agent
How to calculate standard cell potential from standard electrode potentials
E cell = E (positive electrode) - E(negative electrode)
Or
E(reduction) - E(oxidation)
How can the cell potential predict feasibility of a reaction
Positive cell potential indicates the reaction is feasible
Why would a reaction be feasible but not occur
High activation energy, slow rate of reaction
Non-standard conditions (different concentration or not aqueous)
Benefits of fuel cells
No harmful/toxic byproducts, only water produced
Operate continuously
Do not have to recharged
Disadvantages of fuel cells
Constant fuel supply needed
Hydrogen is expensive and hard to store, very combustible
