Redox reactions

• What is a redox reaction?

  where one substance is reduced and another is oxidised
• What are oxidising agents?

  + oxidise other species

  + accept electrons

  + are reduced
• What are reducing agents?

  + reduce other species

  + donate electrons

  + are oxidised
• Which reaction produces the molten iron and what is the equation?

  the thermit reaction:

  Fe2O3 + 2Al -> Al2O3 + 2Fe
• What happened to iron (III) oxide in the thermit reaction?

  + Iron (III) oxide, Fe2O3, is reduced to iron, Fe

  + iron(III) oxide is reduced

  + Iron(III) oxide is the oxidising agent
• What happened to the aluminium powder, Al, in the thermit reaction?

  + aluminium powder is oxidised to aluminium oxide, Al2O3

  + aluminium is oxidised

  + aluminium is the reducing agent
• What are the common oxidising agents?

  + concentrated sulfuric acid (H2SO4)

  + potassium manganate (VII) (KMnO4)

  + potassium dichromate(VI) (K2Cr2O7)

  + manganese(IV) oxide (MnO2)

  + chlorine (Cl2)

  + Hydrogen peroxide (H2O2)
• What are the common reducing agents?

  + Hydrogen (H2)

  + Zinc (Zn)

  + Carbon (C)

  + Carbon monoxide (CO)

  + lithium tetrahydrido-aluminate (III) (LiAlH4)

  + sodium tetrahydrido-borate (III) (NaBH4)
• What are the two types of electrochemical cells?

  + galvanic (or voltaic)

  + electrolytic
• How do Galvanic cells work in principle?

  Galvanic cells derive its energy from spontaneous redox reactions
• How do electrolytic cells work in principle?

  involve non-spontaneous reactions and thus require external electron source like DC battery or AC power source
• What doe both galvanic and electrolytic cells consist of?

  + two electrodes ( anode and cathode)

  + electrodes can be made of same or different metals

  + electrolyte in which two electrodes immersed
• How do galvanic cells differ from electrochemical cells?

  Galvanic:

  + oxidation occurs at anode and reduction occurs at cathode

  + anode is the negative terminal as it is the source of electrons for current

  Electrochemical:

  + reduction occurs at anode and oxidation at cathode

  + cathode is negative terminal as source of electrons for current is cathode
• What are the two common redox titrations?

  + potassium managante (VII) KMnO4 (aq) under acidic conditions

  + sodium thiosulfate (Na2S2O3 (aq)) for determination of iodine (I2(aq))
• Which ions are reduced in Manganate (VII) titrations?

  MnO4- (aq) ions are reduced and are oxidising agents
• How is the meniscus need for manganate (VII) titrations?

  + burette readings read from the top of meniscus

  + titre is difference between two readings

  + titre is the same as reading the bottom of meniscus provided the top is used for both initial and final burette readings
• Why is potassium manganate solution used in redox titrations?

  + used to find concentration of all sorts of reducing agents

  + always used in acidic solution

  + acts as an oxidising agent
• What is the change in oxidation number of manganese during the reaction?

  the oxidation number falls from +1 to +2

  MnO4- + 8H+ + 5e- => Mn2+ + 4H2O
• How many moles of MnO4- ions react with Fe2+ ions and what is the equation?

  + 1 mole of MnO4- ions react with 5 moles of Fe2+ ions

  + MnO4- + 5e- + 8H+ -> Mn2+ + 5Fe3+ + H2O
• What is the oxidation equation for titrating KMnO4 with hydrogen peroxide solution to oxygen?

  H2O2 -> O2 + 2H+ + 2e-
• What is the oxidation reaction of titrating KMnO4 with ethanedioic acid?

  + ethanedioic acid to carbon dioxide (has to be done hot)

  + COOH-COOH -> 2CO2 + 2H+ + 2e-
• What is the oxidation reaction of titrating KMnO4 with sulfite ions (IV)?

  Oxidation: Sulfite ions (IV) are oxidized to sulfate ions (VI) by KMnO4.
• What are thiosulfate ions?

  thiosulfate ions (S2O3)2- is a reducing agent
• What is the half equation of the reduction of thiosulfate ions to tetrathionate ions?

  (2S2O3)2- (aq) -> S4O62- (aq) + 2e-
• What is thiosulfate often used for?

  to find the concentration of aqueous iodine
• How is starch used for the end point determination?

  + when endpoint is approached and iodine has faded to a pale straw colour, a small amount of starch indicator is added

  + deep blue-black colour forms to assist with identification of the end point

  + blue-black colour fades as more sodium thiosulfate is added

  + at endpoint all iodine has reacted and blue-black colour disappears
• What are the steps for a redox titration to measure the purity of iodine in a sample?

  + record mass of iodine sample

  + react iodine with thiosulfate to produce iodide and tetrathionate

  + add starch indicator to solution (do not add too early or starch break down)

  + iodine combines with starch to from black complex (not chemical reaction)

  + when all iodine has reacted with thiosulphate colour changes from black to clear - endpoint of titration

  + record volume of thiosulphate that has reacted
• What is an electrochemical cell?

  converts chemical energy into electrical energy using a redox reaction
• What are metal/ metal ion half cells?

  + metal rod is immersed in its solution of ions and voltage is created between metal rod and ionic solution

  + voltage = and electrode potential and is measured in volts

  + equilibrium exists between the metal and its ions
• What are metal ion/ metal ion half cells?

  + contain an aqueous solution with ions of the same element in two different states of matter

  + there is no metal to transport electrons

  + inert platinum electrode is needed to transfer electrons into or out of the half cell
• What are non-metal/ non-metal half cell?


  + platinum electrode dipped into non-metal ions in solution

  + non-metal gaseous element can be bubbled over the electrode

  + electrode is in contact with element and aqueous ions so redox equilibrium is achieved on its surface
• What is the standard hydrogen electrode?

  + to compare ability of half cells to lose or gain electrons

  + electrode potentials for each half cells are obtained using the standard hydrogen electrode as a reference

  + the E0 of the SHE is exactly is 0V
• What are the properties of the SHE?

  + Hydrogen gas at a pressure of 1 atm

  + solution of H+ ions with concentration of 1 mol/dm3

  + temperature at 298K

  + platinum electrode covered with platinum black in contact with solution of ions

  + equilibrium between hydrogen and its ions
• How is the standard electrode potential measured?

  + to measure the standard electrode potential, the half cell is connected to a SHE

  + electrode are connected by a wire

  + solution are connected by a salt bridge, allowing ions to flow

  + salt bridge contains concentrated electrolyte that does not react with the solutions e.g. a strip of filter paper soaked in KNO3 (aq)
• What is the standard electrode potential E0?

  the emf of a half-cell compared with a standard hydrogen electrode
• How are all electrode reaction half-equations written by the IUPAC rules?

  written as reduction reactions
• Which electrode is the more reactive metal that loses electrons and is oxidised?

  negative electrode - reducing agent
• What does: || represent?

  solutions joined via a salt bridge
• How are metal electrode represented?


  e.g. Zn (s) | Zn2+ (aq)
• How are gas electrode represented?

  e.g. Pt(s) | Fe2+ (aq), Fe3+ (aq)