redox and electrode potentials Flashcards
what is meant by oxidation (1)
when a species loses electrons
what is the oxidation number of chromium in Cr2O72- (10
+6
what is the oxidation number of chromium in Cr3+ (1)
+3
write the half equation for Cr3+ oxidising to CrO42- ion in alkaline solution (2)
- Cr3+ + 8OH- ->CrO42- +4H2O+3e-
what colour change is seen from the reduction of dichromate (VI) ions to chromium ions (1)
orange to green
construct the half equation for the oxidation of ethanol in the prescence of water to form ethanoic acid and H+ ions (2)
CH3CH2OH +H2O ->CH3COOH +4H+ +4e-
produce and equation for the oxidation of ethanol by dichromate ions (2)
2Cr2O72- + 3CH3CH2OH + 16H+ -> 4Cr3+ + 3CH3COOH + 11H2O
suggest why iron tablets have an outer coating that is insoluble in water but breaks down slowly in acid (1)
- so that iron is gradually released in the stomach when stomach acid break down the coating
when doing redox titration why is it not necessary to know the exact conenctration of sulfuric acid (1)
it’s in excess
when filtering solutioin into a volumetric flask what can you do to get out the most accurate results (1)
- the residue should be washed with some distilled water used to make the solution up to 100cm3
explain why an indicator is not used when titrated with potassium mangante (VII) solition (1)
- at the end point the purple colour of the unreacted potassium managante (VII) is seen so no indicator is required
Give the standard conditions required for the standard hydrogen electrode (3)
- hydrogen pressure 100kpa
- [H+] = 1moldm-3
- temperature = 298k
describe how the standard hydrogen electrode is used with a half cell to measure the standard electrode potential of the hald cell (2)
- The SHE is connected to the half cell (with standard conditions)
- the stanndard electrode potential is read of a voltmeter
what is a suitable solution to use in the zinc half cell (2)
- zinc sulfate/zinc nitratee
- at 1moldm-3
describe a suitable salt bridge (1)
- filter paper soaked in KNO3 solution
describe the purpose of a salt bridge (1)
complete the circuit
what is the direction of electron flow (1)
left ot right
what is the electrode at which reduction occurs in a zinc-copper cell (1)
- copper electrode/cathode/positive electrode
describe how the NO3- and H+ hald cell would be set up under standard conditions (3)
- a beaker containing a mixutre of NO3- and H+ ions each at 1moldm-3
- and HNO2 at 1moldm-3
- at 298k and with a platinum electrode
write the ionic equation for the reaction between Fe3+ ions and I- ions and for the reaction between Fe2+ ion and I- ions and calculate the standard electrode values for each of these reactions (6)
- 2Fe3+ + 2e- -> 2Fe2+ + I2
- 0.77-0.54 = 0.23V
- Fe2+ +2I- -> Fe + I2
- -0.44-0.54 = -0.98V
if Fe 3+ ions are mixed with iodide ions explain the final iron species that will be formed (2)
- the answer shows that the positive electrode potential value is only for the first equation and so the final product will be Fe 2+
what observations would be made if some potassium manganate (VII) solution is mixed with an excess of hydrogen peroxide solution (2)
- the solution goes from purple to colourless
- and effervescence is seen
some oxygen gas is bubbled through an acidified solution of V2+ ions explain which vandium species will be present when no further reactions occurs
wrtie and ionic equations and calculate the electrode potential values for each reaction that occurs
- 2V2+ +O2 +2H= ->2V3+ +H2O2
- 0.26+0.68= 0.94V
- 2V3+ +2H2O + O2 -> 2VO2+ + 2H+ + H2O2
- -0.34 =0.68=0.34V
- the final species will be VO2+ which will not be oxidised to VO2+ as the potetial for thi s change will be negative
a voltmeter is attatched to the storage cell suggest why the reading is not equal to the value calculated (1)
cell not in standard conditions
how does a fuel cell differ from a storage cell (2)
- a fuel in a fuel cell is supplied continuously
- the voltage from a fuel cell is constant
