practical: redox titrations Flashcards
(10 cards)
write the full equation including half equations for redox reaction between KMnO4 and Fe2+
6e^- + 8H+ + MnO4^- —-> Mn2+ + 4H2O
Fe2+ —> Fe3+ + e- (x5)
8H+ + MnO4- + 5Fe2+ —> 5Fe3+ + Mn2+ +4H2O
write the full equation including half equations for redox reaction between K2Cr2O7 and Fe2+
6e- + 14H+ + Cr2O7^-2 —-> 2Cr^3+ + 7H2O
Fe2+ —> Fe3+ + e- (x6)
14H+ + Cr2O7^2- + 6Fe2+ —> 6Fe3+ +2Cr^3+ + 7H2O
what is a suitable acid for for both KMnO4 and K2Cr2O7
dilute H2SO4 for both but dilute HCL for only K2Cr2O7
what indicator is used for KMnO4 and K2Cr2O7
KMnO4: none -its self-indicating
K2Cr2O7: barium diphenylaminesulphonate
what is the colour change for both the recations of KMnO4 and K2Cr2O7 with Fe2+
Mno4-: colourless to first hint of pink
Cr2O7^-2: goes purple
suggest what you could do to improve the reliabilty of the results
titration evaluation
-read aty eye level
-remove funnel from burrette
-rinse conical flask with deionised water near the end point
in a similair experiment the student carried out the titration after leaving the iron solution overnight. what difference may you expect in the results
-Fe2+ is oxidised to Fe3+ by oxygen in the air
-this will lead to a lower volume of KMnO4 therefore a lower titre.
the iron(ii) concentration could ahve been estimated using potassium dichromate(vii) instead of potassium manganate(vii). what was the advantage of using potassium manganate(vii)
-its self indicating: MnO4^- is purple but Mn2+ is colourless
in a similiar experiment the iron(ii) solution was contaminated with Cl- ions. expalin why its better to use KCr2O7 over KMnO4 for this?
MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O
2Cl- —-Cl2 +2e-
16H+ 2MnO4 + 10Cl- —-> 2Mn2+ +8H2O + 5Cl2
———————————————————-
-KMnO4 can oxidise Cl into Cl2
-Cl2 is toxic
-K2Cr2O7 cannot oxidise cl into Cl2 so safer to use
