Chapter 15: Redox Titrations
1. (A) Which of the following is true of a redox titration?
(i) Since the redox reaction is spontaneous, the equilibrium constant does not have to be large for an effective titration. (ii) The titration effectiveness is increased when the two half-reaction potentials are far apart. (iii) Without an indicator the equivalence point cannot be detected. (iv) For an effective redox titration, the reducing agent must always be in the buret. (v) The equivalence point in a redox titration is found at the highest voltage reached during the titration. (B) In some pretreatment situations, the analyte must be reduced prior to titration. Of the following, which does not show an analyte being converted to its reduced form(s)? (i) CrO42- ( Cr2+
(ii)MnO4- ( Mn2+
(iii) Fe3+ ( Fe2+
(iv)Mo3+ ( MoO42-
(v) UO22+ ( U3+/U4+
(C) Which of the following, if any, provides a true statement about a redox Gran plot? (i) The redox Gran plot is useful only if a constant ionic strength is maintained (ii) The Y-axis data is obtained from VT x 10-nAEcell/0.05916V (iii)The Y-axis intercept of the line in a proper redox Gran plot shows the volume of added titrant at the equivalence point. (iv) In order to determine the volume of titrant needed to reach the equivalence point, several precise measurements near the equivalence point are required. (v) none of the above are true statements
2. (A) Methylene blue is a common redox reaction indicator. A useful solution can be prepared by dissolving 0.10 gram in 100 mL of water. The Eo’ value for this indicator is 0.53V. (n = 2) What is the useful voltage range of this indicator? (B) Starch indicators are useful when iodine titrations (organic oxidations) are performed. Why is it considered an advantage in these titrations to have iodine as the titrant instead of the analyte?
3. An environmental chemist is going to field test the chemical oxygen demand factor for a stream in Yellowstone Park. (A) The dichromate that will be used for the redox titration of the stream was standardized before leaving on the hike. Based on the following data, what is the molarity of the potassium dichromate solution? 1.352 grams of K2Cr2O7 (294.19g/mol) are placed in enough water to form 250.0 mL of solution. (B) A 25.0 mL of stream water was treated with 10.00 mL of acidified K2Cr2O7 from part “A” and warmed. After the reaction mixture was cooled, the excess K2Cr2O7 was titrated with 0.2505 M Fe2+. This back titration requires 35.67 mL. How many moles of K2Cr2O7 reacted with the original organic material in the stream? What is the COD for the stream sample? (Use 3/2 ratio for the reaction between oxygen and dichromate.) (Overall titration reaction: 6 Fe2+ + Cr2O72- + 14H+ [pic] 6Fe3+ + 2Cr3+ + 7 H2O)
4. High nitrite ion (NO2-) levels in ground water can cause many health problems (nitrite can bond to hemoglobin). One method to determine nitrite levels can be titration with Ce4+. Ce4+ (aq) + e- ( Ce3+ (aq) Eo = 1.72 V
NO3- (aq) + 2e- ( NO2- (aq) Eo = 0.940 V
(A) Balance the redox equation ___ NO2- + __ Ce4+ [pic] ___ NO3- + ___Ce3+ (B) How many mL of 0.00400 M Ce4+ solution would be required to titrate 100.0 mL of 0.00100 M NO2- solution to the equivalence point? (C) At the equivalence point show the relationships indicated here by placing the proper coefficient in place of “?”:
[? NO2-] = [Ce4+]; [ NO3-] = [?Ce3+]
(D) Assume that the titration is carried out with an apparatus similar to the one shown here using a S.C.E. reference electrode (+0.241V); What is the E cell after the addition of 5.00 mL of the Ce4+ solution? (E) What is the Ecell voltage at the halfway point to the equivalence point? (F) What is the Ecell at the equivalence point?
(G) The redox indicator Tris(5-nitro-1,10-phenanthroline)iron has been suggested to use with this particular titration. Justify, with calculations, either accepting or rejecting this...
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