Redox

Question 1

Loss of electrons, oxidation number going up.

Answer 1

Oxidation

Question 2

Gain of electrons, oxidation number going down.

Answer 2

Reduction

Question 3

is based on an oxidation-reduction reaction between the analyte and titrant. These most commonly use a potentiometer or a redox indicator to determine the endpoint. Frequently either the reactants or the titrant have a color intense enough that an additional indicator is not needed.

Answer 3

Redox Titration

Question 4

The vertical axis in oxidation/reduction titration curves is generally an electrode potential instead of logarithmic functions.

Answer 4

Redox Titration Curves

Question 5

At the equivalence point, the conc. cannot be obtained from the stoichiometric reaction. Fortunately, equivalence-point potentials are readily obtained by taking advantage of the fact that the two reactant species have known conc. ratios at chemical equivalence.

Answer 5

Equivalence Point Potentials

Question 6

Derivation of Redox Titration Curves

Answer 6

a. initial potential
b. potential after initial addition
c. equivalence-point potential
d. potential after the excess addition

Question 7

➢substances that change color upon beig oxidized or reduced. The color change is independent of the chemical nature of the analyte and titrant and depend instead upon the changes in the electrode potential of the system that occur as the titration progresses

Answer 7

General Redox Indicator

Question 8

Types of General Redox Indicator

Answer 8

a. Iron(II) Complexes of Orthophenanthrolines
b. Diphenylamine and its Derivatives
c. Iodine Solutions

Question 9

known as phenanthroline that form stable complexes with iron(II) and certain other ions.

Answer 9

Iron(II) Complexes of Orthophenanthrolines

Question 10

widely used in oxiddddation/reduction reactions involving iodine as an oxidant or iodide ion as the reductant.

Answer 10

Iodine Solutions

Question 11

the best known specific indicator which forms a dark blue complex with triiodide ion.

Answer 11

Starch Indicator

Question 12

sticks or coils of the metals can be immersed directly in the analyte solution. After reduction is judged complete, the solid is removed manually and rinsed with water. It is necessary to filter the solution to remove granular or powdered forms of the metal. An alternative to filtration is the use of reductor.

Answer 12

Auxilliary Reducing Reagents

Question 13

= a number of metals are good reducing agents and have been used for preredction of analytes included among these are

Answer 13

Zn, Al, Cd, Pb, Ni, Cu, and Ag (in the presence of chloride ion).

Question 14

Auxilliary Oxidizing Reagents

Answer 14

1. Sodium Bismuthate (NaBiO₃)
2. Ammonium Peroxydisulfate
3. Sodium Peroxide and Hydrogen Peroxide

Question 15

• powerful oxidizing agent capable of converting manganese(II) quantitatively to permanganate ion.

• a sparingly soluble salt but its exact composition is uncertain.

• oxidations are performed by suspending the bismuthate in the analyte solution and boiling for a brief period.

• the half-reaction for the reduction of sodium bismuthate can be written as

NaBiO3(s) + 4H+ + 2e- BiO+ + Na+ + 2 H2O

Answer 15

Sodium Bismuthate (NaBiO3)

Question 16

• powerful oxidizing agent that converts Cr(III) to dichromate, Ce(III) to Ce(IV), and Mn(II) to permanganate in acidic solutions.

• the half-reaction is
S2O82- + 2e- 2SO42- E°=2.01 V

• the oxidations are catalyzed by traces of silver ion.

• the excess reagent is readily decomposed by a brief period of
boiling:
2S2O8 + 2H2O 4SO42- + O2(g) + 4 H+

Answer 16

Ammonium Peroxydisulfate

Question 17

• a convenient oxidizing agent either as a solid sodium salt or as a dilute solution of the acid.

• the half-reaction for hydrogen peroxide in acidic sol’n H2O2 + 2H+ + 2e- 2H2O E°=1.78 V

• after oxidation is complete, the solution is freed of excess reagent by boiling:
2H2O2 2H2O + O2(g)

Answer 17

Sodium Peroxide and Hydrogen Peroxide

Question 18

Standard solutions of most reducing agents tend to react with atmospheric oxygen. For this reason, reductants are seldom used for the direct titrations of oxidizing analytes; indirect methods are used.

Answer 18

Application of Standard Reductants

Question 19

Standard Reductants:

The two most common indirect methods are based upon:

Answer 19

A. Iron(II) Solutions
B. Sodium Thiosulfate

Question 20

• readily prepared from iron(II) ammonium sulfate (Mohr’s salt) or
  form closely related iron(II) ethylenediamine sulfate (Oesper’s
  salt)
• air oxidation of iron(II) takes place rapidly in neutral solutions but
  is inhibited in the presence of acid with the most stable
  preparations beig 0.5 M in H2SO4
  .

Answer 20

. Iron(II) Solutions

Question 21

• moderately strong reducing agent that has been widely used to
  determine oxidizing agents by indirect procedure that involves
  iodine as an intermediate.
• with iodine, thiosulfate ion is oxidize quantitatively to
  tetrathionate ion, the half-reaction being
  2S2O32- ⇔ S4O62- + 2e-
• the end point is the disappearance of the iodine color (for
  colorless analyte)

Answer 21

Sodium Thiosulfate

Question 22

Thiosulfate tend to decompose to give sulfur and
hydrogen sulfite ion:
S2O32- + H+ ⇔ HSO3- + S(s)

provided the analyte solution is colorless, the disappearance of the
iodine color can serve as the indicator in titrations with sodium
thiosulfate.

Answer 22

Stability of Sodium Thiosulfate Solutions