KEY2CHEM

Redox Titrations 

A redox titration allows for the quantitation of amounts of oxidized and reduced species in a redox reaction. At the equivalence point in a redox titration, the moles of oxidized species = moles of reduced species. The permanganate ion (\(\require{mhchem}\ce{MnO4-}\)) is commonly used in redox titrations because its deep purple color fades as it is reduced to \(\require{mhchem}\ce{Mn^2+}\).


Example 1.

 

An iodometric titration is a reliable redox titration in which iodine (\(\require{mhchem}\ce{I2}\)) reacts with thiosulfate ion (\(\require{mhchem}\ce{S2O3^{2-}}\)) in acidic solution to generate \(\require{mhchem}\ce{I-}\) and \(\require{mhchem}\ce{S4O6^{2-}}\). Write a balanced equation for this redox titration.

A. \(\require{mhchem}\ce{I2(aq) + 2 S2O3^{2-} -> 2 I^{-}(aq) + S4O6^{2-}(aq)}\)

B. \(\require{mhchem}\ce{I2(aq) + S2O3^{2-} -> I^{-}(aq) + S4O6^{2-}(aq)}\)

C. \(\require{mhchem}\ce{I2(aq) + 2 S2O3^{2-} + H+(aq)-> 2 I^{-}(aq) + S4O6^{2-}(aq)}\)

 

 

Solution

A. \(\require{mhchem}\ce{I2(aq) + 2 S2O3^{2-} -> 2 I^{-}(aq) + S4O6^{2-}(aq)}\)

The atoms in this redox reaction can be balanced using simple coefficients in front of the species, followed by balancing the number of electrons. Adding \(\require{mhchem}\ce{H2O}\) or \(\require{mhchem}\ce{H+}\) is not required to balance.

 


Example 2.

 

A breathalyzer is a device used to measure blood alcohol content, based on exhaled alcohol. If the redox reaction occurring in a breathalyzer is

 \(\require{mhchem}\ce{3 C2H5OH + 2 Cr2O7^{2-} + 8H+-> 2 Cr^{3+} + 3 CH3COOH +11 H2O}\)

which species is reduced?

A. \(\require{mhchem}\ce{C2H5OH}\)

B. \(\require{mhchem}\ce{Cr2O7^{2-}}\)

C. \(\require{mhchem}\ce{H+}\)

 

 

 

 

 

Solution

B. \(\require{mhchem}\ce{Cr2O7^{2-}}\)

Dichromate ion (\(\require{mhchem}\ce{Cr2O7^{2-}}\)) is reduced to \(\require{mhchem}\ce{Cr^{3+}}\) and ethanol (\(\require{mhchem}\ce{C2H5OH}\)) is oxidized to acetic acid (\(\require{mhchem}\ce{CH3COOH}\)).


Example 3.

 

Which is true at the equivalence point in a redox titration?

A. There is a color change.

B. The moles of oxidized species equal the moles of reduced species.

C. There must be an acid and a base present.

 

 

 

 

Solution

 

B. The moles of oxidized species equal the moles of reduced species.

At the equivalence point in a redox titration, the moles of oxidized species equal the moles of reduced species.