Section 13.4 Redox Stoichiometry p. 596 - 600

Redox titration is a laboratory technique where one chemical species is progressively added to another chemical species (one solution is dripped into another solution) until complete reaction occurs and one of the reacting species undergoes a colour change.

The colour change occurs because excess substance being added is present. Equivalence point is reached when exact numbers of moles of reactants have been used up.   Endpoint is reached when the next drop of excess reactant is added, noted by a colour change in the resulting mixture.

Typical Situations: "Unknown" is a reducing agent. "Known" therefore is an oxidizing agent that a) is strong! b) shows the endpoint by a colour change of the mixture.

Typical Titrants Used: 1) KMnO4(aq):   MnO4–(aq) + 8 H +(aq) + 5 e –  Mn 2+(aq) + 4 H2O(l) purple almost colourless acid added to unknown very faint pink     When all the unknown reducing agent has reacted with added MnO4–(aq) , one more drop of MnO4–(aq) turns the mixture pink! (endpoint)

Cr2O7 2–(aq) + 14 H +(aq) + 6 e –  2 Cr 3+(aq) + 7 H2O(l) 2) K2Cr2O7(aq):   Cr2O7 2–(aq) + 14 H +(aq) + 6 e –  2 Cr 3+(aq) + 7 H2O(l) orange acid added to unknown faint green one extra drop of orange Cr2O7 2–(aq) yields a brown orange colour     After completion of reaction, one extra drop of Cr2O72–(aq) causes the orange colour to persist! (endpoint - Stop Titrating!)

Activity: Practice Questions: #1-5, p.598 Questions: #1-6, p.600