Redox titrations 218-219

Question 1

What is oxidation?

Answer 1

Oxidation is loss of electrons, or an increase in oxidation number.

Question 2

What is reduction?

Answer 2

Reduction is the gain of electrons or decrease in oxidation number.

Question 3

What are two common oxidation states of iron. Which state is most stable?

Answer 3

Iron has two common oxidation states in its compounds, Fe²⁺ and Fe³⁺. The iron(II) oxidation state is less stable than the iron(II) oxidation state.

Question 4

-Write equation for iron(II) being oxidised to iron(III). -What would oxidise iron?

Answer 4

Fe²⁺(aq) → Fe³⁺(aq) + e^-

In the presence of air, or when in contact with another oxidising agent, Iron would be oxidised.

Question 5

Write the equation for the reduction of Mn⁷⁺ to Mn²⁺. (where a reactant is MnO₄^-)

Answer 5

MnO₄^-(aq) + 8H⁺(aq) + 5e^- → Mn²⁺(aq) + 4H₂O(l)

Question 6

Write the equation for the reaction between iron(II) and acidified manganate(VII).

Answer 6

MnO₄^-(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O

Question 7

Explain how redox titrations differ from acid-base titrations.(chemically, not in terms of process.)

Answer 7

• An acid-base reaction involves the transfer of protons from the acid to the base and the formation of water and salt.
• A redox titration involves the transfer of electrons from one species to another.

Question 8

Explain why potassium manganate(VII) is used as a common oxidising agent.

Answer 8

Aqueous potassium manganate(VII) contains MnO₄^-(aq) and is a common oxidising agent that is self-indicating:

• MnO₄^-(aq) → Mn²⁺(aq)

purple → almost colourless

Mn²⁺(aq) ions are very pale pink in colour. In manganate(VII) titrations, the solutions used are very dilute and the Mn²⁺(aq) ions are present in low concentrations. This means that the very pale pink colour from Mn²⁺(aq) cannot be seen and the solution appears colourless. The MnO₄^-(aq) ions have such a deep purple colour that this masks any other colour present.

Question 9

When MnO₄^- is used to oxidise solution containing iron(II) ions. The solution is acidified with what acid?What acid cannot be used and why?

Answer 9

The solution is acidified with sulfuric acid. Hydrochloric acid cannot be used because its reacts with MnO₄^-. This would not only invalidate the titration results but would also cause you some distress - chlorine gas is given off.

Question 10

Explain, briefly, how you would carry out a redox titration involving potassium manganate(VII) solution and iron(II) solution.

Answer 10

Typically these reactions are carried out with potassium mangante(VII) solution in the burettte and the iron(II) solution in the conical flask. As the manganate (VII) solution is added to the iron(II) solution, it is decolourised. The end point of the titration is when excess MnO₄^- ions are present. - this shows up as the first hint of a permanent pink colour in the solution on the conical flask.