TM: Oxidation States + Titrations
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What type of reaction occurs when a transition metal changes oxidation states?
redox
State the four different ions of vanadium, their oxidation state and their colour.
V²⁺ → 2+ (violet) V³⁺ → 3+ (green) VO²⁺ → 4+ (blue) VO₂⁺ → 5+ (yellow)
How is vanadium (V) ions (VO₂⁺) reduced?
using zinc in an acidic solution
State the colour change when vanadium (v) is changed to (iv)
yellow to blue
State the colour change when vanadium (iv) is changed to (iii)
blue to green
State the colour change when vanadium (iii) is changed to (ii)
green to violet
Which ions have the largest redox potential?
least stable ions
What does the redox potential of a transition metal depend on?
• ligand type • pH (more acidic = larger potential)
What is meant by redox potential?
a value that predicts how easily an atom can be reduced to a lower oxidation state
State which type of solution is required for the reduction and oxidation of a transition metal to take place.
reduction → acidic solution oxidation → alkaline solution
In terms of reactions and ions, explain what happens during a Tollen’s test.
the Ag⁺ in [Ag(NH₃)₂]⁺ is reduced to Ag metal by the aldehyde
Give the equation for the reduction of manganate (vii) ions in a redox titration.
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
What reagent is used for manganate titrations and why?
→ dilute sulfuric acid → provides sufficient H+ ions to allow the reduction of oxidising agent
State and explain what is observed when there is an insufficient volume of sulfuric acid during a manganate titration.
→ solution is not acidic enough → MnO₂ will be produced instead of Mn²⁺ → which has a brown colour that will mask the colour change (so more manganate is used)
Explain why you cannot use concentrated HCL for a titration between Fe²⁺ with MnO₄⁻.
→ Cl⁻ ions would be oxidised to Cl₂ by MnO₄ → leads to greater volume of manganate being used
Explain why you cannot use ethanoic acid for a titration between Fe²⁺ with MnO₄⁻.
→ too weak (does not provide enough H+ ions)
Give the half equation for the oxidation of Fe during its titration with MnO₄⁻ ions.
Fe²⁺ (aq) → Fe³⁺ (aq) + e⁻
State the colour change during a titration of Fe²⁺ with MnO₄⁻ (if MnO₄⁻ is in the burette).
colourless to purple/pink
Explain why you cannot use nitric acid for a titration between Fe²⁺ with MnO₄⁻.
→ it is also an oxidising agent → oxidises Fe²⁺ to Fe³⁺ → leads to smaller volume of manganate being used
Give the half equations for the titration between manganate (vii) and ethanedioate ions.
C₂O₄²⁻ → 2CO₂ + 2e⁻ MnO₄⁻ + 8H⁺ +5e⁻ → Mn²⁺ + 4H₂O
What reaction conditions are needed for the titration between manganate (vii) and ethanedioate ions and why?
→ solution needs to be heated to about 60°C → initial reaction has high activation energy as both ions are negative
Write an equation between iron and dilute sulphuric acid.
Fe + H₂SO₄ → FeSO₄ + H₂