Variable Oxidation States

What is the equation to work out the total oxidation state of metal?

Total oxidation state of metal = Total oxidation state of metal complex - Total oxidation state of ligands

What are the 4 common oxidation states of Vanadium and their ions?

• +5 → VO₂⁺ (Vanadate (V) ion)

• +4 → VO²⁺

• +3 → V³⁺

• +2 → V²⁺

What is the colour of VO₂⁺?

Yellow

What is the colour of VO²⁺?

Blue

What is the colour of V³⁺?

Green

What is the colour of V²⁺?

Violet

When can the different oxidation states of Vanadium be seen?

When a solution of ammonium vanadate (V) is reduced using zinc in acidic conditions

Half-equation for oxidation of Zn

Zn → Zn²⁺ + 2e^-

What is the half-equation for the reduction of VO₂⁺ to VO²⁺?

VO₂⁺ + 2H⁺ + e^- → VO²⁺ + H₂O

What is the overall equation for the reduction of VO₂⁺ to VO²⁺ by zinc in acidic conditions?

2VO₂⁺ + 4H⁺ + Zn → 2VO²⁺ + Zn²⁺+ 2H₂O

What is the half-equation for the reduction of VO²⁺ to V³⁺?

VO²⁺ + 2H⁺ + e^- → V³⁺ + H₂O

What is the overall equation for the reduction of VO²⁺ to V³⁺ by zinc in acidic conditions?

2VO²⁺ + 4H⁺ + Zn → 2V³⁺ + Zn²⁺+ 2H₂O

What is the half-equation for the reduction of V³⁺ to V²⁺?

V³⁺ + e^- → V²⁺

What is the overall equation for the reduction of V³⁺ to V²⁺ by zinc in acidic conditions?

2V³⁺ + Zn → 2V²⁺ + Zn²⁺

What is Tollen’s reagent used to distinguish between?

Aldehydes and ketones

What is the formula for the transition metal complex in Tollens’ reagent?

[Ag(NH₃)₂]⁺

What happens to an aldehyde when it is warmed with Tollens’ reagent?

Oxidised to a carboxylic acid

What happens to the silver ions in Tollens’ reagent when it is warmed with an aldehyde

The silver ions are reduced to a silver metal

Why do ketones not give a reaction when warmed with Tollens’ reagent?

Ketones cannot be oxidised

Give the equation for the reduction of Tollens’ reagent

[Ag(NH₃)₂]⁺ + e^- → Ag + 2NH₃

What is redox potential a measure of

How easily an ion or atom is reduced to a lower oxidation state

What ions have more positive redox potentials? What does this mean?

• Less stable ions

• More likely to be reduced

What 2 factors affect redox potential?

• Ligand

• pH

What ligand surrounds the metal ion when standard electrode potentials are measured? Why?

• Water

• Standard electrode potentials are measured in aqueous solution

What are redox titrations use to find?

The concentration of a reducing or oxidising agent

What solution is used in the burette in Redox Titrations and why?

• Potassium Manganate (VII)

• Strong oxidising agent

What is the procedure for a redox titration of Potassium Manganate against Iron(II) sulfate? (5 steps)

1. Fill a burette with a standard solution of potassium manganate(VII)

2. Pipette a known volume of iron(II) sulfate solution into a conical flask

3. Add an equal volume of dilute sulfuric acid

4. Add the potassium manganate(VII) until the end point is reached (colour change)

5. Perform a rough titration and then sufficient accurate ones until titres are concordant

What is MnO₄^- reduced to?

Mn²⁺

Write the equation for the reduction of MnO₄^- in the presence of H⁺ ions

MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O

What must the difference in concordant results be?

Between 0.1cm³ of each other

KMnO4^- colour

Purple

Mn²⁺ colour

What is the end point colour change of redox titrations involving KMnO4^- solution?

Colourless → Pale pink

What are Iron(II) ions oxidised to? Give the equation

• Iron(III) ions

• Fe²⁺ → Fe³⁺ + e^-

What are C₂O₄^2- ions oxidised to? Give the equation

• CO₂

• C₂O₄^2- → 2CO₂ + 2E-