Transition Metals: Some Manganese Chemistry

AQA, Edexcel, OCR A level Chemistry (2017)

Principles of transition metal chemistry

Learning Objectives related to manganese chemistry.

AQA

The redox titrations of Fe²⁺ and C₂O₄^2— with MnO₄^—

Students should be able to perform calculations for these titrations and similar redox reactions.

A homogeneous catalyst is in the same phase as the reactants.

When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species.

To explain, with the aid of equations, how Mn²⁺ ions autocatalyse the reaction between C₂O₄^2— and MnO₄^—

Edexcel

15/35. To know the role of Mn²⁺ ions in autocatalysing the reaction between MnO₄^— and C₂O₄^2— ions

14/18. To be able to carry out both structured and non-structured titration calculations including Fe²⁺/ MnO₄^—.

Some Manganese Chemistry

Manganese chemistry at A level and college level revolves around the use of potassium manganate(VII) (MnO₄^—) in redox titrations and the use of manganese(II) ions (Mn²⁺) as a homogenous catalyst.

A) Manganate(VII) redox titrations

These are usually styled back titrations.

Let’s look at an example of one typically used to measure the percentage purity of an ethandioate (oxalate) salt.

You can check out the actual practical here from the RSC.

A student weighed out a 2.29g sample of impure hydrated potassium iron(III) ethandioate, K₃[Fe(C₂O₄)₃]3H₂O and dissolved it in water.  This solution was added to a 250ml volumetric flask and made up to 250ml with distilled water.

A 25ml aliquot was pipetted into a conical flask and an excess of sulphuric acid was added.  The mixture was warmed to 65^oC and titrated with 0.02M KMnO₄ solution.  26.40ml of the manganate(VII) solution was needed for complete reaction.

Calculation steps

1: Find the number of moles of the manganate(VII)

Use n=cV, so moles MnO₄^—  =   

=   5.28 × 10^—4 moles

2: Construct the manganate(VII)—ethandioate equation.

2MnO₄^-(aq) + 16H⁺(aq) + 5C₂O₄^2-(aq)  ⟶  2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)

So if 2 moles of manganate(VII) react with 5 moles of ethandioate ions

then 5.28 × 10^—4 moles of manganate(VII) react with 5/2 × 5.28 × 10^—4 moles of ethandioate ions.

3. The total moles of ethandioate is 10 times the answer to 2 because a 25ml aliquot is a tenth of the 250ml volumetric flask.

Total moles ethandioate =  10 × 5/2 × 5.28 × 10^—4  =  132 × 10^—4 

4. Therefore total moles of the compound potassium iron(III)ethandioate is

total moles = 

44  ×  10^—4 

since one mole of the compound produces 3 moles of ethandioate ions.

5. Next calculate the molar mass of the hydrated potassium iron(III)ethandioate

It turns out to be 491g.mol^—1

6. From the M_r of the potassium iron (III) ethandioate we can find the mass of pure material in the 2.29g.

Use n= m/M_r  and rearrange to give: 

mass (m)  = amount (n)  ×  molar mass (M_r)

therefore

mass (m)   =   44  ×  10^—4   ×  491    =  2.16g

Lastly, percentage purity of the potassium salt is given by

% purity =  mass of pure salt/mass of impure salt   ×  100

therefore

% purity =  2.16g/2.29   ×  100  =  94.3%

B) Manganese(II) ions (Mn²⁺) as an homogenous catalyst, an example of autocatalysis.

2MnO₄^-(aq) + 16H⁺(aq) + 5C₂O₄^2-(aq)  ⟶  2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)

In this reaction the Mn²⁺ ions catalyse the reaction once they form.

Initially, the reaction is very slow because the reaction is between two negatively charged ions.  These two ions will more likely repel each other and there will, therefore, be a very high activation energy for this reaction.

The action of Mn²⁺ as a catalyst is two fold.

First, the Mn²⁺ reacts with the manganate(VII) ion producing unstable Mn³⁺ ions.

4Mn²⁺   +  MnO₄^—   +  8H⁺    =   5Mn³⁺    +   4H₂O

Second, these Mn³⁺ ions then oxidise the ethandioate ions to form carbon dioxide and regenerate the Mn²⁺ ions. 

2Mn³⁺    +   C₂O₄^2—     =   2CO₂    +   2Mn²⁺

A plot of concentration of reactants vs time looks something like this:

The reaction rate is given by the gradient to the curve at each concentration.

As the catalyst Mn²⁺ concentration increases, the reaction begins to speed up.

But eventually the concentration of the reactants falls and the rate slows down to the point where the reaction stops.