Transition Metal Redox and Titrations

Transition metals have variable oxidation states, making them highly effective in redox chemistry. In this lesson, we cover their roles as catalysts in industrial and biological systems and study how redox titrations are used to determine iron concentration quantitatively.

Transition Metals as Catalysts

Transition metals make excellent catalysts because they can easily donate and accept electrons due to their variable oxidation states, which provides alternative reaction pathways with lower activation energies.

1. Heterogeneous Catalysis

In heterogeneous catalysis, reactants are adsorbed onto active sites on the solid catalyst surface. The bonds in the reactant molecules are weakened, bringing the reactants close together in the correct orientation. After the reaction, the product molecules desorb from the surface.

Key industrial examples include:

• Iron (\(\text{Fe}\)) in the Haber Process: \(\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \xrightarrow{\text{Fe(s)}} 2\text{NH}_3(\text{g})\)
• Vanadium(V) oxide (\(\text{V}_2\text{O}_5\)) in the Contact Process: \(2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \xrightarrow{\text{V}_2\text{O}_5\text{(s)}} 2\text{SO}_3(\text{g})\)

2. Homogeneous Catalysis

Homogeneous catalysts act by changing oxidation state. A classic exam example is the reaction between peroxodisulfate (\(\text{S}_2\text{O}_8^{2-}\)) and iodide (\(\text{I}^-\)) ions, catalyzed by iron(II) or iron(III) ions:

Without a catalyst, this reaction is extremely slow because it requires two negatively charged ions to collide. The electrostatic repulsion between the anions creates a very high activation energy.

Iron ions catalyze the reaction by providing a two-step mechanism that avoids anion-anion collisions:

• Step 1: Iron(II) reduces peroxodisulfate to sulfate, becoming oxidised to iron(III): \[ \text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{Fe}^{2+}(\text{aq}) \rightarrow 2\text{SO}_4^{2-}(\text{aq}) + 2\text{Fe}^{3+}(\text{aq}) \]
• Step 2: The newly formed iron(III) then oxidises iodide ions to iodine, regenerating the iron(II) catalyst: \[ 2\text{Fe}^{3+}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{Fe}^{2+}(\text{aq}) + \text{I}_2(\text{aq}) \]

Because each step involves a collision between oppositely charged species (an anion and a cation), the activation energy for both steps is much lower, significantly increasing the rate of reaction.

Potassium Manganate(VII) Redox Titrations

Potassium manganate(VII), \(\text{KMnO}_4\), is a powerful oxidising agent used in redox titrations to find the concentration of reducing agents, most commonly iron(II) ions (\(\text{Fe}^{2+}\)).

Redox Half-Equations

In acidic conditions, the purple manganate(VII) ion (\(\text{MnO}_4^-\)) is reduced to the virtually colourless manganese(II) ion (\(\text{Mn}^{2+}\)):

Iron(II) is oxidised to iron(III):

Multiplying the iron half-equation by 5 and combining them yields the overall redox equation:

Titration Methodology

1. Fill the burette with a standard solution of potassium manganate(VII) (\(\text{KMnO}_4\)).
2. Pipette a known volume (typically \(25.0\text{ cm}^3\)) of the \(\text{Fe}^{2+}\) solution into a conical flask.
3. Acidify the conical flask by adding an excess of dilute sulfuric acid (\(\text{H}_2\text{SO}_4\)).
4. Titrate by adding the purple \(\text{KMnO}_4\) solution from the burette. The purple colour fades instantly upon entering the flask as \(\text{Mn}^{2+}\) forms.
5. End Point: The end point is reached when a single drop of \(\text{KMnO}_4\) remains unreacted, turning the solution in the flask permanently pale pink. No indicator is needed because \(\text{KMnO}_4\) is self-indicating.

Potassium Dichromate(VI) Redox Titrations

In some circumstances, potassium dichromate(VI), \(\text{K}_2\text{Cr}_2\text{O}_7\), is used as an oxidising agent instead of manganate. The reduction half-equation of the orange dichromate ion to green chromium(III) is:

Combining this with the iron(II) half-equation gives a reacting mole ratio of 1:6 for dichromate to iron(II):