Homogenous catalysts

What are homogenous catalysts?

a catalyst in the same phase as the reactants.

Usually aqueous/gaseous reactions.

These are much less common in industry, due to the difficulty in separatign products and catalysts

They form intermediates durign a reaction

reaction between iodide ions, I- (aq) with peroxodisulfate ions, S2O8 2-

• The iodide ions are oxidised to form I2

• S₂O₈^2- + 2I^-  → I₂ + 2SO₄^2-

• ^{the reaction is very slow. Both reactants are negatively charged so repel each other (high Ea)}

• ^{Fe 2+ is added as a catalsyt ot speed up rate of reaction}

mechanism for reaction of iodide ions with S2O82-

1) S2O8 2- (aq) + 2Fe2+ (aq)—>2SO4 2- (aaq) + 2Fe3+ (aq)

The reactants are oppositely charged, so attract, rather than repel.

Fe2+ is oxidised to form Fe3+

2) 2I- (aq) + 2Fe3+ —>I2(aq) + 2Fe2+ (aq)

Fe 3+ oxidises the I- to form iodine

Fe2+ has been reduced back to Fe3+ - the catalyst is regenerated

• As all of the reactants and products and the catalyst are in the aqueous phase this reaction is an example of homogeneous catalysis

how can Fe3+ be used as a catalyst

1) 2I^- + 2Fe³⁺  →  I₂   +  2Fe²⁺  

2) S₂O₈^2- + 2Fe²⁺  →  2SO₄^2-   + 2Fe³⁺  

reverse order of equations

manganate (MnO4-) reaction with ethanedioate ions, C2O42-

• MnO4- ions oxidise the ethanedioate ions. This forms carbon dioxide and water, in acidic conditions

• 2MnO4-(aq) + 16H+ (aq) + 5C2O4 2- 9aq)—> 2Mn2+ (aq) + 8H2O (l) + 10 CO2 (g)

• The reaction is used in a titration to confirm the cocnentraiton of KMnO4 - potassium manganate Vll

Activation energy of reaction btw MnO4- and C2O4 2-

• The reactants both have negative charges, so they repel eachother. Ea is high and rate of reaction is slow.

• The catalyst for this reaction is Mn2+, one of the products

• As the titration proceeds, teh rate of reaction increases

reaction mechanism for reaction between manganate(VII) ions and oxalate (ethandioate) ions

• As more manganese(II) is formed the reaction speeds up

• Like to the role of iron(II) in the previous section, manganese(II) ions take part in a redox cycle between two different oxidation states (+2 →  +3 → +2)

4Mn²⁺ (aq)  +  MnO₄^– (aq) + 8H⁺ (aq)   →   5Mn³⁺ (aq)  + 4H₂O (aq)

2Mn³⁺ (aq)  +  C₂O₄^2- (aq)  →  2CO₂ (g) +  2Mn²⁺ (aq)

• The manganese(II) is not present in the beginning of the reaction, but as it is formed is speeds up the reaction and is re-generated during the redox cycle

• This reaction is easily followed on a colorimeter as the rate at which the purple manganate(VII) ion is consumed accelerates with time

autocatalysis meaning

the product of the reaction acts as a catalyst for the reaction