1.9.7

Homogeneous Catalysts

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Homogeneous Catalysis

A homogeneous catalyst is a catalyst which is in the same phase as the reactant. For example, the catalyst is in aqueous solution, and the reactants are in the same aqueous solution.

How do catalysts work?

How do catalysts work?

  • Catalysts work by changing oxidation state.
    • This is why transition metals are often excellent catalysts: they can change oxidation state very easily.
  • Catalysts will donate or accept electrons to oxidise or reduce species in a reaction.
    • By acting as a go-between in this exchange, they speed up the reaction.
  • Catalysts are always regenerated, so if it donates electrons to a reactant, it must accept electrons from the other.
Homogeneous catalysis

Homogeneous catalysis

  • A homogeneous catalyst is one which does all this in the same phase as the reactants.
    • It will be either reduced or oxidised by one reactant and then the reverse by the other.
  • The enthalpy profile for this is shown on the next slide.
Diagram
Key points

Key points

  • The enthalpy profile for the catalysed reaction has two peaks.
  • This is because there are two steps to the catalysed reaction:
    • First, the catalyst is either oxidised or reduced.
    • Then, the reverse happens.
  • Both of these steps have a lower activation energy than the uncatalysed reaction, so this pathway is faster.

Autocatalysis

Autocatalysis is the name given to a reaction in which a product is the catalyst.

Effect of autocatalysis

Effect of autocatalysis

  • An autocatalytic reaction will usually speed up over time.
    • The concentrations of the reactants decrease, so you might expect a reduced rate.
    • But the production of the catalyst outweighs this, and so the rate increases.
An example

An example

  • Mn2+ is autocatalytic for the reaction between C2O42- ions and MnO4- ions.
  • The reaction equation is:
    • 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g)
Catalytic action

Catalytic action

  • Mn2+ is catalytic because it reacts with MnO4- to make Mn3+.
    • 4Mn2+(aq) + MnO4-(aq) + 8H+(aq) → 5Mn3+(aq) + 4H2O(l)
  • The Mn3+ then reacts with the C2O42-:
    • 2Mn3+(aq) + C2O42-(aq) → Mn2+(aq) + 2CO2(g)
    • The Mn2+ is regenerated in this step.

Catalytic Fe2+

Fe2+ is a catalyst for the reaction between S2O82- and I-.

S<sub>2</sub>O<sub>8</sub><sup>2-</sup> and I<sup>-</sup>

S2O82- and I-

  • In the reaction between the peroxodisulfate ion (S2O82-) and the iodide ion (I-), Fe2+ can act as a homogeneous catalyst.
  • The uncatalysed reaction is:
    • S2O82-(aq) + 2I-(aq) → I2(aq) + 2SO42-(aq)
    • This reaction has a high activation energy and is slow because both ions are negatively charged.
    • They repel each other, so making them collide takes a lot of energy.
The role of Fe<sup>2+</sup>

The role of Fe2+

  • Fe2+ is a catalyst for this reaction. It is positively charged, so doesn’t have any problem with colliding with an anion.
  • The first step is its oxidation by reaction with S2O82-(aq).
  • The equation is:
    • S2O82-(aq) + 2Fe2+(aq) → 2SO42-(aq) + 2Fe3+(aq)
  • The second step is reduction of Fe3+(aq) by the iodide.
    • 2Fe3+(aq) + 2I-(aq) → 2Fe2+(aq) + I2(aq)
  • This regenerates the Fe2+.
Jump to other topics
1

Physical Chemistry

1.1

Atoms, Molecules & Stoichiometry

1.1.1Relative Masses1.1.2The Mole1.1.3Mass Spectrometry1.1.4Empirical & Molecular Formulae1.1.5Balanced Equations1.1.6Molar Volume1.1.7End of Topic Test - Amount of Substance
1.2

Atomic Structure

1.2.1Fundamental Particles1.2.2Isotopes & Mass Number1.2.3Electron Shells, Sub-Shells & Orbitals1.2.4Electron Configuration1.2.5Ionisation Energy1.2.6Factors Affecting Ionisation Energies1.2.7Trends of Ionisation1.2.8Specific Impacts on Ionisation Energies1.2.9Electron Affinity1.2.10End of Topic Test - Atomic Structure1.2.11A-A* (AO2/3) - Atomic Structure
1.3

Chemical Bonding

1.3.1Ionic Bonding1.3.2Covalent & Dative Bonding1.3.3Shapes of Molecules1.3.4Intermolecular Forces1.3.5Intermolecular Forces 21.3.6Electronegativity1.3.7Bond Length, Bond Energy, & Bond Polarity1.3.8Metallic Bonding1.3.9Physical Properties1.3.10End of Topic Test - Bonding1.3.11A-A* (AO2/3) - Bonding
1.4

States of Matter

1.4.1Simple Covalent Molecules1.4.2The Ideal Gas Equation1.4.3States of Matter1.4.4Particle Diagrams1.4.5The Liquid State1.4.6Giant Ionic Lattices1.4.7Giant Metallic Lattices1.4.8Giant Covalent Structures1.4.9Graphene & Fullerene1.4.10Recycling
1.5

Chemical Energetics

1.5.1Calorimetry1.5.2Bond Enthalpies1.5.3Enthalpy Changes1.5.4Hess' Law1.5.5Born-Haber Cycles1.5.6Born-Haber Cycle Calculations1.5.7Entropy1.5.8Free Energy1.5.9End of Topic Test - Energetics
1.6

Electrochemistry

1.6.1Electrochemical Cells1.6.2Electrochemical Series1.6.3Commercial Application1.6.4Nernst Equation1.6.5Redox
1.7

Equilibria

1.7.1Dynamic Equilibrium & Le Chatelier1.7.2Kc1.7.3Kp1.7.4pH1.7.5The Ionic Product of Water1.7.6Weak Acids & Bases1.7.7Introduction to Solubility Equilibria1.7.8Solubility Equilibria Calculations1.7.9Free Energy of Dissolution1.7.10pH and Solubility1.7.11Common-Ion Effect1.7.12End of Topic Test - Kp & Electrochemistry1.7.13A-A* (AO2/3) - Electrochemical Cells
1.8

Partition Coefficient

1.8.1Kpc
1.9

Reaction Kinetics

1.9.1Collision Theory1.9.2Orders, Rate Constants & Equations1.9.3Rate Graphs1.9.4Rate Determining Step1.9.5Maxwell-Boltzmann Distribution1.9.6Catalysts1.9.7Homogeneous Catalysts1.9.8Heterogeneous Catalysts1.9.9End of Topic Test - Kinetics1.9.10End of Topic Test - Rate Equations1.9.11A-A* (AO2/3) - Rate Equations
2

Inorganic Chemistry

2.1

The Periodic Table

2.1.1The Periodic Table2.1.2Trends in the Periodic Table2.1.3End of Topic Test - Periodicity & Trends2.1.4Ceramics2.1.5Period 3 Elements & Oxides2.1.6Period 3 Oxides2.1.7End of Topic Test - Period 3
2.2

Group 2

2.2.1Group 2 Chemistry
2.3

Group 17

2.3.1Introduction to Halogens2.3.2Reactions of Halogens2.3.3Chlorine & Chlorate(I)2.3.4End of Topic Test - Group 172.3.5A-A* (AO2/3) - Periodicity, Group 2 & 17
2.4

Transition Metals

2.4.1General Properties2.4.2Titrations2.4.3Shapes of Complex Ions2.4.4Variable Oxidation States2.4.5Ligands2.4.6Colours of Ions2.4.7Stereoisomerism of Complex Ions2.4.8Cisplatin2.4.9Ligand Substitutions & Kstab2.4.10End of Topic Test - Transition Metals2.4.11A-A* (AO2/3) - Transition Metals
2.5

Nitrogen & Sulfur

2.5.1Nitrogen & Ammonia2.5.2Formulated vs Manure Fertilisers2.5.3Nitrogen Containing Fertilisers2.5.4Fertilizers & Eutrophication2.5.5Common Atmospheric Pollutants & Their Properties2.5.6Catalytic Converters2.5.7Atmospheric Oxides
3

Organic Chemistry & Analysis

3.1

Introduction to Organic Chemistry

3.1.1Naming Conventions3.1.2Mechanism3.1.3Isomerism3.1.4End of Topic Test - Introduction to Organic Chem
3.2

Hydrocarbons

3.2.1Fractional Distillation3.2.2Cracking3.2.3Combustion3.2.4Chlorination3.2.5End of Topic Test - Alkanes3.2.6Introduction to Alkenes3.2.7Reactions of Alkenes3.2.8Polymerisation Reactions3.2.9End of Topic Test - Alkenes3.2.10Arenes3.2.11Evidence for Structure of Arenes3.2.12Reactions of Benzene3.2.13End of Topic Test -Arenes
3.3

Halogen Derivatives

3.3.1Properties of Haloalkanes3.3.2Substitution Reactions of Haloalkanes3.3.3Environmental Impacts of Haloalkanes3.3.4End of Topic Test - Halogenoalkanes
3.4

Hydroxy Compounds

3.4.1Types of Alcohol3.4.2Oxidation 13.4.3Oxidation 23.4.4Elimination3.4.5Acids & Esters3.4.6Acyl Chlorides & Acylation3.4.7Phenols & Their Reactions3.4.8End of Topic Test - Alcohols
3.5

Carbonyl Compounds

3.5.1Carbonyls3.5.2Hydrogen Bonding & Carbonyls3.5.3Aldehydes & Ketones3.5.4Tri-Iodomethane Reaction3.5.5End of Topic Test -Carbonyls
3.6

Carboxylic Acids & Derivatives

3.6.1Properties of Carboxylic Acids3.6.2Esters3.6.3Acyl Chlorides3.6.4End of Topic Test - Carboxylic Acids
3.7

Nitrogen Compounds

3.7.1Nature & Preparation of Amines3.7.2Basicity & Nucleophilicity3.7.3Amides3.7.4Reactions of Amino Acids3.7.5Amino Acids
3.8

Polymerisation

3.8.1Properties of Addition Polymers3.8.2Condensation Polymerisation3.8.3Effects of Side-Chains on Polymers3.8.4Hydrolysis of Ester & Amide Groups3.8.5Proteins3.8.6DNA3.8.7Adhesive & Conducting Polymers3.8.8End of Topic Test - Amines & Proteins
3.9

Analytical Techniques

3.9.1Chromatography3.9.2High-Performance Liquid Chromatography3.9.3Gas Chromatography3.9.4IR Spectroscopy3.9.5Uses of IR Spectroscopy3.9.6Mass Spectrometry3.9.7Mass Spectrometry Analysis3.9.8Nuclear Magnetic Resonance3.9.9Carbon-13 NMR3.9.10Proton NMR I3.9.11Proton NMR II3.9.12End of Topic Test - Analytical Techniques3.9.13A-A* (AO2/3) - Analytical Techniques
3.10

Organic Synthesis

3.10.1Chirality & Drug Synthesis3.10.2Organic Synthesis3.10.3Examples of Synthetic Routes3.10.4End of Topic Test - Organic Synthesis

Practice questions on Homogeneous Catalysts

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which of these best describes a homogeneous catalyst?Multiple choice
  2. 2
    Which of the following best describes autocatalysis?Multiple choice
  3. 3
    What does the catalytic activity of Mn<sup>2+</sup> involve?Multiple choice
  4. 4
    Why is the uncatalysed reaction between S<sub>2</sub>O<sub>8</sub><sup>2-</sup> and I<sup>-</sup> slow?Multiple choice
  5. 5
    First step of the catalysis by Fe<sup>2+</sup>:Fill in the list
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Catalysts

Heterogeneous Catalysts