4.2.9

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+.
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1

Principles of Science I

1.1

Structure & Bonding

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1.2

Properties of Substances

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1.3

Cell Structure & Function

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Cell Specialisation

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1.5

Tissue Structure & Function

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Working with Waves

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1.7

Waves in Communication

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2

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Titration

2.1.1pH Curves & Titrations2.1.2pH Curves & Titrations 22.1.3Buffer Solutions
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Colorimetry

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2.3

Chromatography

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3

Science Investigation Skills

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Scientific Processes

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3.2

Data Handling & Analysis

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3.3

Enzymes in Action

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3.4

Diffusion

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Plants & Their Environment

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3.6

Energy Content in Fuels

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3.7

Electrical Circuits

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4

Principles of Science II

4.1

Extracting Elements

4.1.1Atom Economy
4.2

Relating Properties to use of Substances

4.2.1Group 2 Chemistry4.2.2End of Topic Test - Group 24.2.3Electrolysis4.2.4Electrolysis Examples4.2.5Extracting Metals4.2.6Transition Metals4.2.7Variable Oxidation States4.2.8Heterogeneous Catalysts4.2.9Homogeneous Catalysts
4.3

Organic Chemistry

4.3.1Naming Compounds4.3.2Functional Groups4.3.3Isomerism4.3.4Hydrocarbons4.3.5Structure of Alkanes4.3.6Boiling Points of Alkanes4.3.7Alkenes4.3.8Curly Arrows & Mechanisms4.3.9Cracking4.3.10Combustion4.3.11Free Radical Substitution of Alkanes4.3.12Electrophilic Addition of Alkenes
4.4

Energy Changes in Industry

4.4.1The Kelvin Scale4.4.2Enthalpy Changes4.4.3Calorimetry4.4.4Bond Enthalpies4.4.5Hess' Law
4.5

The Circulatory System

4.5.1The Circulatory System4.5.2Blood Vessels4.5.3Blood Transfusion & the ABO Rhesus System4.5.4The Heart4.5.5The Cardiac Cycle4.5.6Cardiac Output4.5.7Coordination of Heart Action4.5.8Heart Dissection4.5.9Controlling Heart Rate4.5.10Electrocardiograms4.5.11Cardiovascular Disease4.5.12Investigating Heart Rates
4.6

Ventilation & Gas Exchange

4.6.1Human Gas Exchange4.6.2Ventilation4.6.3Measuring Gas Exchange4.6.4Controlling Ventilation Rate4.6.5Lung Disease4.6.6Lung Disease Data
4.7

Urinary System

4.7.1Kidneys & Control of Water Balance4.7.2Kidneys & Osmoregulation4.7.3Kidneys & Blood Pressure4.7.4Urine4.7.5Kidney Failure & Urinalysis4.7.6Dialysis4.7.7Transplants
4.8

Cell Transport

4.8.1Cell Membrane Structure4.8.2Diffusion4.8.3Osmosis4.8.4Active Transport
4.9

Thermal Physics

4.9.1Power & Efficiency4.9.2Work & Energy4.9.3Conservation of Energy4.9.4Pressure4.9.5First Law of Thermodynamics4.9.6Second Law of Thermodynamics4.9.7Heat Engines, Heat Pumps & Refrigerators4.9.8Non-Flow Processes4.9.9p-V Diagrams4.9.10Ideal Gases4.9.11Ideal Gases 24.9.12Thermal Energy Transfer4.9.13Thermal Energy Transfer Experiments
4.10

Materials

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4.11

Fluids

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5

Contemporary Issues in Science

5.1

Contemporary Issues in Science

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5.2

Analysing Scientific Information

5.2.1Validity5.2.2Peer Review

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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Heterogeneous Catalysts

Naming Compounds