Homogeneous Catalysts

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?

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

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.

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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

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

Mn²⁺ is autocatalytic for the reaction between C₂O₄^2- ions and MnO₄^- ions.
The reaction equation is:
- 2MnO₄^-_(aq) + 16H⁺_(aq) + 5C₂O₄^2-_(aq) → 2Mn²⁺_(aq) + 8H₂O_(l) + 10CO_2(g)

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 C₂O₄^2-:
- 2Mn³⁺_(aq) + C₂O₄^2-_(aq) → Mn²⁺_(aq) + 2CO_2(g)
- The Mn²⁺ is regenerated in this step.

Catalytic Fe²⁺

Fe²⁺ is a catalyst for the reaction between S₂O₈^2- and I^-.

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

Illustrative background for S<sub>2</sub>O<sub>8</sub><sup>2-</sup> and I<sup>-</sup> ?? "content

S₂O₈^2- and I^-

In the reaction between the peroxodisulfate ion (S₂O₈^2-) and the iodide ion (I^-), Fe²⁺ can act as a homogeneous catalyst.
The uncatalysed reaction is:
- S₂O₈^2-_(aq) + 2I^-_(aq) → I_2(aq) + 2SO₄^2-_(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.

Illustrative background for The role of Fe<sup>2+</sup>

Illustrative background for The role of Fe<sup>2+</sup> ?? "content

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 S₂O₈^2-_(aq).
The equation is:
- S₂O₈^2-_(aq) + 2Fe²⁺_(aq) → 2SO₄^2-_(aq) + 2Fe³⁺_(aq)
The second step is reduction of Fe³⁺_(aq) by the iodide.
- 2Fe³⁺_(aq) + 2I^-_(aq) → 2Fe²⁺_(aq) + I_2(aq)
This regenerates the Fe²⁺.

1Principles of Science I

1.1Structure & Bonding

1.1.1Atomic Model

1.1.2Electron Shells, Sub-Shells & Orbitals

1.1.3Ionic Bonding

1.1.4Representing Ionic Bonds

1.1.5Covalent Bonding

1.1.6Representing Covalent Bonds

1.1.7Metallic Bonding

1.1.8Intermolecular Forces

1.1.9Intermolecular Forces 2

1.1.10End of Topic Test - Bonding

1.1.11Relative Masses

1.1.12The Mole

1.1.13Molar Calculations

1.1.14Molar Calculations 2

1.1.15Empirical & Molecular Formulae

1.1.16Balanced Equations

1.1.17Percentage Yield

1.1.18End of Topic Test - Amount of Substance

1.2Properties of Substances

1.2.1The Periodic Table

1.2.2Ionisation Energy

1.2.3Factors Affecting Ionisation Energies

1.2.4Trends of Ionisation

1.2.5Trends in the Periodic Table

1.2.6Polarity

1.2.7Metals & Non-Metals

1.2.8Alkali Metals

1.2.9Alkaline Earth Metals

1.2.10Reactivity of Alkaline Earth Metals

1.2.11Redox

1.2.12Transition Metals

1.2.13Redox Reactions of Transition Metals

1.3Cell Structure & Function

1.3.1Introduction to Cells

1.3.2Prokaryotes

1.3.3Organelles of Eukaryotic Cells

1.3.4Organelles of Eukaryotic Cells 2

1.3.5Organelles in Plant Cells

1.3.6Microscopy

1.3.7End of Topic Test - Cell Structure

1.4Cell Specialisation

1.4.1Cell Specialisation

1.4.2Blood Cells

1.4.3Plant Cells

1.4.4Gametes

1.5Tissue Structure & Function

1.5.1Human Gas Exchange

1.5.2Blood Vessels

1.5.3Atherosclerosis

1.5.4Skeletal Muscle

1.5.5Slow & Fast Twitch Fibres

1.5.6Neurones

1.5.7Speed of Transmission

1.5.8Action Potentials

1.5.9End of Topic Test - Neurones & Action Potentials

1.5.10Synapses

1.5.11Types of Synapse

1.5.12Medical Application

1.5.13End of Topic Test - Synapses

1.5.14Chemical Brain Imbalances

1.5.15Effect of Drugs on the Brain

1.6Working with Waves

1.6.1Progressive Waves

1.6.2Wave Speed & Phase Difference

1.6.3Longitudinal & Transverse Waves

1.6.4Stationary Waves

1.6.5Stationary Waves 2

1.6.6Applications of Stationary Waves

1.6.7Interference

1.6.8Diffraction

1.6.9Diffraction Gratings

1.6.10Application of Diffraction Gratings

1.7Waves in Communication

1.7.1Refraction at a Plane Surface

1.7.2Internal Reflection & Fibre Optics

1.7.3Applications of Total Internal Reflection

1.7.4Properties

1.7.5Gamma, X-Ray & UV Light

1.7.6Visible Light

1.7.7Infrared, Microwave & Radio Waves

1.7.8Intensity

1.7.9Digital & Analogue Signals

1.7.10Communication Channels

2Practical Scientific Procedures and Techniques

2.1Titration

2.1.1pH Curves & Titrations

2.1.2pH Curves & Titrations 2

2.1.3Buffer Solutions

2.2Colorimetry

2.2.1Measuring Rates

2.2.2Beer-Lambert Law

2.3Chromatography

2.3.1Overview of Chromatography

2.3.2Thin-Layer Chromatography

2.3.3Column Chromatography

2.3.4Gas Chromatography

3Science Investigation Skills

3.1Scientific Processes

3.1.1Aims, Hypotheses & Sampling

3.1.2Pilot Studies & Design

3.1.3Ethics

3.1.4Variables & Control

3.1.5Demand Characteristics & Investigator Effects

3.1.6Features of Science

3.1.7Scientific Report

3.1.8Scientific Report 2

3.2Data Handling & Analysis

3.2.1Types of Data

3.2.2Descriptive Statistics

3.2.3Presentation & Display of Data

3.3Enzymes in Action

3.3.1Overview of Protein Structure

3.3.2Enzymes

3.3.3Factors Affecting Enzyme Activity

3.3.4Enzyme-Controlled Reactions

3.4Diffusion

3.4.1Diffusion

3.5Plants & Their Environment

3.5.1Factors Affecting Plant Growth

3.5.2Measuring Number & Distribution of Species

3.6Energy Content in Fuels

3.6.1Fractions

3.6.2Burning Hydrocarbons

3.6.3Combustion

3.6.4Energy

3.6.5Energy at Home

3.6.6Calorimetry

3.7Electrical Circuits

3.7.1Circuit Diagrams & Components

3.7.2Diodes, LDRs & Thermistors

3.7.3Resistance

3.7.4Electrical Work

3.7.5Ohm's Law

3.7.6Domestic Uses of Electricity

3.7.7Fuses

4Principles of Science II

4.1Extracting Elements

4.1.1Atom Economy

4.2Relating Properties to use of Substances

4.2.1Group 2 Chemistry

4.2.2End of Topic Test - Group 2

4.2.3Electrolysis

4.2.4Electrolysis Examples

4.2.5Extracting Metals

4.2.6Transition Metals

4.2.7Variable Oxidation States

4.2.8Heterogeneous Catalysts

4.2.9Homogeneous Catalysts

4.3Organic Chemistry

4.3.1Naming Compounds

4.3.2Functional Groups

4.3.3Isomerism

4.3.4Hydrocarbons

4.3.5Structure of Alkanes

4.3.6Boiling Points of Alkanes

4.3.7Alkenes

4.3.8Curly Arrows & Mechanisms

4.3.9Cracking

4.3.10Combustion

4.3.11Free Radical Substitution of Alkanes

4.3.12Electrophilic Addition of Alkenes

4.4Energy Changes in Industry

4.4.1The Kelvin Scale

4.4.2Enthalpy Changes

4.4.3Calorimetry

4.4.4Bond Enthalpies

4.4.5Hess' Law

4.5The Circulatory System

4.5.1The Circulatory System

4.5.2Blood Vessels

4.5.3Blood Transfusion & the ABO Rhesus System

4.5.4The Heart

4.5.5The Cardiac Cycle

4.5.6Cardiac Output

4.5.7Coordination of Heart Action

4.5.8Heart Dissection

4.5.9Controlling Heart Rate

4.5.10Electrocardiograms

4.5.11Cardiovascular Disease

4.5.12Investigating Heart Rates

4.6Ventilation & Gas Exchange

4.6.1Human Gas Exchange

4.6.2Ventilation

4.6.3Measuring Gas Exchange

4.6.4Controlling Ventilation Rate

4.6.5Lung Disease

4.6.6Lung Disease Data

4.7Urinary System

4.7.1Kidneys & Control of Water Balance

4.7.2Kidneys & Osmoregulation

4.7.3Kidneys & Blood Pressure

4.7.4Urine

4.7.5Kidney Failure & Urinalysis

4.7.6Dialysis

4.7.7Transplants

4.8Cell Transport

4.8.1Cell Membrane Structure

4.8.2Diffusion

4.8.3Osmosis

4.8.4Active Transport

4.9Thermal Physics

4.9.1Power & Efficiency

4.9.2Work & Energy

4.9.3Conservation of Energy

4.9.4Pressure

4.9.5First Law of Thermodynamics

4.9.6Second Law of Thermodynamics

4.9.7Heat Engines, Heat Pumps & Refrigerators

4.9.8Non-Flow Processes

4.9.9p-V Diagrams

4.9.10Ideal Gases

4.9.11Ideal Gases 2

4.9.12Thermal Energy Transfer

4.9.13Thermal Energy Transfer Experiments

4.10Materials

4.10.1Density

4.10.2Stress & Strain

4.10.3Properties of Materials

4.10.4Elasticity

4.10.5Plasticity

4.10.6Energy in Materials

4.10.7Young Modulus

4.11Fluids

4.11.1Fluid Flow

4.11.2Viscosity

4.11.3Density

4.11.4Continuity Equation

4.11.5Bernoulli's Equation

4.11.6Non-Newtonian Fluids

5Contemporary Issues in Science

5.1Contemporary Issues in Science

5.1.1Sustainability

5.1.2Genetic Engineering

5.1.3Uses of Genetic Modification

5.1.4Stem Cell Therapy

5.1.5Cloning

5.1.6Cloning 2

5.1.7Nanotechnology

5.1.8Ecosystems

5.2Analysing Scientific Information

5.2.1Validity

5.2.2Peer Review

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