Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Physical Chemistry → Chemical kinetics ↓


Transition state theory


Transition state theory (TST) is a model in chemical kinetics that helps explain how chemical reactions occur. It provides a framework for understanding at what rate and under what conditions reactions occur. This theory is particularly useful in understanding reaction rates and the factors that affect them. TST is also known as activated complex theory.

Basic concepts of transition state theory

The basic premise of TST is that, for a chemical reaction to occur, reactant molecules must pass through a high-energy state known as a "transition state" or "activated complex." This transition state represents a point of maximum energy through which the system must pass during the transition from reactants to products.

Energetic landscape of the reaction

Imagine a chemical reaction as a trip on a road. The reactants start at one point and have to cross a hill (the transition state) before they can reach the products on the other side. The highest point on this hill is the transition state, and the energy required to reach this peak is called the activation energy.

Reactants ---|----(Transition State)----|--- Products
^^^ Activation Energy

In this scenario, energy is required to move from the reactants to the transition state, and energy is released as the system moves from the transition state to the products.

Important terms related to transition state theory:

  • Activation Energy ((E_a)): The minimum energy required to form a transition state from reactants.
  • Activated complex: Transient configuration of atoms formed in the transition state.

The role of temperature

According to TST, temperature plays an important role in reaction rates. As the temperature increases, the kinetic energy of molecules also increases. This means that more molecules have enough energy to reach the transition state, which increases the reaction rate.

The Arrhenius equation is often used to express this dependence:

k = A * e^(-Ea/RT)

Where:

  • (k) is the rate constant of the reaction.
  • (A) is the pre-exponential factor, often related to the frequency of collisions.
  • (E_a) is the activation energy.
  • (R) is the gas constant.
  • (T) is the temperature in Kelvin.

Visualization of transition state theory

To make this concept more clear, here is a simplified illustration of the energy profile for a hypothetical exothermic reaction:

Reactants Transition state Products (E_A)

This energy profile shows the increase in energy required to reach the transition state and the energy released during the transformation of the system into products.

Reaction coordinates

The concept of the reaction coordinate is important in TST. It represents a mathematical pathway that transitions from reactants to products. As we move along this reaction coordinate, we pass through the reactant state, the transition state, and the product state. The exact position of the transition state on this pathway is where the potential energy reaches a maximum.

Example: Reaction of hydrogen and iodine

A classic example often used to illustrate TST is the reaction between hydrogen and iodine:

H 2 + I 2 → 2HI

In this reaction, hydrogen and iodine molecules must collide with sufficient energy to form the activated complex. This complex has characteristics of both H 2 + I 2 and HI, which represents a higher energy state. If sufficient energy is supplied, the activated complex will proceed to form the product HI.

Factors affecting transition state and rate

Several factors affect the transition state and the overall reaction rate:

  • Nature of the reactants: Reactants with stronger bonds or more complex structures often have higher activation energies and more stable transition states.
  • Concentration: Higher concentrations of reactants can lead to more frequent successful collisions, leading to the formation of the transition state.
  • Presence of a catalyst: Catalysts lower the activation energy by providing an alternative mechanism or pathway with a different transition state or states.

Example of catalytic effect

A good way to represent the effect of a catalyst is through a diagram that shows the effect on the activation energy:

Reactants Products without catalyst with catalyst

The dashed line shows the potential energy curve when the catalyst is used. It shows that the activation energy decreases, causing the reaction to proceed more quickly.

Limitations of transition state theory

While TST provides a useful framework, it also has its limitations. These include:

  • Assumption of thermodynamic equilibrium: One of the key assumptions is that there is a quasi-equilibrium between the reactants and the activated complex. Real-world reactions may not always satisfy this criterion.
  • Over-simplified models: TST often reduces complex multi-step reactions to simpler pathways. In reality, reactions may involve multiple transition states and intermediates.

Despite these limitations, the TST remains an important theoretical model in chemical kinetics, widely used in research and education.

Conclusion

Transition state theory provides profound insight into how chemical reactions proceed. By focusing on the process of reaching and overcoming the transition state, it gives chemists the tools to manipulate and predict reaction rates. From the role of temperature to the effects of catalysts and concentrations, TST remains an essential model for understanding and optimizing chemical reactions. As research and technology advance, further refinements to this theory continue, opening the door to new applications and a better understanding of the molecular dynamics that govern reactions.


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Transition state theory

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