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 ↓


Collision theory


Collision theory is a fundamental concept in chemical kinetics, a branch of physical chemistry concerned with understanding the rates of chemical reactions. This theory provides a framework for predicting how different variables affect the speed of a reaction by focusing on the interactions between reactant particles. By examining these interactions, scientists can gain information about the conditions necessary for a reaction to occur. In this detailed discussion, we will explore the principles of collision theory, the factors that affect reaction rates, and illustrative examples using visual diagrams and textual explanations.

Fundamentals of collision theory

The core of collision theory states that for a reaction to occur, the reactant particles – that is, atoms, molecules or ions – must collide. However, not every collision results in a chemical change. The following prerequisites must be met for a successful reaction:

  1. Collision frequency: The more collisions there are between reactant particles, the greater the probability of a reaction. Higher concentrations of reactants increase the probability of collisions.
  2. Proper orientation: The colliding particles must be correctly oriented relative to each other in order to form a new bond. If the orientation is wrong, the particles will simply collide with each other without reacting.
  3. Sufficient energy: The kinetic energy of the colliding particles must be equal to or greater than the activation energy (Ea) needed to break the bonds and initiate the reaction. This energy threshold is important to overcome the energy barrier for the reaction.

Activation energy

Activation energy is the minimum energy required for a chemical reaction to occur. It is like the barrier that reactant particles have to cross to turn into products. In graphical terms, if you look at a reaction on an energy diagram, activation energy is the peak between reactants and products.

        Reactants --( E_a )--> Products

where E_a is the activation energy.

Factors affecting the reaction rate from the perspective of collision theory

Various factors change the rate of chemical reactions by affecting the frequency, orientation or energy of collisions. Let us look at these factors in detail:

1. Concentration of reactants

Increasing the concentration of reactants increases the number of particles per unit volume, which increases the frequency of collisions. As a result, this increases the probability of an effective collision, speeding up the reaction.

2H_2 + O_2 → 2H_2O - Increasing the concentration of hydrogen gas or oxygen will increase collisions and cause water to form more quickly.

H2 O2

Increasing the concentration results in more blue and red circles, indicating more frequent collisions of H2 and O2.

2. Temperature

Increasing the temperature of the reaction mixture increases the kinetic energy of the reactant particles. Higher kinetic energy means that the particles collide more forcefully and more frequently, thereby exceeding the activation energy - a key requirement for reactions to proceed.

Reactions such as the decomposition of hydrogen peroxide: 
2H_2O_2 → 2H_2O + O_2

These events occur more rapidly at higher temperatures because of the increase in effective collisions.

H2O2 O2 High temperature

3. Pressure

While pressure mostly affects reactions involving gaseous reactants, increasing the pressure decreases the volume, effectively increasing the concentration. This results in collisions occurring more frequently.

For example, in the synthesis of ammonia via the Haber process:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

High pressure favors the forward reaction because of the increased frequency of collisions between nitrogen and hydrogen molecules.

N2 H2

Compression causes collisions between N2 and H2 to occur more frequently.

4. Catalyst

Catalysts are substances that increase the rate of a reaction without causing permanent changes in the substance itself. They work by lowering the activation energy of the reaction, increasing the number of particles that have enough energy to react.

Consider the decomposition of hydrogen peroxide catalyzed by iodide ions:

2H_2O_2(aq) → 2H_2O(l) + O_2(g)

The presence of iodide ions facilitates this process at a lower activation energy.

        Without activator:
        Reactants -(high E_a)-> Products
        With Activator:
        Reactants -(low E_a)-> Products

5. Surface area

For solid reactants, increasing the surface area leads to more collisions. Finely powdered solids react more quickly than their bulk counterparts because there is more area available for the reaction to take place.

The reaction between calcium carbonate and hydrochloric acid is a classic example of this:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) CaCO3 part CaCO3 Powder

As shown, the large surface area provided by powdered calcium carbonate leads to rapid reactions.

Conclusion

Collision theory provides a comprehensive view of the challenges and conditions necessary for successful chemical reactions. By emphasizing the importance of collision frequency, proper orientation, and sufficient energy, this theory contributes to our understanding of how and why reactions proceed at their respective rates. Adjustments in concentration, temperature, pressure, presence of catalysts, and surface area can all significantly affect reaction velocities by changing the probability and nature of particle encounters. Each factor plays a different role in aligning with the principles of collision theory, making it an important aspect of chemical kinetics and physical chemistry. Through this detailed investigation, we gain a more thorough understanding of the dynamics and complexities present in chemical processes.


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Collision theory

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