Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → General chemistry → Chemical reactions ↓


Reaction energy profiles


Chemical reactions involve the conversion of reactant molecules into products. Understanding how energy changes during this conversion is important for predicting the behavior of chemical systems. Energy profiles provide information about the energy changes that occur during reactions and help visualize the steps in a reaction pathway. In this explanation, we will explore the importance of reaction energy profiles, describe their main features, and discuss their application in understanding chemical reactions.

Introduction to energy profiles

An energy profile is a graphical representation of the energy changes that occur during a chemical reaction. The profile typically plots the energy of the system on the vertical axis against the progress of the reaction on the horizontal axis. By examining the profile, we can understand the stability of the reactants and products and identify any intermediate states during the reaction.

Components of the energy profile

Reactants and products

In a chemical reaction, reactants are transformed into products. In terms of energy, reactants exist at a certain energy level, and when they turn into products, they either absorb or release energy. This energy change is represented in the energy profile:

Reactants --> Products

Activation energy

Activation energy is the minimum energy required to initiate a chemical reaction. It represents the energy barrier that must be crossed to convert reactants into products. In an energy profile, activation energy is represented as the height of the peak that the reactant molecules have to cross:


+--------------------
Energy 
|      ____ 
|     /    ___ Products 
|    /    
|   ____/ 
+--------------------
Progress of Reaction 
  Reactants

Transition state

The transition state, also known as the activated complex, is the point at which the system has the maximum energy on a reaction pathway. In the transition state, bonds in the reactants are in the process of breaking while new bonds in the products are being formed. It is represented by the topmost point on the energy profile.

Exothermic and endothermic reactions

The energy change of the reaction can be either exothermic or endothermic:

  • Exothermic reaction: Energy is released as heat. In the energy profile, the products have a lower energy level than the reactants, creating a downward slope.
    • 
+--------------------
Energy 
| ____ Reactants 
|         
|       ____ Products    
|         __/
+--------------------
Progress of Reaction
  • Endothermic reaction: Energy is absorbed. In this case, the energy level of the products is higher than that of the reactants, creating an upward slope.
    • 
+--------------------
Energy 
|        ____ Products 
|       /    
|      / ____ Reactants 
|     /__/
+--------------------
Progress of Reaction

Potential energy surface (PES)

The potential energy surface is a more advanced representation that maps the potential energy of a system as a function of atomic positions. While the energy profile provides a one-dimensional view of energy changes along a single reaction coordinate, the PES provides a multidimensional landscape. By exploring the PES, chemists can predict reaction pathways, intermediates, and possible alternative pathways.

Saddle point

Within the PES framework, the transition state corresponds to a saddle point, which is a point of energy maximum along the reaction coordinates, but minimum along the other coordinates.

Transition state theory

Transition state theory provides a framework for understanding how and why chemical reactions occur at the molecular level. It describes reaction rates in terms of the energy barrier that must be crossed to transform reactants into products. This theory is important for explaining the role of transition states and activation energies in determining reaction kinetics.

When considering transition states, chemists often use the Arrhenius equation to relate the rate constant of a chemical reaction to temperature and activation energy:

k = Ae^(-Ea/RT)
  • k is the rate constant.
  • A is the pre-exponential factor, indicating the frequency of collisions.
  • Ea is the activation energy.
  • R is the universal gas constant.
  • T is the temperature in Kelvin.

Catalysts and their effects

Catalysts play an important role in chemical reactions by providing alternative reaction pathways with lower activation energy. This allows reactions to proceed more quickly or at lower temperatures than reactions that would occur without a catalyst. The effect of a catalyst is clearly visible in the energy profile:


+--------------------
Energy 
|      ____ Without Catalyst
|     /    
|    /   
|   /    ____ With Catalyst
|  /    /
| /    
+--------------------
Progress of Reaction

The use of a catalyst often results in a new, lower peak, corresponding to a decrease in the activation energy, while the overall energy change between reactants and products remains unchanged.

Examples of reaction energy profiles

Combustion of methane

The combustion of methane is an exothermic reaction that can be represented by the following equation:

CH4 + 2O2 --> CO2 + 2H2O

The energy profile of this reaction shows that the products (carbon dioxide and water) are at a lower energy level than the reactants (methane and oxygen), resulting in the release of energy.


+--------------------
Energy 
| ____ CH4 + 2O2 
|         
|       ____ CO2 + 2H2O
|         __/
+--------------------
Progress of Reaction

Photosynthesis

Photosynthesis, an endothermic process, converts carbon dioxide and water into glucose and oxygen using sunlight:

6CO2 + 6H2O + light --> C6H12O6 + 6O2

In this reaction, the energy profile indicates that energy must be absorbed for the reaction to proceed, and the products are at a higher energy level than the reactants.


+--------------------
Energy 
|        ____ C6H12O6 + 6O2 
|       /    
|      / ____ 6CO2 + 6H2O 
|     /__/
+--------------------
Progress of Reaction

Conclusion

Reaction energy profiles are essential tools for visualizing and understanding the energy changes that occur during chemical reactions. By examining these profiles, students and chemists alike can gain valuable insights into reaction mechanisms, compare reaction pathways, and predict the effects of catalysts and other factors on reaction rates. Whether representing simple processes such as combustion or complex biological reactions such as photosynthesis, reaction energy profiles are indispensable for the study and understanding of chemical phenomena.


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Reaction energy profiles

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