Chemical reactions are the fundamental basis on which all chemical engineering is built. They not only allow the transformation of raw materials into high value-added products, but also determine the feasibility, efficiency and safety of industrial processes. In the design of chemical plants, the proper selection of the type of reaction, together with its optimal operating conditions, defines the technical and economic success of the project.
Understanding the various classes of reactions and their behavior under controlled conditions is essential for making informed decisions in areas such as catalysis, energy production, synthesis of organic and inorganic compounds, water treatment and biotechnological processes. This article explores the main types of chemical reactions from an applied perspective, highlighting how each finds a specific role in modern chemical engineering.
Most significant types of reactions in chemical engineering
A chemical reaction is a process in which atoms, molecules or ions of certain substances are transformed into atoms, molecules or ions of different chemical substances; the initial substances that are transformed are called reactants and the new substances that are produced are called products.
The chemical reactions are usually the most significant stages of the processes, and the ones that decide their economic viability; the combination of economic factors and the chemical and physical aspects constitutes the most genuine part of the chemical engineer’s engineering, called chemical reactor design. The following image represents the conceptualization of the application of chemical reactions in the field of chemical engineering.
Thus, “Chemical Reaction Engineering” is the discipline that, by itself, gives Chemical Engineering its own entity within Engineering; the study of the “chemical unit operation” is of central importance in the curriculum of the chemical engineer as it constitutes the heart of all chemical process systems together with the corresponding physical stages that may accompany it, its operation frequently and decisively determines the load, size and configuration of the equipment and associated preparation, conditioning and separation operations).
The following are the most important reactions in the field of chemical engineering, as well as their application at industrial level.
Combination or synthesis reactions
Combination or synthesis reactions consist of the union of two or more reactants to form a single, more complex product. From an industrial point of view, this type of reaction is widely used in the synthesis of basic and specialty chemicals. A paradigmatic example is the Haber-Bosch process, whereby ammonia is produced from nitrogen and hydrogen.
This reaction, represented by N₂ + 3H₂ → 2NH₃, takes place at high pressures and temperatures, in the presence of iron catalysts, and forms one of the bases of the fertilizer industry worldwide. Chemical engineering is responsible for optimizing the conditions for this synthesis to maximize yield, minimize energy costs and ensure safety in high-pressure systems. Other synthesis reactions include the production of methanol, sulfuric acid and various polymers that require controlled conditions for efficient and safe conversion.
Decomposition reactions
In decomposition reactions, a complex compound is broken down into two or more simpler products. This type of reaction is essential in processes where the controlled breakdown of large or complex molecules is required, such as in the petrochemical, energy and materials industries. A prime example is thermal pyrolysis, which involves breaking down heavy hydrocarbons (such as diesel or crude oil) by heat, without oxygen, to obtain lighter products such as ethylene, propylene, benzene and other olefins.
This process is key to obtaining plastic precursors and fine chemicals. We also find applications in the thermal decomposition of compounds such as calcium carbonate to obtain quicklime (calcium oxide), used in steelmaking and construction. In all cases, chemical engineering carefully studies the conditions of temperature, residence time and reactor design to ensure that decomposition occurs in a controlled and efficient manner without generating unwanted by-products.
Displacement or simple substitution reactions
Displacement or simple substitution reactions occur when a chemical element reacts with a compound and replaces one of its constituent elements. These reactions are of great relevance in metallurgical engineering and in the production of metallic materials, where reducing agents are used to extract metals from their oxides. A classic example is the obtaining of iron in blast furnaces, where iron oxide reacts with carbon monoxide (Fe₂O₃ + 3CO → 2Fe + 3CO₂), releasing the pure metal and generating carbon dioxide as a by-product.
In other cases, these reactions are used for solution purification, as in the displacement of copper by iron in hydrometallurgical processes. The chemical engineer must understand the reactivity of the elements involved and design processes where this type of reaction occurs with high selectivity and stoichiometric control, avoiding parallel reactions and optimizing the consumption of reagents.
Double displacement reactions
In double displacement reactions, two compounds exchange their ions or components with each other, generating two new products. These reactions are fundamental in solution chemistry and industrial water treatment, where they are used to precipitating pollutants, neutralize acids or adjust pH. For example, the neutralization of an acid with a strong base such as hydrochloric acid with sodium hydroxide (HCl + NaOH → NaCl + H₂O) is a double shift reaction that is essential in pH control in numerous chemical processes.
In addition, these reactions allow the precipitation of heavy metals through the formation of insoluble salts, facilitating their separation from the liquid effluent. In chemical engineering, this type of reaction is also considered in the design of mixing reactors, in the formulation of pharmaceutical products and in the manufacture of ceramic compounds by solution synthesis.
Combustion reactions
Combustion reactions are exergonic and are characterized by the combination of a fuel with oxygen, releasing energy in the form of heat and light. They are the basis of thermal generation in numerous industrial processes, from boilers for steam generation to furnaces for heat treatment of materials. In chemical engineering, combustion reactions are not only used to produce energy, but also to evaluate the energy efficiency of processes, control pollutant emissions (CO, NOₓ, particulates) and optimize the design of combustion systems.
For example, complete combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is used as the basis for generating steam in chemical plants, and its efficiency depends on proper air mixing, flame temperature and residence time. Precise control of these variables allows maximizing thermal performance and reducing the environmental impact of the processes.
Redox reactions (oxidation-reduction)
Redox reactions involve the transfer of electrons between chemical species, and are essential in processes where the oxidation state of an element needs to be modified. These reactions play a leading role in electrochemical production, corrosion control and metal recovery. An emblematic case is the electrolysis of brine to obtain chlorine, sodium hydroxide and hydrogen, a reaction widely used in the chlor-alkali industry. They are also fundamental in the design of galvanic cells and batteries, where chemical energy is transformed into electrical energy.
In chemical engineering, the mastery of redox reactions also makes it possible to develop cathodic protection technologies to prevent corrosion of metal pipes, equipment and structures exposed to aggressive media. For this purpose, sacrificial anodes or impressed currents are used to modify the electrochemical potential of the system while preserving the integrity of the base material.
Acid-base reactions
Acid-base reactions, governed by proton transfer (H⁺), have critical applications in pH regulation, chemical synthesis, and biochemical processes. In many cases, the success of a reaction or the stability of a product depends on maintaining pH within an optimal range. For example, in fermentative processes or enzymatic synthesis, pH control by buffers is crucial to maintain catalytic activity and avoid denaturation of the compounds. Furthermore, these reactions are fundamental in the neutralization of industrial effluents prior to discharge, where the pH is adjusted to comply with environmental regulations.
These are also used in the formulation of pharmaceuticals, detergents and cosmetics, where acidity or alkalinity directly influences the efficacy and stability of the final product. In all cases, chemical engineering allows the design of dosing systems, sensors and automatic control to ensure that acid-base reactions occur accurately and reproducibly.
Thermodynamic and kinetic fundamentals
The analysis of any chemical reaction or industrial process requires a clear understanding of its thermodynamic and kinetic fundamentals. Thermodynamics makes it possible to determine whether a reaction is spontaneous under certain conditions, based on parameters such as the variation of Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS). This discipline defines the limits of equilibrium and the potential direction of the processes, but does not provide information on the rate at which they occur.
Chemical kinetics, on the other hand, studies the rate of reactions and the factors that affect it, such as temperature, concentration of reactants, presence of catalysts and reaction mechanism. Understanding kinetics is essential for sizing reactors, optimizing residence times and ensuring the desired performance.
Kinetic and thermodynamic control
Thermodynamic and kinetic approaches are fundamental for predicting the actual behavior of a process, identifying optimal operating conditions and ensuring both technical feasibility and economic efficiency of systems in chemical engineering. The following image shows a comparative graph between kinetic (in blue) and thermodynamic (in red) control of a chemical reaction, illustrating how the yield of the product [AB] varies as a function of time.
Typical progress curves of product concentrations (AB) in reactions under kinetic (blue) or thermodynamic (red) control. The substrate analog B* contains a high-energy leaving group (*) and its removal temporarily establishes more favorable equilibrium conditions. As the initial driving force dissipates, the equilibrium between AB and A+B becomes predominant and the product concentration converges to that of the corresponding thermodynamically controlled reaction.
Importance of kinetic and thermodynamic analysis.
According to the above mentioned, identifying the type of reaction is essential to analyze its thermodynamic feasibility (ΔG, chemical equilibrium) and its reaction rate (kinetics). Thermodynamics allows knowing if a reaction is spontaneous under certain conditions, while kinetics determines how fast it occurs and if it is feasible in an industrial environment. These two branches of chemical engineering are key to selecting suitable reactors (batch, continuous, piston flow, CSTR), designing catalytic systems, and establishing the operating parameters that guarantee maximum conversion with minimum energy.
A thermodynamically feasible process can be useless if its kinetics are too slow, or if it generates unwanted by-products. Therefore, the chemical engineer not only studies what reacts with what, but how, how much and when to do it in order to achieve efficiency, safety, and competitiveness in each process.
Applications of chemical reactions
The following table presents some of the most important applications for each of the above-mentioned reactions.
| Type of Reaction | Industrial Applications in Chemical Engineering |
|---|---|
| 1. Combination or Synthesis | – Ammonia production (Haber-Bosch) for fertilizers. – Manufacturing of methanol, sulfuric acid, and polymers. – Synthesis of pharmaceuticals and organic compounds. |
| 2. Decomposition | – Hydrocarbon pyrolysis to produce ethylene, propylene, and light fuels. – Quicklime (CaO) production from calcium carbonate. – Thermal decomposition of explosives, fertilizers, or hazardous waste. |
| 3. Single Displacement or Substitution | – Metal recovery (Fe, Cu, Zn) by chemical reduction in blast furnaces and hydrometallurgical processes. – Removal of metallic impurities in aqueous solutions. – Preparation of metallic salts. |
| 4. Double Displacement | – Water treatment by contaminant precipitation (e.g., barium sulfate to remove Ba²⁺). – Neutralization of acidic or basic effluents. – Synthesis of salts and pharmaceuticals via ion exchange. |
| 5. Combustion | – Heat and steam generation in industrial boilers. – Waste incineration processes. – Thermal design of reactors and furnaces. – Energy control of endothermic processes using burners. |
| 6. Redox (Oxidation-Reduction) | – Electrolysis for obtaining chlorine, hydrogen, and caustic soda. – Cathodic protection to prevent corrosion in pipelines and metallic structures. – Electrochemical metal recovery. – Energy generation in batteries and fuel cells. |
| 7. Acid-Base | – pH regulation in chemical reactors and fermenters. – Neutralization of industrial effluents prior to discharge. – Formulation of cosmetic, pharmaceutical, and food products. – Preparation of buffer solutions for biochemical processes. |
From the classroom to the reactor: The value of chemical knowledge
Knowing the types of chemical reactions is not only part of a chemical engineer’s academic curriculum: it is a strategic tool that allows solving real problems, designing sustainable processes and continuously improving industrial productivity. Each reaction represents a controlled transformation opportunity, and its correct application determines the success of a plant, the quality of the final product, energy efficiency and operational safety.
Today’s industry demands engineers capable of integrating theoretical fundamentals with practical decisions. Choosing the right type of reaction, calculating its balances, modeling its kinetic and thermodynamic behavior, and applying control measures is not an isolated task, but part of a global vision that connects science, technology and sustainability. In this sense, mastering chemical reactions not only makes it possible to produce more and better, but also to innovate, reduce environmental impact and adapt to the challenges of an increasingly demanding chemical industry.
Therefore, a thorough understanding of the types of reactions and their concrete application is an competence of the contemporary chemical engineer. Transforming this knowledge base into action is what makes chemistry a powerful tool for designing a more efficient, safer and sustainable future.
Conclusions
Understanding the types of chemical reactions is not only a theoretical competency, but a critical skill for chemical engineering professionals. Each type of reaction has practical implications for process design, material selection, operational parameter control and environmental sustainability.
Applying this knowledge enables engineers to transform raw materials into useful products in a safe, efficient and responsible manner. From the simplest to the most complex reactions, mastering chemical reactivity is a strategic factor driving innovation and continuous improvement in industry. In a world that demands cleaner, more economical and sustainable processes, a thorough understanding of chemical reactions is the foundation for building the future of engineering.
References
- Himmelblau, D. M., & Riggs, J. B. (2012). Basic principles and calculations in chemical engineering (8th ed.). Pearson Education.
- Smith, J. M., Van Ness, H. C., & Abbott, M. M. (2005). Introduction to chemical engineering thermodynamics (7th ed.). McGraw-Hill Education.
- Levenspiel, O. (1999). Chemical reaction engineering (3rd ed.). John Wiley & Sons.
