Everyday example of an exothermic chemical reaction: burning a candle releases heat and light.
Hess's law , also known as the law of heat addition, states that the total enthalpy change of a chemical reaction is independent of the path the reaction takes, as long as the initial and final states are the same. It is a fundamental principle in thermodynamics that is used to calculate reaction enthalpies indirectly by adding the intermediate enthalpies.
Fundamentals of Hess's law
Hess's law is a direct consequence of the principle of conservation of energy , which ensures that energy is neither created nor destroyed, but rather transformed.
To apply Hess's law, it is essential to understand the following concepts:
- chemical reactions : processes in which initial substances (reactants) are transformed into new substances (products);
- thermal energy : form of energy associated with the movement of particles, released or absorbed during a reaction;
- heat of reaction : amount of heat exchanged during a chemical reaction, usually expressed in kJ/mol ;
- law of conservation of mass : principle that states that the total mass of the reactants is equal to the total mass of the products;
- material balance : process of equalizing the reactants and products in a chemical reaction;
- reaction route : intermediate path by which reactants are converted to products;
- algebraic method : procedure used to add the enthalpies of different intermediate reactions to obtain the total enthalpy;
- algebraic sum of enthalpies : key mathematical operation in the application of Hess's law.
Chemical reactions and energy
Exothermic and endothermic reactions
Exothermic reactions release energy to the medium in the form of heat, resulting in a negative enthalpy change. In contrast, endothermic reactions absorb energy from the medium, with a positive enthalpy change. These differences are crucial to understanding how systems exchange energy with their environment.
Internal energy and thermodynamics
The internal energy of a system is the sum of all the potential and kinetic energies of its particles. In the context of thermodynamics, fundamental principles, such as the first law of thermodynamics, describe the conservation of energy in closed systems.
Heat and heat capacity
Heat is the form of energy transferred between systems due to a temperature difference. Heat capacity and specific heat are properties that determine how much heat is needed to change the temperature of a substance.
Innovation in energy production is committed to sustainable solutions that integrate technology and nature.
Enthalpy: properties and concepts
Enthalpy is a fundamental quantity in thermodynamics, used to describe heat exchanges at constant pressure. In the context of Hess's law, we work with different types of enthalpy changes and their properties:
- standard state : reference conditions (1 atm, 25 °C) used to measure thermodynamic properties;
- standard enthalpy change of formation : energy associated with the formation of one mole of substance from its elements in their most stable states;
- standard enthalpy change of combustion : energy released when one mole of substance burns completely in the presence of oxygen;
- enthalpy tables – resources containing tabulated values of standard enthalpy changes for different substances and reactions;
- energy diagrams : Graphical representations illustrating the energy levels of reactants, intermediates, and products;
- activation energy : minimum energy required to start a chemical reaction;
- chemical bond : forces that hold atoms together in a molecule and whose formation or breaking involves energy changes.
Thermal and energy analysis tools
Calorimetry
Calorimetry is an experimental technique that measures the heat exchanged in chemical or physical processes. Tools such as the bomb calorimeter allow enthalpy changes to be determined with high precision, while differential scanning calorimetry (DSC) analyzes thermal transitions.
Born-Haber cycle
The Born-Haber cycle is a graphical method used to calculate energy changes in processes such as the formation of ionic compounds. This approach illustrates how the different steps contribute to the total enthalpy change.
Phase diagrams
Phase diagrams represent the temperature and pressure conditions under which a substance changes state. These diagrams also provide key information about latent heat and phase transitions.
Dynamics and chemical equilibrium
The study of chemical dynamics and equilibrium provides key information about the behavior of chemical reactions. These concepts are also useful in interpreting and applying Hess's law:
- chemical equilibrium : state in which the rates of the forward and reverse reactions are equal, keeping the concentrations of reactants and products constant;
- equilibrium constant : value that describes the ratio between the concentrations of products and reactants at equilibrium;
- reversible reactions : processes that can proceed in both directions until equilibrium is reached;
- chemical dynamics : branch of chemistry that studies the rates of reactions and the factors that affect them;
- chemical kinetics : study of the rate of reactions and the mechanisms by which they occur;
- rate law : equation that relates the rate of a reaction to the concentrations of the reactants;
- reaction order : exponent to which the concentration of a reactant is raised in the rate equation;
- catalysts : substances that increase the rate of a reaction by decreasing its activation energy;
- inhibitors : substances that slow the rate of a reaction.
Practical applications of Hess's law
energy production
Hess's law is fundamental in the design and optimization of industrial processes related to energy production. In thermoelectric plants, it is used to accurately calculate the energy released during the combustion of fossil fuels. Furthermore, its application is key in the analysis of the efficiency of fuel cells, where controlled chemical reactions generate electrical energy.
Design of new materials
In materials chemistry, Hess's law allows predicting the enthalpy changes associated with the formation of novel compounds. This is essential to develop efficient catalysts, energy storage materials such as lithium-ion cells and batteries, and compounds with specific properties for advanced technological applications.
Medicine and pharmacology
In the medical field, it is used to evaluate the thermal stability of drugs and bioactive compounds. This analysis ensures that pharmaceutical products maintain their effectiveness under varying environmental conditions, optimizing both their development and storage.
chemical engineering
Hess's law facilitates the design of safer and more efficient industrial chemical processes. In particular, it is applied to the calculation of the energy required or released in complex reactions, such as those that occur in the synthesis of ammonia or in the production of plastics. This allows you to minimize energy consumption and maximize profitability.
Sustainable development
In the context of sustainability, Hess's law helps analyze the chemical reactions involved in clean technologies, such as carbon dioxide capture and storage. It is also used to model biomass conversion processes into renewable fuels, contributing to the transition towards cleaner energy sources.
The fusion process illustrates how phase changes involve energy exchanges.
Phases and phase changes
The study of phases and phase changes also finds application in the context of Hess's law, since phase transitions involve energy exchanges:
- phase : state of matter (solid, liquid, gas) with homogeneous properties;
- phase changes : transitions between states of matter, such as fusion, vaporization, or solidification;
- boiling point : temperature at which a substance changes from a liquid to a gas;
- melting point : temperature at which a substance changes from a solid to a liquid;
- sublimation : direct change from solid to gas without passing through the liquid state;
- deposit : direct change from gas to solid;
- latent heat of vaporization : amount of heat necessary to convert a liquid into a gas at constant temperature;
- latent heat of fusion : amount of heat required to convert a solid to a liquid at constant temperature;
- phase diagram – graph showing the temperature and pressure conditions under which a substance exists in different phases;
- Gibbs phase rule : equation relating the number of phases, components, and degrees of freedom in a system.