Understanding Thermochemistry, Systems, Reactions, and Formulas

Thermochemistry is a chemical study that explains the changes in energy in the definition of chemical reactions . The study of this energy is very necessary because by knowing the energy, we can estimate the application and efficiency of the reaction.

Humans also apply thermochemistry in their bodies, for example cells in humans that carry out combustion reactions to organic molecules such as sugar, fat and other food sources by then producing heat and energy that are useful for carrying out activities for these humans. In the industrial field, thermochemistry is also very important to design an efficient reaction so as to increase the productivity of the industry. In this article, we will discuss thermochemistry , systems in chemistry, energy, reaction enthalpies, and their laws .

table of contents

  • Thermochemistry
    • Systems in Chemistry
      • Open system
      • Closed System
      • Isolated System
      • Energy
    • Enthalpy of Reaction
      • Exothermic Reaction
      • Endothermic reaction
    • Hess’s Law
    • Spread this:
    • Related Posts:


Thermochemistry is a branch of chemistry that studies the heat of reactions. In a chemical reaction always involves energy for the reaction and the reaction can also produce energy from certain heat.

For example, when you light a match, in that event there is a change in energy and matter through the existence of chemical reactions, namely combustion reactions.

Systems in Chemistry

In thermochemistry, it is very important to define systems and environments because this becomes a basic concept in the study of thermochemistry. Basically the system is an object that is being observed or studied while the environment is the outside of the system that is still interacting with the system.

This type of system and environment can affect the thermochemistry of a reaction. In conjunction, there are three types of systems in chemistry:

Open system

As the name implies, an open system is a system that freely allows the exchange of material or energy from the system with its environment.

This open system can be exemplified that is when you boil water using an open pan, energy and substances can easily move to the environment as well as heat energy flowing into the environment and substances in the form of water vapor that also move into the environment from the system in the pan.

Closed System

A closed system is a chemical system that allows the system to exchange energy with the environment but cannot exchange material because the system is closed.

This system for example is when you boil water with a closed pan where in that condition the heat can still be exchanged from the system to the environment, but the material or substances from the system can not come out so called closed.

Isolated System

In addition to the two systems above, there is one type of system that is an isolated system in which the exchange of energy and matter is not possible. An example of this system is the thermos for storing hot water where material cannot move to the environment nor can an energy or heat system which is isolated or cannot also move into the environment.


Energy is a potential that can be used to do work such as moving objects, lifting objects, generating electricity, raising the temperature or heat of a system, and others. Energy becomes a power that can drive change.

The changes that occur due to the involvement of energy where this energy can be in the form of light, heat, electricity, etc. An energy cannot be destroyed or created, energy can only move from one form for example to another, this is the basic principle of energy conservation.

In chemistry, energy is always involved in the occurrence of chemical reactions because to cause a substance or reactant to react with each other reactants, it requires a minimum energy which is usually called activation energy.

The activation energy is the minimum energy that must be achieved by a substance so that reactions and chemical changes can occur. In addition, energy can also be produced from a chemical reaction. When a reaction produces a certain amount of heat, that heat is the type of energy produced by the reaction.

Enthalpy of Reaction

In thermochemistry, the term most often used is the enthalpy commonly denoted by (). The reaction enthalpy is the amount of heat involved in a chemical reaction where the reaction is carried out under certain conditions.

For example when you do a combustion reaction that is combustion of methane gas (CH 4 ) using O 2 it will produce heat where the heat is the enthalpy of the combustion reaction of methane gas. But besides releasing heat, a reaction can also absorb heat so that the reaction will take its heat from the environment.

The heat of a reaction or reaction enthalpy can also be distinguished for each reaction such as the heat of the formation reaction, the heat of the combustion reaction, the heat of the neutralizing reaction, the heat of the decomposition reaction, and others.

Basically there are two types of chemical reactions namely exothermic reaction and endothermic reaction:

Exothermic Reaction

Exothermic reaction is a reaction in which the reaction will release a heat. This exothermic reaction is characterized by a negative reaction enthalpy value which indicates the amount of heat released from the reaction.

Exothermic reactions can also be known by looking at the temperature of the reaction, when a reaction has a final temperature after the reaction is increased or rising means the reaction is exothermic because the reaction system releases heat which is then received by the environment. Examples of exothermic reactions include burning paper, rusty iron, mixing acids with bases, burning wood, and others.

Endothermic reaction

Endothermic reaction is a reaction in which this reaction will absorb some heat. Endothermic reactions have a positive or enthalpy value which means they absorb some heat from the environment. The sign that a reaction is said to be endothermic is at the final temperature after the reaction has dropped from its initial temperature.

The decrease in temperature is because the reaction system absorbs the temperature from the surrounding environment so that the ambient temperature decreases because it is absorbed. Examples of endothermic reactions include melting ice into water, evaporation of water, dissolving urea in water, and others.

Hess’s Law

Because enthalpy is a state function, the heat involved in a reaction does not depend on the direction of the reaction whether it goes from the reactant to the product or vice versa and depends on the reaction stage whether one step or several steps of the reaction.

Hess’s Law states that if two reactions are combined to produce a third reaction, then the enthalpy number for the first two reactions is equal to the enthalpy of the third reaction.

Examples of reaction enthalpies can be seen in the following reactions:

From this reaction it can be seen that there are two ways in the reaction that form CO. The first reaction involves only one stage, but in the second reaction there are two stages to the product. Based on the energy level diagram, the reaction can be described as follows:

It can be seen that the reaction is exothermic because it produces a negative enthalpy value which indicates the release of heat. The initial state of the reactants can undergo two types of reactions, namely directly into the product or undergo two stages of reaction before the new product is formed.

Based on Hess’s law, when a reaction is formed from two other reactions, the total enthalpy value of the reaction formed is the sum of the two reactions. It was found in a reaction that took place in two stages where the total enthalpy of the two stages of the reaction turned out to be the same as another reaction that lasted one stage.

One-stage reaction: CO (s) + O 2 (g) → CO 2 (g) ∆H = –394 kJ

Two-stage reaction: C (s) + ½ O 2 (g) → CO (g) ∆H = –110 kJ

CO (g) + O 2 (g) → CO 2 (g) ∆H = –284 kJ

Total: C (s) + O 2 (g) → CO 2 (g) ∆H = –394 kJ

Well, that was a series of discussions on a variety of materials about the understanding of thermochemistry, systems in chemistry, energy, enthalpy of reaction, various formulas that became the law. Hopefully, through this article can provide insight and in-depth references for all readers


Leave a Comment