What is Thermochemistry?

Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.    

"open, closed and isolated system"


Energy can be considered as the ability to do work or provide heat. Thermochemistry deals with the energy (typically regarding heat) which is incorporated or produced when a compound undergoes any chemical changes.    

This can be classified into three systems. The systems with their definitions are given below:

  • Open System – it involves the exchange of both mass and energy/heat with its environmental factors.    
  • Closed System – it involves the exchange of energy/heat but not mass.    
  • Secluded (isolated) System – here exchange of mass as well as energy/heat does not happen.

Responses can be Differentiated into Two Types:

  • Exothermic: In this kind of response, heat is produced and supplied to the surroundings.
  • Endothermic: In this kind of response, heat is absorbed from the surroundings.

The same idea may be expressed shortly in an extraordinary way. Energy is either used or acquired in a substance or actual interaction.

Albeit the energy isn't destroyed nor created in both cases; the complete energy of the universe stays constant.

Energy only moves from one place to another. If the energy of the system is dropped for the duration of the response, it is an exothermic response indicating that the system gives the energy to its environment.

Subsequently, the temperature is downgraded; hence, the heat is delivered during the response.

In an endothermic response, the energy is absorbed by the system from its environment.  In this case energy must be supplied to the system in order to start the synthetic cycle. This energy is usually supplied in the form of heat.

When a response happens, two possibilities are there. Either it will give out energy and carries an exothermic reaction or else, it will take up energy and carries an endothermic reaction. The energy incorporated during both these types of responses can be measured using the term enthalpy change which is denoted as ΔH.

"Endothermic and Exothermic reactions"

In the case of exothermic reactions, the energy of the reactants seems to be high when the reaction starts. However, when the response proceeds the energy drops since in an exothermic reaction energy is liberated to the environment. Thus, the products will be expected to have a least possible energy. It should be noted that for every reaction a base energy is required to proceed. This energy is referred as activation energy. On the other hand, in the case of endothermic responses, the reactants will have the lowest possible energy. Thus, for the response to advance (to acquire the activation energy) energy must be supplied from outside. Here, the energy of the products is expected to be high. The difference in the energy of the reactant and product is the energy incorporated in a response.

Hess’s law of Constant Heat Summation 

This is an important law in physical chemistry that was proposed by a Russian chemist named Germain Henri Hess. The law states that the amount of heat consumed or evolved (or the change in enthalpy) in any chemical reaction is a constant quantity that is unaffected by the reaction's direction or the number of steps required to complete it. The equation for Hess's law is given below:

Δ H 0 = ΣΔ H n

Where H n is the amount of heat absorbed or evolved in the individual n steps of the reaction and H 0 is the heat absorbed or evolved.

Nuclear Power (Thermal Energy), Temperature, and Heat 

Nuclear power is a type of motor energy related to the irregular movement of particles and atoms. Expanding the measure of nuclear power builds the temperature of the framework and the other way around.

Heat can be considered as the exchange of nuclear power between two lines with different temperatures. Heat here is denoted as q. The transfer of heat/warmth will take place from a higher motor energy place to a lower motor energy place.


A calorimeter or bomb calorimeter is the equipment which is used to measure the heat change in a system. This gadget possesses a shut holder and a thermometer to measure the temperature.

Here, the measure of energy is expressed in calories (Cal). One Cal is equal to the amount of energy required to raise the temperature of 1 g of water by 1°C or 1 Kelvin. The SI unit of warmth, work, and energy is the Joule (J). One J is the measure of energy required to change the position of an article by 1meterwith a power of 1 Newton.

1cal = 4.184 J


The total heat associated with a response is expressed as enthalpy which is denoted using H. Enthalpy (H) can be defined as the energy which is released or absorbed during a reaction. The direct measurement of enthalpies is very difficult and hence, the term enthalpy change is used. It is denoted with ΔH which is equal to H final - H initial .

Enthalpy Changes

Some of the enthalpy changes which are used frequently in thermochemical equations of chemical reactions is given below:

Standard Enthalpy Change of Combustion/ignition is denoted as - Δ H Co

Standard enthalpy change of ignition can be defined as the enthalpy change associated with the combustion of one mole of substance in the presence of excess oxygen under standard conditions of temperature and pressure.

Standard Enthalpy Change of Formation is denoted as - Δ Η o f

Standard enthalpy change of formation can be defined as the enthalpy change associated with the formation of one mole of substance under standard conditions of temperature and pressure.

Standard Conditions

Thermodynamics is the study of energy changes accompanied with a compound response or simply it depends on “changes in heat." In thermodynamics some standard conditions are there, they are given below,

  • Standard temperature =25 °= 298 K
  • Standard Pressure =1 atm

The standard conditions of temperature and pressure associated with thermodynamics is different from that of gases

When a reaction is taking place either energy can be produced, or energy can be absorbed or use the expression "release" or "absorb" energy.  Heat/warmth is described in this course as enthalpy (H). Heat energy is expressed in the units of joules (J) or kilojoules (kJ).

Key Takeaways

  • While dealing with the exothermic reactions, it should be noted that the enthalpy change is expressed with a negative sign. If it is has not been done right, the value will be wrong since, in the case of enthalpy change positive and negative signs have a great significance.
  • If the reaction is endothermic the enthalpy change is expressed with a positive sign.
  • While dealing with enthalpy the unit joules can be used. On the other hand, when dealing with enthalpy change, the unit must be kg/mol. And when enthalpies are given in joules it must be converted to kilo joules when calculating the enthalpy change.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors. in Chemistry and  Masters in Chemistry.

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