Thermochemistry

Thermochemistry - The branch of Physical Chemistry that deals with the study of the exchange of heat in the reaction. A brief discussion of Thermochemistry is given in the article. 

What is Thermochemistry?

This branch of Chemistry describes the phenomena of thermal energy conversion from one form to another form of energy. In this branch, the effects of heat on the matter are also being studied. When we discuss thermodynamics, the particular item or collection of items that we are interested in is called the system, while everything that is not included in the system we have defined is called the surroundings. System and surroundings are separated by the boundary. 

For example, If the system is one mole of gas in the container, then the system is the one mole of the gas, the inner wall of the container is known as the boundary (separates the system and surrounding), and everything that is present outside of the boundary is considered the surroundings, which would include the container itself.

Types of System

In order to understand thermodynamics in physical chemistry and the impact of different effects of thermal energy on any particular chemical change, we need to define

1. Open System

An open system can exchange both energy and matter with its surroundings. The stovetop is a good example of the open system. As the water vapour and heat can be lost to the atmosphere.

2. Closed System 

Closed system can exchange only energy with its surroundings, not matter. When we put a very tightly fitting lid on the pot, it would be considered as a closed system.

3. Isolated System 

An isolated system cannot exchange either matter or energy with its surroundings. A perfect isolated system is hard to come by, but an insulated drink cooler with a lid is conceptually similar to a truly isolated system. 

What is Thermochemical Reaction?

The balanced chemical reaction indicates the physical state of all the reactants and products and also indicates the heat change known as a thermochemical reaction.

The thermochemical reaction is of two types:

1. Endothermic Reaction 

Those thermochemical reactions in which heat is absorbed. Change in enthalpy for this reaction is positive. A compound formed in the endothermic reaction is known as an endothermic compound. If more heat is absorbed then the product formed will be less stable. Example - decomposition reaction, fusion reaction, vapourisation reaction, sublimation reaction, and photosynthesis.

2. Exothermic Reaction 

Exothermic reactions are the reaction in which the heat or the energy is evolved during the reaction. The change in enthalpy for the exothermic reactions is negative. A compound formed in the exothermic reaction is known as an exothermic compound. If more heat is evolved then the product formed will be more stable. Example- combustion reaction, neutralization reaction, respiration, and fermentation.

Some Important Point Related to Thermochemical Reaction

• In thermochemical reaction, if conditions are not given then change in enthalpy is considered to be ΔH0. 

• If the thermochemical reaction is multiplied by some coefficient then the change in enthalpy is also multiplied by the cell coefficient.

• If the thermochemical reaction is reversed then the numerical value of change in enthalpy remains the same but the sign is changed.

The Heat of Reaction or Enthalpy of Reaction 

The amount of heat change when moles of reactant present in the thermochemical reaction has completely reacted is called the heat of reaction or enthalpy of reaction.

Types of Heat of Reaction

1. The Heat of Combustion or Enthalpy of Combustion 

The amount of heat evolved by the complete combustion of one mole of a compound is known as the heat of combustion.

2. The Heat of Formation or Enthalpy of Formation- 

The amount of heat evolved or absorbed when one mole of a compound is formed from its constituent elements which are in their stable standard or stable state.

The standard heat of formation of all the stable and free elements is taken to be zero. 

3. The Heat of Neutralization or Enthalpy of Neutralization 

Amount of heat evolved when 1 gram equivalent of acid is completely neutralised by 1 gram equivalent of the base in dilute solution.

The heat of neutralisation of strong acid and strong base always remains constant and its value is -13.7 Kcaleq-1 because some amount of heat is used in the dissociation of weak acid or base and this difference amount of heat is known as the heat of dissociation. Exception: HF (weak acid) heat of neutralisation of HF is more than -13.7 KCaleq-1 because of the high hydration energy of fluoride ions.

4. The Heat of Solution or Enthalpy of the Solution 

The amount of heat evolved or absorbed when 1 mole of a compound is dissolved in such an excess amount of solvent that further dilution does not involve any more heat change known as the heat of solution.

5. The Heat of Hydration or Enthalpy of Hydration

Amount of heat released or absorbed when 1 mole of anhydrous or partially hydrated salt reacts with a required number of a water molecule to form hydrated salt.  

6. The heat of Transition or Enthalpy of Transition 

The amount of heat changes when one allotropic form of 1 mole of compound converts into another allotropic form known as the heat of transition.

7. The Heat of Fusion or Enthalpy of Fusion 

Amount of heat required to convert 1 mole of solid substance into a liquid at its melting point known as enthalpy of fusion.

8. The Heat of Vaporisation or Enthalpy of Vaporization- 

The amount of heat required to convert 1 mole of liquid into vapour form is called enthalpy of vaporisation.

9. The Heat of Sublimation or Enthalpy of Sublimation

The amount of heat required to convert 1 mole of solid into gaseous form is called enthalpy of sublimation.

10. Lattice Energy

Amount of heat released when 1 mole of ionic solid is performed from its gaseous ion known as lattice energy.

11. The Heat of Hydrogenation or Enthalpy of Hydrogenation

The amount of heat evolved when 1 mole of unsaturated organic saturated compound reacts with hydrogen to form a saturated organic compound known as the heat of hydrogenation.

12. The Heat of Atomisation or Heat of Atomisation

Amount of energy required to dissociate 1 mole of the stable molecule into a gaseous atom known as the heat of atomisation.

13. Bond Dissociation Energy or Enthalpy of Bond Dissociation

Amount of energy required to dissociate 1 mole of a particular type of bond to separate the atoms in a gaseous state known as bond dissociation energy.

In the case of the diatomic molecule, bond dissociation energy and heat of atomisation is the same.

14. Bond Energy

Bond energy is the average bond dissociation energy in the same type of molecule it is always positive.

Laws of Thermochemistry

1. Lavoisier and Laplace Laws

C(s) + O₂(g)→CO₂      ΔH=-393 kJ

CO₂(g)→C(s) + O₂(g) ΔH= +393 KJ

2. Hess Law of Constant Heat Summation 

In physical or Chemical processes heat of reaction/enthalpy of reaction remains the same whether it takes place in one step or multistep.

Thermochemistry and Calorimetry

The only thermal quantity that can be measured directly is the heat denoted by q that flows into or out of a reaction vessel (system), and that q is numerically equal to ΔH° only under the special condition of constant pressure. Moreover, q is equal to the standard enthalpy change only when the reactants and products are both at the same temperature, normally 25°C. The measurement of heat (q) is known as calorimetry.

An indirect Calorimeter determines heat (q)  which is produced by living bodies by measuring the production of nitrogen compounds and carbon dioxide or from the amount of oxygen taken. A direct Calorimeter can be used to determine heat that is produced by living bodies.

Did You Know?

• If heat required to dissociate bonds is more than the heat evolved in bond formation, then the stability of the reactant is more than the stability of the product.

• The formation of an explosive compound is an endothermic reaction.

• NaCl, KCl, and NH₄Cl do not form hydrated salts.

Conclusion

The main distinction between thermochemistry and thermodynamics is that thermochemistry is the quantitative study of the relationship between heat and chemical reactions, whereas thermodynamics is the study of laws relating to the same.