Enthalpy Change in Thermodynamics

Enthalpy can be defined as the energy or the heat content of the total system. Enthalpy is one of the most crucial and important factors in thermodynamics (the study of the interrelation between heat and other forms of energy). The measurement of change in enthalpy is important for understanding the total mechanics of the chemical reaction. Any chemical (or biochemical) reaction involved an exchange of energy in any form amongst the substance undergoing that reaction and their surroundings. Whenever a reaction occurs, heat energy is either utilized by that system or is given out from that system. All in all, every reaction involves some change in the enthalpy of the total system. Without, change in enthalpy, no reaction around us is possible.

When a system undergoing a chemical reaction requires an inflow of the energy, the enthalpy of the total system increases and when it gives off some of its energy into the surrounding, the enthalpy of the total system decreases. The part of the energy of the molecules that get utilized during a reaction is known as the internal enthalpy or the enthalpy possessed by the system and the energy which is not related to the components or substances undergoing reaction is known as the external enthalpy or the enthalpy of the surroundings.

The total enthalpy change during a chemical reaction is molecularly equal to the amount of heat energy that went into the system from the surroundings or went into the surroundings from the system. In totality, the first law of thermodynamics holds true during every reaction that no amount of heat is either created or destroyed during any reaction. In general, all spontaneous processes or reactions that occur in nature tend to shift their equilibrium towards that side of the reaction where the total enthalpy of the system is decreased i.e., the net amount of energy is lost from the system into the surroundings.

The enthalpy change in the chemical reactions is due to the bond breaking and making process that occurs in order to lead to the formation of some new products from the pre-existing reactants taken at the start of the reaction. Bond breaking process always requires some energy (in the form of activation energy) to excite the electrons of the molecule to their excited state where they collide with the molecules of the other reactant so as to initiate the reaction and proceed it in the forward direction. On the other hand, the breaking and making process always gives out energy from the system into the surrounding since the new final products that are formed in any reaction are always more stable than wither the initial reactants of the reaction or the intermediates formed during the proceedings of the reactions. In mathematical terms, enthalpy is the difference between the potential energies of the total bond energies possessed by the products and the reactants. The change in enthalpy is denoted by ∆H.

So, ∆H = Potential energy of the bonds of products – Potential energy of the bonds of reactants

The enthalpy change in the reaction is an important determinant of whether the reaction that is taking place would be an endothermic reaction or an exothermic one. A chemical reaction becomes endothermic when the bonds of the reactant substances which need to be broken for the initiation of the reaction are very strong and require more energy than what is eventually evolved during the formation of new bonds in the product molecule(s). On the other hand, a chemical reaction becomes exothermic if the formation of new bonds liberates more energy than what was utilized in the initial bond breaking process of the reactants in the form of activation energy. Hence, the overall energy which is required to either break a bond or make a bond, also known as the bond energy, is a very crucial factor which helps in determining the overall change in enthalpy of a particular reaction. However, the total bond energy of a reaction is not always an easy thing to be determined theoretically since when it comes to the actual practicality of the reaction conditions, a lot of factors affect the ongoing reaction. However, with years of research, scientists have collected data about the bond energies and hence the change in enthalpies involved in the common reactions that we encounter almost every day or the reactions which are important industrially and commercially.

With the help of bond energy, we can calculate an estimated change in enthalpy involved in a chemical reaction. For doing this, one needs to follow these steps:

• First, we need to identify which particular bond in the reactant molecules are going to disintegrate or break during the reaction. After identifying such bonds, we need to find the bond enthalpy associated with those bonds which is easily available in the database.

• After finding the bond enthalpy values of all the bonds that are going to break, we need to arithmetically add all those numerical values.

• Similarly, we need to identify which bonds are going to form at the end of the ongoing reaction in the form of new product and find their theoretical numerical values available in the database and add them arithmetically.

• Always remember to assign the correct signs to the bond energies of each side. Since the reactant are requiring the energy to go into the system for bond breakage, energy is supplied to them and hence the sign of the bond energy values is always positive while products involve bond making which releases the energy out of the system and hence their bond energy values are always negative.

• Now combine the bond energies of both the sides. Add the total reactant bond energy value (added from the reactant side) and the total product bond energy value (added from the product side). The final value gives you the total enthalpy change during the reaction.

Now let us try an example if we could find out if can really apply the above-mentioned steps to find the change of enthalpy in any given reaction. The example that we are taking here is the hydrogenation of propene:

CH₃CH=CH₂+ H₂→ CH₃CH₂CH₃

                         (Propene) (Molecular (Propane)

                                                              Hydrogen)

As we can see in this reaction, the double bond present in the propene molecule is getting reduced upon undergoing a reaction with the molecular hydrogen to produce a saturated hydrocarbon molecule of propane. So, in this reaction the double bond present in the reactant molecule would break along with the single bond present in the molecular hydrogenso as to lead the formation of two new C–H bonds (carbon and hydrogen bonds) and one new C–C bond (carbon and carbon bond) in the product. So, now we know that for which bond we need to find the bond energy from the database.

• After searching through the database, we find that the bond enthalpy of the carbon–carbon double bond (C=C) is 610 kJ/mol whereas the bond enthalpy of the hydrogen–hydrogen single bond (H–H) is 436 kJ/mol.

• Now we will add the bond enthalpy associated with both the reactants on the left (reactant) side of the reaction. Since this is the reactant side (which involves bond breaking and requires the flow of energy into the system from the surroundings), the sign of the bond enthalpy value would be taken as negative. This comes out to be 610 + 436 =1,046 kJ/mol. This is the total energy required to break the bonds of the reactants that will take part in the ongoing chemical reaction.

• Similarly from the literature, we found that the bond enthalpy of the carbon–hydrogen (C–H) single bonds is 413 kJ/mol whereas the bond enthalpy of the carbon–carbon single bond (C–C) is 346 kJ/mol.

• Now we will add the bond enthalpy associated with both the bonds in the product on the right (product) side of the reaction. Since, there is a formation of two new C–H bonds (carbon and hydrogen bonds), we would require to add the bond enthalpy value associated with this bond twice and since this is the product side (which involves bond making and the flow of energy is into the surroundings from the system), the sign of the bond enthalpy value would be taken as negative. This comes out to be –413 + (–413) + (–346) =–1,172 kJ/mol. This is the total energy liberated out of the system upon the formation of new bonds in the product.

• Now add the bond enthalpy of both the sides. It gives 1,046 + (–1,172)= –126 kJ/mol, which is the total enthalpy change during the reaction. Since the final value of negative, it is an exothermic reaction.