Enthalpy of Reaction

Chemical reactions cause atoms’ bonds to break, reform, or both in order to absorb or release energy. As a result, the system’s potential energy is altered. Enthalpy is the heat absorbed or released from a system under constant pressure, and the enthalpy of reaction is the change in enthalpy caused by a chemical reaction. The enthalpy of reaction is written as ΔHrxn. The difference between the potential energy of the product bonds and the potential energy of the reactant bonds can be thought of as the enthalpy of reaction.

The amount of heat that should be added or removed during a chemical reaction to keep all of the substances present at the same temperature is known as heat of reaction. If the pressure in the vessel containing the reacting system remains constant, the measured heat of reaction also represents the change in the thermodynamic quantity known as enthalpy, or heat content, that occurs during the reaction i.e. the difference between the enthalpy of the substances present at the end of the reaction and the enthalpy of the substances present at the beginning of the reaction. As a result, the enthalpy of reaction, denoted by the symbol ΔH, is also known as the heat of reaction measured at constant pressure.The reaction is said to be endothermic if the heat of reaction is positive, and exothermic if the heat of reaction is negative.

What is Endothermic Reaction?

Endothermic reactions require external energy to proceed, usually in the form of heat. Endothermic reactions tend to cool their environments because they absorb heat from their surroundings. Endothermic reactions produce products that have a higher energy level than the reactants, so they are generally non-spontaneous. As a result, the enthalpy change in an endothermic reaction is always positive. Heat is required to melt the ice cube, so the process is endothermic. The products of an endothermic reaction have more energy than the reactants. As a result, the change in enthalpy is positive, and the reaction absorbs heat from the environment.

What is Exothermic Reaction?

Exothermic reactions are those that produce heat or light as a byproduct. Since the total energy of the products is less than that of the reactants, energy is released in an exothermic reaction. As a result, for an exothermic reaction, the change in enthalpy, ΔH, is always negative. A strong acid reacts exothermically when it dissociates quickly in the presence of water, releasing heat. 

The products’ total energy is less than the reactants’ total energy in an exothermic reaction. As a result of the negative change in enthalpy, heat is released into the environment.

How to determine Enthalpy of Reaction ?

• Using the formula- by following steps

1. Find products and reactants of the reaction.
2. Find the total mass of reactants (m).
3. Get the specific heat value of the products (s).
4. Find the change in temperature before and after the reaction (∆T ).
5. Use the formula ∆H = m x s x ∆T 

• Using bond energies-

Almost all chemical reactions involve the formation or breaking of atom-to-atom bonds. Because energy cannot be destroyed or created in a chemical reaction, we can estimate the enthalpy change for the entire reaction with high accuracy by adding up the energy required to form or break the bonds being formed (or broken) in the reaction.

• Using enthalpy of Formation-

Enthalpies of formation are a set of  ∆H values that represent the enthalpy changes caused by reactions that are used to make certain chemicals. Similar to bond energies, if you know the enthalpies of formation required to create products and reactants in an equation, you can add them up to estimate the enthalpy.

Bond Enthalpy

The amount of energy stored in a bond between atoms in a molecule is described by bond enthalpy. Chemical bonds form when the thermodynamics are favourable, and breaking them necessitates the addition of energy. Bond enthalpy values are always positive as a result of this. The higher the enthalpy of a bond, the more energy is required to break it and the stronger the bond.

Example of Enthalpy of reaction

Take, for example, the hydrogenation of propane, C3H6 to produce propane, C3H8. 

Propene gas reacts with hydrogen gas to form propane gas in this reaction.

C3H6 + H2 → C3H8

We must first break the carbon and hydrogen bonds between the reactants. Breaking bonds between atoms usually necessitates the addition of energy. The more energy it takes to break a bond, the stronger it is. A new bond and two new bonds are formed to make the propane product. Because breaking bonds requires the addition of energy, forming new bonds always releases energy. The more energy released during the bond formation process, the stronger the bond formed. The resulting system has a lower potential energy than the reactants because the newly formed bonds release more energy than was required to break the original bonds. This indicates that the reaction’s enthalpy is negative.

Conclusion

The enthalpy of reaction is a measure of how much energy a chemical system uses during a reaction. The bond enthalpy is a measure of bond strength and describes how much energy is required to break or form a bond. It’s possible to estimate the total change in potential energy of the system, which Hrxn for a reaction at constant pressure, by adding the bond enthalpy values for all of the bonds broken and formed during a reaction. We can determine whether a reaction is endothermic or exothermic based on whether the enthalpy of the reaction is positive or negative.