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Enthalpy
It is the total of the system’s internal and pV-energy. The total energy of a thermodynamic system is measured by enthalpy. It encompasses both the internal energy required to form a system and the energy required to create space by displacing its environment and establishing its volume and pressure. H=U + PV is how a system’s enthalpy is calculated. The system’s internal energy is denoted by the letter U. P is the pressure at the system’s and its environment interface. V is the system’s volume. It’s a state function. A system’s absolute value of ΔH cannot be determined directly. As a result, the absolute value of enthalpy, H is less informative than the change in enthalpy ΔH. For endothermic processes, ΔH is positive. For exothermic processes, ΔH is –ve. ΔH = H2 – H1
Bond Enthalpy
Bond enthalpy describe the amount of energy stored in a bond between atoms in a molecule (also known as bond-dissociation enthalpy, or bond strength). Anionic or homolytic bonds can be cleaved with the energy required through homolytic or symmetrical cleavage in the gas phase. When a bond is broken through homolytic or symmetrical cleavage, every original participant gets an electron and becomes a radical rather than an ion.
When the thermodynamics are favourable, chemical bonds are formed, and breaking them requires energy. The higher the enthalpy of a bond, the more energy is required to break it and the stronger the bond. We just make the bond enthalpy value harmful to estimate how much energy will be released when building a new bond rather than breaking it. Because bond enthalpy values are valuable, reference tables with average bond enthalpies for common bond types are widely available. While the exact energy shift while establishing and breaking bonds is dependent on nearby atoms in a given molecule, the average values in the tables can still be used as a rough guide.
Whenever a spontaneous process occurs, new products are formed. Some of the processes we know absorb energy, while others evolve. Thus, we always notice a change in enthalpy at the start of a new process. The most common enthalpy changes are explained below: Atomization, sublimation and phase transition, etc.
Enthalpy of Atomisation
Each spontaneous process tends to result in the production of novel products. Some processes, out of all the ones we know about, tend to absorb energy, while others simply result in energy evolution. As a result, when we complete these activities, we usually experience enthalpy or heat change. When the bonds of a compound tend to break, and elements of that complex are reduced to singular atoms, the enthalpy of atomization refers to the amount of change in heat.
The heat of atomization is always a positive number and can never be negative. The symbol for the enthalpy of atomization is ΔaH . In this post, we’ll learn what the enthalpy of atomization is, what different enthalpy changes are, and how to calculate the enthalpy change of atomization. Let’s start with a discussion of the various changes in atomization heat. The amount of energy required when bonds are dissociated, and the constituent atoms are reduced to individual atoms is known as enthalpy of atomization. The transition of enthalpy occurs when one mole of a substance is entirely dissociated through its atoms under normal conditions (298.15 K, 1 bar).
Cl2 (g) → 2Cl (g) ………………ΔaH
Enthalpy of Sublimation
Converting a solid to a gas without passing through the liquid phase is known as sublimation. We need a specific quantity of energy to sublimate a substance, which must be supplied to the substance in the form of heat or work. The enthalpy of sublimation is the energy required to convert a mole of solid into a gas at constant pressure.
C (s) → C (g) ………………ΔsubH
Enthalpy Change During Phase Transition:
When a substance’s phase changes from one form to another, some energy is released or absorbed. For example, when the ice melts into water, energy is required to melt it.
Standard Enthalpy of Vaporisation:
At constant temperature and pressure, the enthalpy of vaporization is the amount of heat it takes to vaporize one mole of a liquid.
H2O (l) → H2O (g) ………………..ΔvapH0
Standard sublimation enthalpy:
The drop in enthalpy that happens when one mole of solid substance sublimes at constant pressure and temperature is known as the standard enthalpy of sublimation(at 1 bar).
C (s) → C (g) ………………ΔsubH0
Factors Affecting Bond Enthalpy
(i) Atomic size- as size increases, bond enthalpy decreases
(ii) Electronegativity – as electronegativity increases, bond enthalpy increases
(iii) Length of the bond – as bond length, increases bond enthalpy decreases
(iv) Bonding electrons – bond enthalpy increases with increased number of bonding electrons
Conclusion
In this article, we came to understand enthalpy of atomisation, sublimation, and phase transition and bond dissociation. The enthalpy of binding and the enthalpy of reaction are two terms that describe how a chemical system consumes energy during reactions. The bond enthalpy is a measure of bond strength and describes how much energy is required to break or build a bond.
