Hess Law

As well as being known by the abbreviation “Hess’ law,” Hess’ law of constant heat summation is an important relationship in physical chemistry that was named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. In accordance with this concept, the total enthalpy change that occurs during the whole course of a chemical reaction is not reliant on the sequence of steps that are taken.

Hess’s law is today recognised to be a statement of the notion that the enthalpy of a chemical process is independent of the path taken from the initial to the final state during the course of the procedure (i.e., enthalpy is a state function). As stated in the first rule of thermodynamics, the enthalpy changes in a system owing to a reaction occurring at constant pressure equals the heat absorbed (or the negative of the heat released), which may be calculated using calorimetry for a wide variety of processes. The values are typically given for reactions that occur at the same temperatures and pressures at the start and end of the reaction (while conditions are allowed to vary during the course of the reactions). Hess’s law can be used to calculate the total amount of energy required for a chemical reaction that can be broken into synthetic steps that are individually easier to characterise than the reaction itself. This allows for the compiling of conventional enthalpies of formation, which may then be used to forecast the enthalpy change in complex syntheses that are more complicated.

A description on Hess’s law

Under the conditions of equal beginning and end states for both reactants and products, Hess’s law asserts that the change in enthalpy in a chemical reaction is the same regardless of whether the reaction occurs in a single step or several steps. Enthalpy is an extensive attribute, which means that the value of enthalpy changes depending on the sample size used to calculate it. As a result, the enthalpy of a reaction changes in proportion to the number of moles involved in the process.

With this in mind, it doesn’t matter whether a chemical change occurs through numerous different channels; the overall enthalpy change is the same regardless of which route the chemical change occurs through (provided the initial and final conditions are the same). The first law of thermodynamics states that if this is not true, then it is possible to break the law.

In the case of a reaction, Hess’s law permits the enthalpy change (H) to be computed even when the reaction cannot be observed directly. Basic algebraic operations based on chemical equations of reactions are used to do this, with previously calculated values for the enthalpies of formation serving as the basis for the calculations.

A net equation, also known as an overall equation, is formed by combining several chemical equations. If the enthalpy changes for all of the equations in the series are known, the sum of the enthalpy changes for the net equation will be the enthalpy change for the net equation. If the net enthalpy change is negative, the reaction is exothermic and hence more likely to occur spontaneously; positive H values correspond to endothermic reactions and are therefore less likely to occur spontaneously. It should be noted that entropy can also play a key part in determining spontaneity, since some reactions with a positive enthalpy change can still be considered spontaneous due to a rise in entropy in the reaction system.

Hess’s Law and Its Importance

It is possible to break down a chemical reaction into many steps and use the conventional enthalpies of formation to determine how much energy is involved in the entire reaction. This is possible because Hess’s law holds true. Equivalent enthalpy tables are compiled using empirical data, which is typically obtained through calorimetry. It is possible to determine if a more complex reaction is thermodynamically advantageous or not by referring to these tables.

Conclusion

Hess’s law states that the total amount of energy changed by a chemical reaction is equal to the total amount of energy changed by all of the individual reactions that make up the reaction. In other words, the enthalpy changes of a chemical reaction (i.e, the heat of reaction under constant pressure) is independent of the pathway between the initial and final states. Essentially, the law is a variation on the first law of thermodynamics as well as the conservation of energy principle.