Application of Hess’s Law

Hess’s Rule

Hess’s Law states that the total enthalpy change for a reaction is equal to the sum of all changes, regardless of how many stages or steps are involved in the reaction. This law demonstrates that enthalpy is a state function, which is important to understand.

Hess’s Law is named after Germain Hess, a Russian chemist and physician who discovered it in 1896. Hess was a pivotal person in the creation of the early concepts of thermochemistry. In honour of Russian chemist and doctor Germain Hess, Hess’s Law, his most well-known publication, is named after him. Hess was a pivotal person in the creation of the early concepts of thermochemistry. His law on thermochemistry was included in his most recognised paper, which was published in 1840 and is still considered to be his best work. Based on the fact that enthalpy is a state function, Hess’s rule allows us to calculate the overall change in enthalpy by simply adding the changes for each step until the product is created. This is known as the enthalpy-to-temperature relationship. It is necessary to complete all steps at the same temperature, and the equations for each step must be balanced.

What is Hess’s Law?

In light of the fact that enthalpy is a state function, we may say that its value remains constant regardless of which path is chosen to achieve the final state from the beginning state. According to Hess’s law, the standard reaction enthalpy for a multistep reaction is independent of the pathway or number of steps taken; rather, it is the sum of the standard enthalpies of intermediate reactions involved at the same temperature, as determined by Hess’s equation.

This law is intended to be used to estimate the enthalpies of neutralisation for numerous acid-base reactions, and then use that information with Hess’s law to determine the reaction enthalpies for two salts in aqueous solution, among other things.

Hess’s Law and Its Application

The Hess law of Heat Summation is used in this application.

It is possible to estimate heat changes that cannot be detected experimentally using Hess’ law of heat summation, which is an efficient method.

1. The change in enthalpy that occurs during a physical change

Carbon and diamond are both allotropes of the chemical element carbon. However, because the process of converting graphite to diamond cannot be carried out, it is impossible to measure the energy change that occurs during the conversion. Nonetheless, using Hess law, it is possible to compute the heat changes associated with this hypothetical physical shift.

In the presence of oxygen, graphite and diamond combine to produce heats of reaction of -393.4kJ and – 395.4kJ, respectively.

C (graphite) + O2 → CO2 ΔHgr = -393.4kJ

C (diamond) + O2 → CO2 ΔHdi = -395.4kJ

Reversing the combustion reaction of diamond we get-

CO2 → C (diamond) + O2 ΔHdi = + 395.4kJ

Now Adding both,

C (graphite) + O2 → CO 2 ΔHgr = – 393.4kJ

C (graphite) → C (diamond) ΔHtr = +2.kJ

Enthalpy change in the allotrope transition of graphite to diamond is an endothermic reaction that is estimated at 2KJ.

2. The change in enthalpy of a chemical reaction

The bond energies of hydrogen, iodine, and hydrogen iodide are 218 kilojoules, 107 kilojoules, and 299 kilojoules.

Calculate the enthalpy of the production of hydrogen iodide. Is the reaction exothermic or endothermic in character?

The reaction that results in the formation of hydrogen iodide from hydrogen and iodine is as follows:

½ H2 + ½ I2 = HI

It is the heat changes that occur when one atom of hydrogen reacts with one atom of iodine in ordinary conditions to generate a mole of hydrogen iodide (as gas). It is necessary to break the molecular bond in order to obtain one atom of hydrogen or iodine.Heating up to create a bond equals the sum of the bond energies of the hydrogen atoms and the hydrogen atoms and the I2-atom bond energies.

the product of [299 -(218 + 107)] = 299-325 = -26kJ

In this case, the reaction is exothermic as a result of the negative formation heat.

3. Enthalpy of formation

When carbon and hydrogen mix, a wide variety of hydrocarbons can be created. As a result, it is impossible to measure the heat of production of benzene empirically. Hess law can be used to calculate the amount of heat that has changed.

6C + 3H2 → C6H6  determine ΔH C6H6 = ?

The heat of production of carbon dioxide and water is -393.5 kJ and -285.8 kJ, respectively in the absence of oxygen. The heat of combustion of benzene is -3301 kilojoules (kJ).

C + O2 → CO2  ΔH1 = -393.5kJ…..1

H2 + O2 → H2O ΔH2 = -285.8kJ……2

C6H6 + 9O2 → 6CO2 + 3H2O  ΔH3 = -3301kJ …….3

6 x Reaction 1 is 6C + 6O2 → 6CO2   6ΔH1 = -2361kJ…..4

3 x Reaction2 is: 3H2 + 3O2 → 3H2O … 3ΔH2 = -857.4kJ……5

Reverse of reaction 3: 6CO2 + 3H2O → C6H6 + 9O2 -ΔH3 = +3301kJ …….6

Adding the three reactions together- 6C + 3H2 → C6H6 ΔH= +82.6kJ

Heat of formation of  C6H6 is 82.6kJ.

4. Bond energy

The amount of energy required to break the atoms participating in a molecular connection into free atoms is known as bond energy, which is a measurement of the bond strength of a chemical bond.

5. Lattice energy

The energy change that occurs when one mole of a crystalline ionic compound is formed from its constituent ions, which are believed to be in a gaseous state at the outset. It’s a measurement of the ionic solids’ cohesive forces.

Hess Law and Its More Application

Hess’ law can be used to compute the enthalpies of the compounds listed in the table below.

• CO(g) and NO2 are examples of unstable intermediates that can be produced (g)
• Heat is transferred during phase transitions and allotropic transitions
• By constructing Born–Haber cycles, or by using other methods, lattice energies of ionic compounds can be computed if the electron affinity to form the anion is known
• Electron affinities are calculated by employing a Born–Haber cycle with a theoretical lattice energy as the starting point

Conclusion

Hess’s law provides for the estimation of the enthalpy shift for any of the reactions, even if the enthalpy shift cannot be known directly. This can be accomplished by performing simple algebraic operations on the Hess’s law equation of the reactions and using the values that were previously determined for the formation enthalpies in order to obtain the desired result.