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Hess Law

Last updated date: 17th Apr 2024
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What is Hess law?

In 1840, Chemist G.H. Hess put forward an important law governing the enthalpies of reaction. According to Hess' law:


“The enthalpy change during a chemical reaction is the same, whether a reaction takes place in one or several steps.”


The enthalpy of a reaction depends upon nature and state reactants and products. It is independent of the path followed by the chemical reaction. The law is, in fact, merely a special case of the first law of thermodynamics (law of conservation of energy).


Proof: A theoretical proof of Hess law can be obtained from the following considerations. 


Let a substance A be converted into B by a process involving a single step reaction which is accompanied by the evolution of Q kJ of heat. In the second process, let A be converted into B in two steps, i.e., A to C and then C to B. Let Q1and Q2 be the heat involved in the two-step respectively, so that Q1+Q2=Q Where Q' is the total heat produced in going from A to B. Suppose Q is greater than Q'. 


Now if we go from A to B, in a single step, Q kJ of heat will be evolved while returning from B to A through C, Q'kJ of heat will be absorbed. In this way, Q-Q'kJ heat will be produced by transforming A to B in a single step and then B to A via C. 


In other words, a large amount of heat can be produced without doing any external work by merely repeating the above cycle several times. However, this is against the law of conservation of energy. Hence, Q' must be equal to Q as required by Hess’ law.   


Illustration of Hess law

Illustration 1) As a further illustration of the law, consider the formation of carbon dioxide. There are two ways in which CO2 can be formed. 

(i) In the first step, by burning carbon in excess of oxygen. 

C(s)+O2CO2(g); ΔrH= -393.5 k J

(ii) In the second step, by burning carbon in a limited supply of oxygen to form CO and then CO is converted to CO2.

C(s)+12O2(g)CO(g);  ΔrH  = -110.54 kJ

CO(g)+12O2(g)CO2(g);  ΔrH  = -393.5 kJ

On adding, we get

C(s)+O2(g)CO2(g);  ΔrH  = -393.5 kJ

Thus, in both cases,  ΔrH   is the same. This proves the law. 


Illustration 2) Now consider the formation of an aqueous solution of ammonium chloride from NH3HCL and water. There are two ways in which the reaction can be brought about. 

(i) NH3(g) + HCL(g) → NH4CL(s); ΔrH  =175.73 kJ

NH4CL(s)+aq → NH4CL(aq); ΔrH  = +16.32 kJ

On adding, 

NH3(g)+HCL(g)+aq → NH4CL(aq); ΔrH  = -159.41kJ

(ii) NH3(g) +aq → NH3(aq);  ΔrH  = -35.15 kJ

HCL (g)+aq →HCL(aq); ΔrH  = -72.38 kJ

NH3(aq)+HCL(aq)  → NH4CL(aq); ΔrH  = -51.36 kJ

On adding NH3(g) + HCL(g)+aq → NH4CL(aq)  ΔrH  = -158.99 kJ

Thus, in both cases, the net enthalpy change is nearly the same. The difference in 0.42kJ may be due to experimental error. 


Hess Law Examples

Example 1) Calculate the enthalpy of formation of methane from the following data:

i) C(s)+O2(g)CO2(g);   ΔrH   = -394 kJ

ii) H2(g)+12O2(g)H2O(l);  ΔrH    = -286 kJ

iii) CH4(g) +2O2(g)CO2(g) +2H2O(l);   ΔrHⒽ    = -890.0 kJ

Solution 1) The required Hess law equation is:

C(s)+2H2(g)CH4(g);  ΔrH    =?

In order to get the required Hess law equation from equation (i), (ii), and (iii), multiply equation (ii) by two and add to equation (i), i.e., 2 x equation (ii) + equation (i)

2H2(g)+O2(g)2H2O(l);   ΔrH    = -572 kJ

C(s)+O2(g)CO2(g);  ΔrH  = -394

(iv) C(s) + 2H2(g)+2O2(g)CO2(g) +2H2O(l)   ΔrH    = -966 kJ

Subtracting equation (iii) from (iv) we get,

C(s)+2H2(g)CH4(g)  ΔrHⒽ  = -76.0 kJ

Thus, enthalpy of formation of methane is -76.0 kJ

Practice more Hess law examples to have a clear underlying of the concept. 

Hess Law

Hess's Law asserts that the total enthalpy change for a reaction is the sum of all changes, regardless of how many stages or steps there are. The fact that enthalpy is a state function is demonstrated by this law.

Hess's Law is named after Germain Hess, a Russian chemist and doctor. Hess was a key figure in the development of thermochemistry's early ideas. Hess's Law, his most well-known publication, is named after Russian chemist and doctor Germain Hess. Hess was a key figure in the development of thermochemistry's early ideas. His law on thermochemistry was included in his most renowned article, which was published in 1840. Hess's law is based on the fact that enthalpy is a state function, allowing us to determine the overall change in enthalpy by simply adding the changes for each step until the product is formed. All steps must be completed at the same temperature, and the equations for each step must be balanced.

Application of Hess Law

Hess' law can be used to calculate the enthalpies of the following substances.

  • Heats associated with the creation of unstable intermediates such as CO(g) and NO (g).

  • In phase transitions and allotropic transitions, heat changes.

  • If the electron affinity to create the anion is known, lattice energies of ionic compounds can be calculated by constructing Born–Haber cycles, or

  • A Born–Haber cycle with a theoretical lattice energy is used to calculate electron affinities.

According to Hess's Law, if you convert reactants A to products B in one step, two steps, or however many steps, the overall enthalpy change will be the same.

I'll give you an easy example. You're on the ground floor of a five-star hotel, and you'd want to go to the third storey. You can do it in one of three ways: (a) take the elevator straight to the third floor from the ground floor. (b) You can take the elevator from the ground floor to the second floor, then take the elevator from the second floor to the third floor after stopping for a bit on the second floor. (c) You can take the elevator from the ground floor to the first floor, then take the elevator from the first floor to the third floor after stopping for a bit on the first floor.No matter which route you choose, the elevator will consume the same amount of energy.

FAQs on Hess Law

1. What is the Significance of Hess Law?

Each and every substance such as atoms or molecules possesses energy inside them. The internal energy that these substances possess depends on the nature of the force that exists in the substance and the temperature. If these substances have to undergo chemical reactions, some of the bonds that are connected to some atoms are broken and some bonds are made new. This break and make of the bonds involve energy.

So, in reactions, the product substances might have either less or more or the same energy than the reacting substances. Accordingly, the reactions may release heat to become exothermic or absorb heat to become endothermic. Having the knowledge of the energy changes in any reaction is useful for manipulating the reactants and the products in a chemical process to our necessity. 


Heat energy changes of reactions that are measured at a constant volume which is known as internal energy change ΔE and the energy measured at constant pressure is known as enthalpy change ΔH.

2. What are the Hess law Examples or the Application of Hess Law?

One of the most useful applications of Hess law is the calculation of the value of rH for a reaction whose rH is unknown or cannot be measured. According to Hess’s law, we can add thermochemical equations to obtain some desired thermochemical equation and its rH. 


Applying this concept, we can calculate the heat of formation, the heat of transition of allotropic modifications, the heat of hydration and heat of various reactions. There is yet much other application of Hess law. 

3. State a few facts about Hess Law?

  1. Hess's Law is named after Germain Hess, a Russian chemist and physician.

  2. In 1840, Hess published his law of thermochemistry, which was based on his research into thermochemistry.

  3. To apply Hess's Law, all of a chemical reaction's component steps must occur at the same temperature.

  4. In addition to enthalpy, Hess's Law can be used to compute entropy and Gibbs energy.

4. Is  it possible to measure the enthalpy change for a reaction ?

It can be difficult, if not impossible, to directly measure the enthalpy change of a reaction in the laboratory. Some reactions occur at such a slow rate that direct measurement is impossible. Fortunately, an indirect approach may be used to determine the enthalpy change for a reaction. If two or more thermochemical equations can be added together to generate a final equation, then the heats of reaction can likewise be summed to give a heat of reaction for the final equation, according to Hess's law of heat summation.

5. What method do you use to calculate Hess's law?

According to Hess's law, the total enthalpy change is independent of the path taken from beginning to conclusion.

As a result, the enthalpy can be calculated as the total of multiple tiny steps.

  1. When altering an equation, there are a few guidelines to follow.

  2. You can solve the problem by reversing the equation. The sign of △ H will be changed as a result of this.

  3. A constant can be used to multiply the equation. The value of  △H must then be multiplied by the same constant.

  4. You can combine the first two rules in any way you like.

6.What exactly is enthalpy? 

The heat content of a system under constant pressure is denoted by (H). The enthalpy change is the heat absorbed or released by a reaction at constant pressure, and it is denoted by the sign (H). All reactions in this article are considered to take place at constant pressure unless otherwise noted. As a result, enthalpy and the first law of thermodynamics, which states that energy cannot be generated or destroyed, are linked.

7. Is it possible to calculate enthalpy?

It's impossible to quantify.

Enthalpy is a psychological concept, not a physical property. Enthalpy is a measurement of energy content that is affected by temperature. High temperatures and pressures, not enthalpies, cause pipes to explode. Because an enthalpy change requires a change in a thermal physical attribute called temperature, enthalpy cannot induce a physical change. When you hear someone remark, "I was hit by a stone, and the stone had a high enthalpy," you could think, "Well, maybe we can quantify enthalpy physically."