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Hess’s Law, also referred to as Hess’s law of constant heat summation or Hess’s law of heat summation, was enunciated by a Swiss-born Russian chemist Germain Henri Hess in 1840. He stated that the heat absorbed or evolved or the change in enthalpy in any chemical reaction is a fixed quantity and at the same time, is independent of the path of the reaction. Let’s know about Hess’s law, its application, equation along with some solved questions.
The energy stored by the molecules, especially the chemical energy, can be released in any form, including heat during the chemical reactions. For example, cooking gas or when coal burns in the air, the energy is released. Now the question arises is how to understand the concept of energy changes and calculate when a chemical reaction occurs? What caused the chemical reaction? For how long will the reaction continue or take place? The answer to all such questions and more comes when we understand the concepts of thermodynamics. Thermodynamics talks about the system and its surroundings, the types of systems available, heat transfer between the system and the surroundings, etc. One such law which helps us in understanding all these concepts and more is Hess’s law.
Hess's Law
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Also called “the law of constant heat summation,” this is one of the most important laws of thermodynamics. This was derived in 1840 by a Russian chemist and physician Germain Hess. The law states that the enthalpy change in the chemical or the physical process is almost the same whether it is carried out in a single step or multi-steps.
The law tries to demonstrate these state functions, explaining that the value of state functions. It does not depend on the path they take for the formation or the dissociation and only depends on the state at the moment (heat, temperature, pressure, etc.).
Since enthalpy is also a state function, it does not depend on which path we follow to reach from the initial state to the final state. Thus, using HESS's LAW, especially in the multi-step reactions, the calculation of reaction enthalpy remains the same and is not dependent on the path taken to conclude.
Applications of the Hess’s Law
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To understand the concept more, let’s take some examples and see how the reactions and enthalpies differ or remain the same if we take a single-step or multi-step process.
If we take the formation of sulphur dioxide from sulphur, this is a multi-step reaction:
- Developing the Sulphur Dioxide gas
S + O2 → SO2 (ΔH1= -70.96)
- Converting this to sulphur trioxide gas
SO2 + ½ O2 → SO3 (ΔH2= -23.47)
- According to Hess's Law, reaction enthalpy is
ΔHr= ΔH1+ ΔH2
=-70.96+-23.47
=-94.95
- Net reaction
S + 3/2 O2 à SO3
ΔHr= -94.95
ΔHr= ΔH1+ ΔH2+ ΔH3……
Calculating the Enthalpy
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There are so many compounds like Co, C2H6 which can’t be determined in a single step and require Hess's Law for its determination. Hess's Law also helps in the determination of Gibbs's free energy and entropy.
ΔGreaction= ∑(ΔGproduct) - ∑(ΔGreactant)
The entropy is calculated using,
ΔSreaction = ∑ΔSproduct - ∑(ΔSreactan)
Process of Calculating the Reaction Enthalpy
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Since we have calculated the standard enthalpy of reactants and the products, we now need to calculate the overall enthalpy of the reaction, and we use Hess's Law for that.
According to Hess's Law,
ΔHreaction= ΔHproduct - ΔHreactan
So basically,
(Sum of the standard enthalpies of the product formation) – (Sum of the standard enthalpies of the reactant formation)
Use of Hess’s Law
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- Calculating the electron affinities in the Born Haber cycle.
- Calculation of heat changes in phase transitions.
- Calculating the heat of unstable compounds like NO (g) and Co (g).
Things to Remember
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- The topic “Hess’s Law” is covered in Chapter 6 Chemical Thermodynamics of NCERT Class 11 Chemistry.
- The topic will carry a weightage of 14 periods and candidates can expect around 4-5 marks in the board exam.
- Hess’s law allows the enthalpy change for a reaction to be determined even when it can not be measured directly.
- It is possible to break a chemical reaction into multiple steps and use the standard enthalpies of formation to determine the overall energy of a chemical reaction due to Hess’s law.
To have a proper understanding of the topic, refer to the video given below:?
Sample Questions of Hess’s Law
Ques. What is the enthalpy change for an adiabatic process? (1 mark)
Answer – For an adiabatic process, ΔH = 0.
Ques:Explain Hess law with an example? (5 marks)
Answer – This law was developed by a Russian physicist named Germain Hess. He stated that the law of constant summation of heat states that the change in the enthalpy during a chemical reaction is not dependent on the path or the direction between the initial state and final state. Therefore, this rule requires calculating the change in enthalpy (ΔH), although it can’t be calculated directly.
Example: We can understand the concept of Hess law by seeing the formation of CO2.
Reaction 1:
C + O2 (g) → CO2
?H= -94.05 (heat of formation)
Reaction 2:
C + ½ O2 → CO (ΔH= -26.417)
CO + ½ O2 → CO2 (ΔH= -67.637)
According to Hess law, irrespective of how CO2 is formed, the change in enthalpy remains the same.
Ques: What do you understand by the term entropy? (1 mark)
Answer – This thermodynamic term is originally derived from a Greek word which means ‘lack of predictability or a ‘change towards something.’ It is often referred to as the degree of randomness or degree of disorder.
Ques: Explain the uses of Hess law? (2 marks)
Answer – There are various uses of the law of summation; some are mentioned below:
This is used to calculate the heat changes in phase transitions.
- This is used to calculate the heat of unstable compounds like NO (g) etc.
- This is used in the Born Haber cycle.
Ques: What does Hess’s law state? (2 marks)
Answer – According to the Hess’s law, in the chemical reaction, the change of enthalpy i.e, the heat of reaction under constant pressure is independent of the direction between the original and final states. The law of Hess requires the change in enthalpy for a reaction that has to determined even though it can not be directly measured.
Ques: Why is Hess’s law Important? (2 marks)
Answer – Hess’s law is significant as it allows the enthalpy shift to be determined for a reaction, even though it can not be directly measured. This is carried out by simple algebraic operations based on chemical equation of reactions using values that are previously defined for the formation of enthalpies.
Ques: What is the basis of Hess’s law? (2 marks)
Answer – Hess’s law states that the increase in enthalpy in any chemical reaction i.e, reaction heat at constant pressure is independent of the process between the initial and the final states.
Ques. What do you mean by entropy? (2 marks)
Answer – Entropy in genrally is a measure of everything in the universe that are in transitions from order to chaos. The term entropy is derived from the Greek word entropy which means “a change toward” or “transformation”.
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