Grade 11

Grade 11Thermodynamics


Hess's law of constant heat summation


Hess's law of constant heat addition, commonly called Hess's law, is an important principle studied in chemistry, particularly in the field of thermochemistry. It deals with the heat involved in chemical reactions and allows chemists to determine the enthalpy changes of reactions even when they cannot be measured directly. The law is named after Russian chemist Germain Hess, who formulated it in 1840.

Understanding Hess's law

To begin understanding Hess's law, it's important to know that in chemistry, a chemical reaction often involves the breaking and forming of bonds, resulting in a change in energy. This energy change is often measured as heat or enthalpy change, represented as ΔH. Hess's law states that the total enthalpy change of a chemical reaction is the same no matter how it is carried out, whether directly in one step or indirectly in multiple steps.

Hess's law can be expressed as follows:

ΔH_total = ΔH_1 + ΔH_2 + ... + ΔH_n

Here, ΔH_total is the total enthalpy change for the reaction, and ΔH_1, ΔH_2, ..., ΔH_n are the enthalpy changes for each step in the reaction pathway.

Thermodynamics and enthalpy

Thermodynamics is the branch of physics that deals with heat and temperature, and their relation to energy and work. In the context of chemical reactions, enthalpy (H) is a measure of the total energy of a thermodynamic system, including internal energy and the energy needed to displace the environment to make way for the system. The change in enthalpy (ΔH) is what we are primarily interested in, since it indicates the heat absorbed or released under conditions of constant pressure.

Importance of Hess's law

Hess's law is important for several reasons:

  • Predicting enthalpy change: This allows us to predict the total enthalpy change for a chemical reaction, even if it is difficult to measure directly.
  • Path Independence: This law states that the enthalpy change for a reaction is path-independent and depends only on the initial and final states.
  • Thermochemical calculations: It helps significantly in thermochemical calculations where direct measurement is impractical.

Visual example of Hess's law

Let's look at an example of the formation of water (H₂O) from hydrogen (H₂) and oxygen (O₂).

Step 1: H₂(g) → 2H(g) ΔH₁
Step 2: O₂(g) → 2O(g) ΔH₂
Step 3: 2H(g) + O(g) → H₂O(g) ΔH₃

Overall reaction: H₂(g) + 1/2 O₂(g) → H₂O(g) ΔH_total
    
        
            
            
            
            H₂(g) + 1/2 O₂(g)
            ΔH₁, ΔH₂
            
            2H(g) + O(g)
            ΔH₃
            
            H₂O(g)
        
    

The overall enthalpy change for water formation can be calculated by adding the enthalpy changes of these steps:

ΔH_total = ΔH₁ + ΔH₂ + ΔH₃

Text example using Hess's law

Suppose we want to determine the enthalpy change when graphite is burned to form carbon dioxide:

Step 1 (known): C(graphite) + O₂(g) → CO₂(g) ΔH₁ = -393.5 kJ/mol
Step 2 (hypothetical): CO₂(g) → C(graphite) + O₂(g) ΔH₂ = +393.5 kJ/mol
Overall reaction: C(graphite) + O₂(g) → CO₂(g) ΔH_total
    

Using Hess's law, the enthalpy change of the direct reaction would be the same as in Step 1, but using a hypothetical inverse step shows how Hess's law applies:

ΔH_total = ΔH₁ + ΔH₂
ΔH_total = -393.5 + 393.5 = 0 (for imaginary inverse)
ΔH_total = -393.5 kJ/mol (for the actual reaction, Step 1)
    

This shows how Hess's law ensures consistency in measuring the enthalpy change, no matter which route is taken.

Application of Hess's law in chemistry

Hess's law finds applications in various fields of chemistry:

  • Determination of enthalpy of formation: It helps in calculating the enthalpy of formation of compounds using data obtained from known reactions.
  • Understanding reaction pathways: It is used to analyze reaction pathways and intermediate steps in complex reactions.
  • Design of chemical processes: Industrial chemists use it to design energy-efficient processes.

Conclusion

Hess's law of constant heat summation is a fundamental principle in thermochemistry that underlies the conservation of energy. By showing that the enthalpy change is independent of the path taken by a reaction, it allows chemists to calculate the heat of reaction in a systematic and predictable way. This law is not simply a theoretical construct; it is a widely used tool in research and industry to solve practical problems involving chemical energy transformations.

Through an understanding of this law, students and chemists can appreciate the beauty and usefulness of thermodynamics concepts in predicting and measuring energy changes associated with chemical reactions.


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