Hess's law of constant heat summation is a fundamental principle in thermodynamics that states that the enthalpy change for a chemical reaction is the same whether it occurs in one step or a series of steps. In other words, the overall enthalpy change of a reaction is independent of the pathway taken.

Hess's law is based on the principle of conservation of energy, which states that energy cannot be created or destroyed, only transferred or converted from one form to another. In the context of chemical reactions, this means that the total energy change of a reaction must be the same regardless of the route taken.

Hess's law can be understood and applied using the concept of enthalpy, which is a measure of the heat energy absorbed or released during a reaction. Enthalpy is a state function, which means it only depends on the initial and final states of a system and is independent of the path taken.

Explanation of Hess's Law:
1. Enthalpy Change:
- Enthalpy change (∆H) is the heat energy released or absorbed during a chemical reaction.
- It is expressed in terms of heat energy per mole of reactant or product.
- Enthalpy change can be positive (endothermic) or negative (exothermic), depending on whether heat is absorbed or released.

2. Hess's Law Statement:
- Hess's law states that the overall enthalpy change (∆H) of a reaction is determined by the difference in enthalpy between the products and reactants, regardless of the pathway taken.
- In other words, if a reaction can occur in multiple steps, the sum of the enthalpy changes for each step will be equal to the overall enthalpy change of the reaction.

3. Application of Hess's Law:
- Hess's law is commonly used to determine the enthalpy change of a reaction indirectly.
- If the enthalpy changes of individual steps in a reaction are known, the overall enthalpy change can be calculated by algebraically summing the enthalpy changes of the steps.

4. Example:
- Let's consider the combustion of methane (CH4) to form carbon dioxide (CO2) and water (H2O).
- This reaction can occur in two steps: methane to carbon monoxide (CO), and carbon monoxide to carbon dioxide.
- The enthalpy changes for these steps are known:
- ∆H1 = -74.8 kJ/mol (methane to carbon monoxide)
- ∆H2 = -283.0 kJ/mol (carbon monoxide to carbon dioxide)
- By applying Hess's law, we can calculate the overall enthalpy change (∆H) for the combustion of methane by summing the enthalpy changes of the steps:
- ∆H = ∆H1 + ∆H2
- ∆H = -74.8 kJ/mol + (-283.0 kJ/mol)
- ∆H = -357.8 kJ/mol

In conclusion, Hess's law of constant heat summation is a powerful tool in thermodynamics that allows us to determine the overall enthalpy change of a reaction by summing the enthalpy changes of individual steps. It is based on the principle of conservation of energy and the fact that enthalpy is a state function. Hess's law