Hess's Law:
Hess's Law states that the total enthalpy change for a chemical reaction is independent of the pathway taken to reach the final products. In other words, the change in enthalpy (thermal effect) of a reaction depends only on the initial and final conditions of the reacting substances, regardless of any intermediate states or reactions that may occur.

Explanation:
Hess's Law is based on the principle of conservation of energy. It states that the total energy change during a reaction is constant and does not depend on the specific steps taken to reach the final products. This law allows us to calculate the enthalpy change of a reaction by considering the enthalpy changes of other reactions that can be combined to give the desired reaction.

Initial and Final Conditions:
The enthalpy change of a reaction depends on the initial and final conditions of the reacting substances. The initial conditions refer to the starting materials or reactants, while the final conditions refer to the products formed after the reaction is complete. These conditions include the physical states (solid, liquid, gas) of the substances, their concentrations, and the temperature and pressure at which the reaction occurs.

Intermediate States:
According to Hess's Law, the enthalpy change of a reaction is independent of any intermediate states that may occur during the reaction. Intermediate states refer to the transient species or reaction intermediates that are formed and consumed during the reaction but do not appear in the overall balanced chemical equation. These intermediates may have different enthalpy changes associated with them, but they do not affect the overall enthalpy change of the reaction.

Significance of Hess's Law:
Hess's Law is a crucial concept in thermodynamics as it allows us to determine the enthalpy change of a reaction indirectly. By combining known enthalpy changes of other reactions, we can calculate the enthalpy change of a reaction that cannot be directly measured. This is particularly useful when direct measurements are not feasible or when the reaction occurs under extreme conditions.

Conclusion:
In summary, according to Hess's Law, the thermal effect of a reaction depends only on the initial and final conditions of the reacting substances. The law disregards any intermediate states or reactions that may occur during the reaction. It provides a powerful tool for calculating enthalpy changes and understanding the relationship between different reactions.