Question

State and explain Hess's law of constant heat summation.

Hint:

Treat thermochemical equations like algebraic equations. You can add, subtract, or multiply them by coefficients, and the corresponding \( \Delta H \) values will follow the same operations.

Answer 1

Step 1: Understanding the Concept:
Enthalpy (\( H \)) is a state function, meaning its value depends only on the initial and final states of the system, not on the path taken. Hess's Law is a direct consequence of this property.
Step 2: Detailed Explanation:
Consider a reaction where reactants \( A \) are converted to products \( B \).
Path 1: Direct conversion:
\[ A \rightarrow B \quad \Delta H = \Delta H_{direct} \]
Path 2: Indirect conversion through intermediates \( C \) and \( D \):
Step 1: \( A \rightarrow C \quad \Delta H_1 \)
Step 2: \( C \rightarrow D \quad \Delta H_2 \)
Step 3: \( D \rightarrow B \quad \Delta H_3 \)
According to Hess's Law:
\[ \Delta H_{direct} = \Delta H_1 + \Delta H_2 + \Delta H_3 \]
This law is extremely useful for calculating enthalpies of reactions that are difficult to measure experimentally, such as the enthalpy of formation of \( CO \) from \( C \) and \( O_2 \).
Step 3: Final Answer:
The total enthalpy change of a reaction is independent of the pathway followed, provided the initial and final states are the same.