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:
Hess's Law is a consequence of the fact that enthalpy (\(H\)) is a state function.
Step 3: Detailed Explanation:
Statement: The total enthalpy change for a chemical reaction is the same regardless of whether the reaction occurs in one step or in several steps.
Explanation: Consider a reaction \(A \rightarrow D\) with enthalpy change \(\Delta H\).
If the reaction occurs via steps:
1. \(A \rightarrow B\) (\(\Delta H_1\))
2. \(B \rightarrow C\) (\(\Delta H_2\))
3. \(C \rightarrow D\) (\(\Delta H_3\))
Then, according to Hess's Law:
\[ \Delta H = \Delta H_1 + \Delta H_2 + \Delta H_3 \]
This law allows us to calculate enthalpies of reactions that are difficult to measure directly by summing enthalpies of other known reactions.
Step 4: Final Answer:
Hess's Law states \(\Delta H_{total} = \sum \Delta H_{steps}\).