Question

Identify the law for the statement "Overall the enthalpy change for a reaction is equal to sum of enthalpy changes of individual steps in the reaction"

• A. First law of thermodynamics
• B. Hess's law
• C. Avogadro's law
• D. Second law of thermodynamics

Hint:

The statement describes the path independence of enthalpy change, which is Hess law.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The statement says the total enthalpy change for a reaction equals the sum of the enthalpy changes of its individual steps. We must name the law that says this.

Step 2: Key Concept:
Enthalpy is a state function, so its change depends only on the initial and final states, not on the path. This is the content of Hess's law of constant heat summation.

Step 3: Detailed Explanation:
If a reaction can be done in one step or as a series of steps, the overall \(\Delta H\) is the same in both cases:
\[ \Delta H_{\text{overall}} = \Delta H_1 + \Delta H_2 + \Delta H_3 + \dots \]
This lets us calculate enthalpies that cannot be measured directly, for example the formation of CO from carbon and oxygen.

Step 4: Why the other options are wrong.
The first law of thermodynamics (A) deals with conservation of energy, \(\Delta U = q + W\). Avogadro's law (C) says equal volumes of gases at the same temperature and pressure hold equal numbers of molecules. The second law (D) concerns entropy and the direction of spontaneous change.

Final Answer:
The statement is Hess's law, option (B). \[ \boxed{\text{Hess's law (B)}} \]