Hess's law states that

Options

A. the standard enthalpy of an overall reaction is the sum of the enthalpy changes in individual reactions.
B. enthalpy of formation of a comound is same as the enthalpy of decomposition of the compound into constituent elements, but with opposite sign.
C. at constant temperature the pressure of a gas is inversely proportional to its volume
D. the mass of a gas dissolved per litre of a solvent is proportional to the pressure of the gas in equilibrium with the solution

Explanation

**Step-by-Step Solution:**

1. **Understanding Hess's Law**: Hess's Law is based on the principle of conservation of energy, which is a fundamental concept in thermodynamics. It states that the total enthalpy change of a reaction is the same, regardless of whether the reaction occurs in one step or multiple steps.

2. **Illustrating the Concept**: Consider a hypothetical reaction where a substance A transforms into substance D. This transformation can occur directly (A to D) or through intermediate steps (A to B, B to C, and C to D).

3. **Defining Enthalpy Changes**: Let’s denote the enthalpy changes for each step:
- From A to B: ΔH₁
- From B to C: ΔH₂
- From C to D: ΔH₃
- The overall reaction from A to D can be represented as ΔH_total.

4. **Applying Hess's Law**: According to Hess's Law, the total enthalpy change for the reaction from A to D (ΔH_total) is equal to the sum of the enthalpy changes for the individual steps:
\[ ΔH_{total} = ΔH₁ + ΔH₂ + ΔH₃ \]

5. **Conclusion**: This means that regardless of the pathway taken (direct or through intermediates), the total enthalpy change remains the same. Therefore, Hess's Law can be summarized as: "The standard enthalpy change of an overall reaction is equal to the sum of the enthalpy changes for the individual steps of the reaction."

6. **Identifying the Correct Statement**: Given the options, the correct statement that aligns with Hess's Law is: "The standard enthalpy of an overall reaction is the sum of the enthalpy change in individual reactions."