Question

The enthalpy change of a reaction does not depend on: (a) Initial and final enthalpy change of reaction (b) State of reactants and products (c) Different intermediate reactions (d) Nature of reactants and products

Short answer

(c) Different intermediate reactions

Step-by-step solution

Understand the Problem

The problem is asking us to determine which factor the enthalpy change of a reaction is independent of. Enthalpy change is a thermodynamic property that measures the heat absorbed or evolved during a chemical process at constant pressure.

Review Concept of Enthalpy Change

According to Hess's Law, the total enthalpy change for a reaction is the same regardless of the pathway taken, because enthalpy is a state function. This means it depends only on the initial and final states, not the path or intermediate steps.

Analyze the Options

Each option should be evaluated: (a) Enthalpy change does depend on initial and final enthalpy but not on how it changes during the reaction, so this isn’t independent. (b) The state of reactants and products affects enthalpy because phase changes involve enthalpy changes, so this isn’t independent. (c) Intermediate reactions are part of the path, not the initial or final state, making them irrelevant to the net enthalpy change. (d) The nature of reactants and products influences the energy content, so enthalpy is affected.

Select the Correct Option

Based on the concept that enthalpy is a state function and does not depend on the path taken, the correct choice is (c) Different intermediate reactions do not affect the overall enthalpy change.

Key concepts

Thermodynamic Properties

Thermodynamic properties are key elements that help describe the physical state of a system, such as temperature, pressure, and volume. Another crucial thermodynamic property is enthalpy, symbolized as \( H \). Enthalpy accounts for the heat content of a system at constant pressure, which is why it plays a significant role in chemical reactions.
In a chemical reaction, the enthalpy change \( \Delta H \) represents the difference in enthalpy between products and reactants. This change measures the heat absorbed or released. If \( \Delta H \) is negative, the reaction releases heat, making it exothermic. Conversely, a positive \( \Delta H \) means the reaction absorbs heat, categorizing it as endothermic.
Understanding thermodynamic properties helps predict reaction behaviors and evaluate energy changes within chemical processes, making it a foundational concept for any student diving into thermodynamics.

Hess's Law

Hess's Law is a fundamental concept in chemistry that governs the prediction of enthalpy changes in reactions. According to Hess's Law, the total enthalpy change for a given reaction is the same, no matter how many steps or pathways the reaction takes to proceed. This principle is a direct consequence of enthalpy being a state function.
To apply Hess's Law effectively, it's essential to understand that you can split complex reactions into simpler steps, calculate the enthalpy changes for each step, and then sum them up to find the overall enthalpy change. The pathway independence implied by Hess's Law is extremely useful:

• Enables calculation of enthalpy changes in reactions where direct measurement is challenging.
• Allows chemists to use known enthalpy values of simpler reactions to predict the enthalpy of a more complex one.

Hess's Law illustrates the importance of understanding state functions and is an invaluable tool for students in mastering the energetics of reactions.

State Functions

State functions are properties that depend solely on the current state of a system, not on how it reached that state. This is what makes them so unique and useful in chemistry and physics. Common examples of state functions include enthalpy \( H \), internal energy \( U \), and entropy \( S \).
The concept behind state functions is that they help simplify complex systems. By focusing only on the initial and final states, state functions allow for predictions about energy changes without worrying about the specific processes or intermediate steps taken.
This is crucial when working with enthalpy changes in reactions. Since enthalpy is a state function, it means that you only need to know the initial and final states of the reaction to determine the total enthalpy change. This understanding underscores why option (c) in the exercise is correct: intermediate steps or different pathways do not influence the net enthalpy change.