Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

The enthalpy change for a reaction does not depend upon the (a) physical state of reactants and products (b) use of different reactants for the same product (c) nature of intermediate reaction steps (d) difference in initial or final temperatures of in volved substances

Step-by-Step Solution

Answer

The enthalpy change for a reaction does not depend upon the nature of intermediate reaction steps (option c).

Step 1: Understand the Problem

The question is asking about what enthalpy change for a reaction does not depend on. Enthalpy change refers to the heat absorbed or released under constant pressure during a reaction.

Step 2: Review Each Option

Option (a) refers to the physical state of reactants and products, which affects enthalpy change because gases, liquids, and solids have different energy levels. Option (b) talks about different reactants producing the same product, which can affect enthalpy if the pathways differ. Option (c) addresses the nature of intermediate steps, meaning how the reaction proceeds in terms of steps from reactants to products. Option (d) is about temperature differences, which do affect the enthalpy due to the heat capacity of substances.

Step 3: Focus on the Nature of Reaction Steps

The enthalpy change is a state function, which means it only depends on the initial and final states and not on the pathway taken (intermediate steps) to get there.

Step 4: Identify the Correct Option

Since enthalpy change doesn't depend on how you transition between states, option (c) - the nature of intermediate reaction steps - is the feature it does not depend on.

Key Concepts

Physical State of Reactants and Products

The physical state of a substance, whether it is a solid, liquid, or gas, can significantly affect its energy level. When considering the enthalpy change in a chemical reaction, it's important to remember that different states have different enthalpies. This is due to the varying amounts of energy required for molecules to exist in these states. For instance, converting water from liquid to vapor requires energy in the form of heat, known as the enthalpy of vaporization.

Therefore, when calculating the enthalpy change, it's crucial to account for the physical state of both reactants and products. The transition of substances from one state to another will involve either the absorption or release of energy. This variance in energy between states directly influences the overall enthalpy change during the reaction.

It's helpful to understand this concept with practical examples such as melting ice or boiling water. Each physical transformation leads to different enthalpy changes, adding complexity to a simple reaction. This is why the physical states are indeed a factor that must be considered while evaluating enthalpy changes in reactions.

Nature of Intermediate Reaction Steps

The journey from reactants to products in a chemical reaction involves multiple intermediate steps. Each of these steps can influence the energy profile of the reaction pathway but does not affect the enthalpy change. This might sound confusing at first.

The key is to understand that enthalpy change is what is known as a 'state function'. This means it only cares about where the reaction starts and where it ends, not how it gets there. So, intermediate steps, no matter how complex or varied, do not have an impact on the value of enthalpy change.

Imagine plotting these steps as points on a graph, where the y-axis represents energy. While the pathway taken might zigzag up and down, the initial and final points on this graph determine the enthalpy change.

• The sum of the enthalpy change of all intermediate steps will always equal the enthalpy change between the initial and final states.
• This principle holds no matter how many intermediate steps are involved.

This characteristic can be a great asset when analyzing or predicting the energy changes in reactions, offering a simplified view amidst complicated reaction mechanisms.

State Function

In thermodynamics, enthalpy is classified as a state function. But what does this mean for calculations and understanding reactions? A state function is a property of a system that only depends on the current state of the system, not on how that state was reached.

This property is immensely useful because it implies that the enthalpy change of a reaction can be determined solely by looking at the initial and final states. It doesn't matter if a reaction goes through a single step or a hundred intermediary stages; the total enthalpy change remains unaffected.

• State functions simplify many complex behaviors by emphasizing what you have now versus what you started with.
• It ensures consistency and reliability in thermodynamic calculations.

Understanding enthalpy as a state function equips students and chemists alike with a powerful tool. It allows predictions and calculations even in intricate reactions, offering a backbone of reliability amidst the chaos. Emphasizing state functions in your approach simplifies determining the enthalpy change and fosters deeper insights into thermodynamics.