Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

If the value of \(\mathrm{AH}\) in a reaction is positive, then the reaction is called (a) exothermic (b) endothermic (c) polymorphic (d) polytropic

Step-by-Step Solution

Answer

The reaction is endothermic.

Step 1: Understanding Enthalpy Change

In chemistry, the symbol \( \Delta H \) represents the enthalpy change of a reaction. This measures the heat absorbed or released during a chemical reaction at constant pressure.

Step 2: Determine the Significance of Positive \( \Delta H \)

When \( \Delta H \) is positive, it indicates that the system absorbs heat from the surroundings. This means that the reaction requires an input of energy to proceed.

Step 3: Identify the Reaction Type

A reaction where \( \Delta H \) is positive is called an endothermic reaction, as it absorbs heat.

Key Concepts

Understanding Enthalpy Change

Enthalpy change is a crucial concept in understanding how heat energy is transferred during chemical reactions. When chemists talk about enthalpy, they are referring to the total heat content of a system. This is especially important when reactions occur at constant pressure, which is common in laboratory and real-world scenarios.

The symbol \( \Delta H \) is used to denote the change in enthalpy during a reaction. It tells us how much heat is either absorbed or released when reactants turn into products. A positive or negative sign in \( \Delta H \) provides insights into the nature of the reaction and how energy is moving. Understanding \( \Delta H \) helps predict whether a reaction will need more heat to proceed or if it will release heat. This knowledge is essential for controlling and scaling chemical processes, from laboratory experiments to industrial applications.

It's also important to consider the surroundings' effect on the reaction as they play a critical role in energy transfer, shaping the overall enthalpy change experienced.

Positive Delta H: A Sign of Heat Absorption

Positive \( \Delta H \) is a key indicator that a reaction is endothermic. In such reactions, heat is absorbed from the surroundings.

This means that the reaction requires external energy input to keep going. For instance, when cooking an egg, heat from the stove is absorbed by the egg, changing its structure. In chemical terms, the process needed energy from outside heat sources.

A positive \( \Delta H \) value signifies a greater enthalpy of products compared to reactants:

• This increase necessitates energy from outside, taken from surroundings, thus making \( \Delta H \) positive.
• Substances can only react if this outside energy is sufficient to overcome activation barriers.

By understanding \( \Delta H \), we can anticipate whether a reaction will need supplemental heat to initiate and progress.

Heat Absorption in Reactions

Endothermic reactions are fascinating because they involve the consumption of heat energy from the surroundings. This characteristic is vital for many processes in both daily life and industrial applications.

Consider photosynthesis: plants absorb sunlight (heat energy) to convert carbon dioxide and water into glucose and oxygen, showcasing a natural example of heat absorption in reactions.

Here’s how heat absorption plays a role:

• The absorbance of heat means that endothermic reactions may feel cold to the touch as they draw energy from the surroundings.
• For practical applications, sometimes heat sources must be provided continuously to sustain these reactions.
• This external heat requirement highlights the need for careful control and monitoring in processes where heat absorption is crucial.

Understanding these reactions helps explain many biological, chemical, and physical transformations that are essential to life and technology.