Problem 173 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: Finely divided solid reactants react much faster as compared to massive reactants. Reason: Finely divided solid reactants possess a large surface area.

Step-by-Step Solution

Answer

Both assertion and reason are true; the reason correctly explains the assertion.

Step 1: Understanding the Assertion

The assertion suggests that finely divided solid reactants, such as powders, react more quickly than massive or bulk reactants. The key here is to focus on the difference between finely divided and massive reactants.

Step 2: Exploring the Reason

The reason given is that finely divided solids have a larger surface area available for reaction compared to a chunk of the same material. The increased surface area allows more reactant molecules to collide and react at the same time.

Step 3: Relating the Reason to the Assertion

The reason supports the assertion because more surface area in finely divided reactants leads to faster reaction rates. Therefore, the assertion is true because the increased surface area, as stated in the reason, means more opportunities for reactions to occur simultaneously.

Step 4: Conclusion

Both the assertion and the reason are true. Moreover, the reason correctly explains the assertion. Finely divided solids react faster due to their larger surface area facilitating more frequent collisions.

Key Concepts

Surface Area

Surface area is integral to chemical reactions, especially with solid reactants. When a reactant has a larger surface area, it provides more spots for molecules to collide. Imagine a loaf of bread compared to the same loaf sliced into thin pieces. While the loaf's overall mass doesn't change, slicing it greatly increases its surface area. Similarly, when a solid reactant is broken down into smaller pieces or powdered, its surface area increases, leading to more exposure and contact with other reactants. An increased surface area means:

• More frequent collisions between reactant molecules.
• Higher reaction rates due to more active collision points.

This concept directly explains why finely divided solids often react more vigorously than their bulk counterparts. Understanding this can provide valuable insights into how to efficiently achieve desired reaction speeds.

Solid Reactants

Solid reactants are materials in a solid phase that participate in chemical reactions. In courses from chemistry, solids are distinct from liquids and gases based on particle arrangement. In a solid, particles are tightly packed, which often limits their reactivity. Reacting solid particles primarily occurs at the surface as particles within the bulk remain trapped and inert. Thus, the nature of solid reactants plays a pivotal role in determining the speed and efficiency of a reaction. For instance:

• Finely ground solid powders have more significant reactivity than blocks due to increased surface area.
• The way solids are prepared and used in a reaction can drastically change outcomes.

Practical scenarios often involve maximizing the surface interaction of solid reactants by smashing, grinding, or powdering, hence facilitating quicker reactions. Consider this when setting up experiments that involve solid-phase chemistry.

Collision Theory

Collision theory provides insights into how reactions occur and why different factors affect reaction rates. According to this theory, chemical reactions result from the collisions of reactant particles. These collisions must have enough energy and correct orientation to break bonds and form new ones, leading to product formation. Here’s why collision theory is important:

• Helps in understanding why reactions are faster when reactants are in powdered form.
• Explains how temperature, surface area, concentration, and catalysts influence reaction rates.

When the surface area of solid reactants increases, more molecules are available for effective collisions. Thus, more collisions mean a higher chance for a reaction. Understanding collision theory aids students in predicting and controlling reaction rates in laboratories and industrial settings, allowing for optimal conditions and improved efficiencies in reactions.