Question

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

Short answer

The reason correctly explains the assertion; larger surface area increases reaction rate.

Step-by-step solution

Understanding the Assertion

First, let's understand the assertion: 'Finely divided solid reactants react much faster as compared to massive reactants.' This suggests that smaller particles of a solid (like powder) react more quickly than larger chunks of the same material due to their size.

Analyzing the Reason

Now, examine the reason given: 'Finely divided solid reactants possess a large surface area.' When a solid is finely divided, more of its surface is exposed to the surroundings, increasing the surface area available for reactions.

Connecting the Assertion and Reason

Relate the assertion and reason: The increased surface area from being finely divided allows more molecules of the reactant to be in contact with the reactant molecules, hence increasing the reaction rate. Therefore, the reason correctly explains why the assertion is valid.

Evaluating the Explanation

Conclude that the reason provided is a correct explanation for the assertion. Finely divided solid reactants do indeed react faster because the larger surface area allows for more interaction with other reactants.

Key concepts

Surface Area

Surface area plays a crucial role in determining the rate at which chemical reactions occur, especially for solid reactants. When a solid material is exposed to a reaction medium, only its surface can interact with the reactant molecules. A larger surface area means more molecules are in contact at once.
This results in a faster reaction.

• Maximizing surface area can be achieved by breaking down the material into smaller pieces.
• More surface exposure increases the speed of the reaction.

Imagine two cubes of sugar: one whole and one ground into a fine powder.
The powdered sugar dissolves more quickly in water due to its greater surface exposure.

Finely Divided Solids

Finely divided solids are materials that have been broken down into tiny particles. These small particles expose a greater surface area relative to their mass. This characteristic makes finely divided solids highly reactive.

• They speed up chemical processes.
• Industrial processes often use finely divided forms to enhance efficiency.

For instance, in the presence of a catalyst, finely divided platinum is more effective in speeding up reactions than bulk platinum, because of the increased surface contact.

Solid Reactants

Solid reactants are substances in a solid state that partake in a chemical reaction. They may not react as quickly as gases or liquids unless their surface area is optimized.
The physical state limits the area available for reaction.

• Reducing the particle size of solid reactants can make them react faster.
• Grinding or powdering are common methods to achieve this.

This is why powdered sulfur will react with oxychloride solutions more rapidly than bigger sulfur lumps.

Chemical Reaction

A chemical reaction involves the transformation of reactants into one or more products, with bonds being broken and formed. In solid-gas reactions, the reaction rate depends on how well the solid and gas interact.

• The surface area of solids is crucial for reactions with gases.
• More surface contact leads to faster chemical changes.

Consider rust formation: iron reacts with oxygen in air. Rust forms faster if the iron is finely divided, rather than as a large lump.

Reaction Mechanism

The reaction mechanism describes the step-by-step process by which a chemical reaction occurs. It provides insight into the pathway and the intermediate stages of a reaction. Involving finely divided solids in a reaction generally leads to a simpler mechanism because they offer large reactive surfaces.

• More surface area enables distinct stages to occur more rapidly.
• Intermediates have less distance to travel over the surface.

For example, in combustible reactions involving powdered coal, the mechanism is accelerated due to the vast exposure provided by the small particles.