Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

According to the collision theory of reaction rates, an increase of the temperature at which the reaction oc curs will inturn increase the rate of the reaction. This caused due to (a) greater number of molecules are having the activation energy (threshold energy) (b) greater velocity of reaction molecules (c) greater number of collisions (d) none of these

Step-by-Step Solution

Answer

(a) greater number of molecules are having the activation energy (threshold energy)

Step 1: Breaking Down Collision Theory

Collision theory explains that chemical reactions occur when reactant molecules effectively collide. For a collision to be effective, molecules must have sufficient energy (activation energy) and a proper orientation during the collision.

Step 2: Understanding Temperature's Role

Increasing the temperature generally increases the kinetic energy of molecules. This results in more molecules having sufficient energy to exceed the activation energy threshold, leading to a greater frequency of effective collisions.

Step 3: Analyzing Each Option

(a) At higher temperatures, more molecules possess the activation energy, making this option correct. (b) Increased velocity of molecules is true at higher temperatures, but this alone doesn't directly address collision effectiveness. (c) Higher temperature results in more collisions, but the emphasis in collision theory lies on effective collisions. (d) Since options (a), (b), and (c) all contain aspects of the temperature effect, option (d) 'none of these' is incorrect.

Step 4: Selecting the Best Answer

Given the information from collision theory, the increase in temperature predominantly affects the number of molecules that reach the activation energy threshold, making option (a) the most relevant and correct.

Key Concepts

Reaction Rates

Reaction rates quantify how quickly a chemical reaction occurs. They are influenced by the frequency and effectiveness of molecular collisions. A high reaction rate indicates that a large number of reactant molecules are transforming into products in a short amount of time. The rate can be affected by several factors including:

• Concentration of reactants: Higher concentration usually leads to more collisions.
• Surface area: Greater surface area of a solid reactant increases reaction rate.
• Presence of catalysts: Catalysts lower the activation energy, increasing the rate without being consumed.

When analyzing reaction rates in the context of collision theory, it's important to note that not all collisions result in a reaction. Only those collisions where molecules have enough energy and the correct orientation will lead to a chemical change.

Activation Energy

Activation energy is the minimum energy required for a reaction to occur. It serves as a barrier that reactant molecules must overcome to transform into products. Every reaction has its own specific activation energy, dictated by the nature of the reactants and the process involved in the reaction. The concept can be likened to pushing a boulder over a hill; the energy needed to reach the top is the activation energy. In terms of collision theory:

• Only molecules with kinetic energy equal to or greater than the activation energy can react upon collision.
• Effective collisions, i.e., those leading to a reaction, require both sufficient energy and proper molecular orientation.

A catalyst can lower the activation energy, making it easier for reactions to occur. This is why adding a catalyst often speeds up a reaction.

Temperature Influence on Reactions

Temperature plays a critical role in the rate of a chemical reaction. As the temperature increases, the kinetic energy of the molecules also increases. This increase in kinetic energy results in several effects crucial to understanding reaction dynamics:

• More molecules possess the requisite activation energy to react, leading to a higher number of effective collisions.
• The speed of the molecules increases, resulting in more collisions in a given time frame.

Higher temperatures mean that more molecules are moving faster and with more energy, increasing the probability of overcoming the activation energy barrier. However, not all reactions speed up with an increase in temperature; for endothermic reactions, a temperature rise can be particularly beneficial. Maintaining the correct temperature is crucial for controlling reaction rates safely and effectively.