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

The correct answer is (a) greater number of molecules are having the activation energy.

Step 1: Understanding the Problem

We need to determine why an increase in temperature increases the rate of a reaction according to collision theory. This involves looking at factors like activation energy and molecular collisions.

Step 2: Analyzing Collision Theory

Collision theory posits that molecules must collide with sufficient energy (activation energy) and proper orientation to react. Thus, increasing temperature can affect these factors.

Step 3: Effect of Temperature on Activation Energy

Higher temperature gives more molecules enough energy to overcome the activation energy barrier. Therefore, more molecules have the requisite energy at higher temperatures.

Step 4: Evaluating Options

Looking at the options: (a) suggests more molecules have the activation energy - well aligned with our understanding. (b) discusses velocity, which affects the energy of collisions but not directly the rate. (c) mentions more collisions, which is a consequence but not the primary explanation. (d) is not applicable. The best explanation is (a).

Step 5: Conclusion

An increase in temperature increases the number of molecules with enough energy to overcome the activation energy barrier, thus increasing the rate of reaction.

Key Concepts

Reaction Rates

When we talk about reaction rates in chemistry, we are discussing how quickly a chemical reaction occurs. Reaction rates are crucial as they determine how fast products form from reactants. This can be particularly important in industrial processes or in situations where a quick reaction might be necessary for safety or efficiency.

The rate of a reaction depends on several factors, including:

• Concentration of reactants: More concentrated reactants lead to more frequent collisions.
• Surface area: Larger surface areas can enhance reaction rates by allowing more collisions.
• Catalysts: Catalysts speed up reactions by providing an alternative pathway with a lower activation energy.

In essence, reaction rates are about how often reactant molecules collide and how many of these collisions are successful in leading to a reaction. To increase reaction rates, we typically look for ways to boost the frequency and energy of these molecular collisions.

Activation Energy

Activation energy refers to the minimum energy that reacting molecules need in order to successfully collide and cause a reaction. It is often visualized as a hill that the reactants must climb over to become products.

Molecules must possess at least this amount of energy during a collision to convert reactants into products. If they don't, they simply bounce off each other without any reaction occurring. \[ E_a \]

Here, \( E_a \) symbolizes activation energy.

Several factors can influence activation energy:

• Temperature: Higher temperatures can impart more energy to molecules, helping them reach or exceed activation energy.
• Presence of a catalyst: Catalysts work by lowering the activation energy needed for a reaction.

Understanding activation energy helps in predicting whether a reaction will occur at a certain temperature and how quickly it will proceed.

Temperature Effect on Reactions

Temperature plays a vital role in the rates of chemical reactions. According to collision theory, increasing the temperature increases the kinetic energy of molecules. As molecules move faster, they collide more often and with greater energy.

This has several effects:

• More frequent collisions: The number of collisions between molecules rises.
• Increased energy of collisions: More molecules have the necessary activation energy at higher temperatures.
• Accelerated reaction rates: The overall speed of these reactions goes up as more collisions become successful.

In practical terms, think of cooking: higher heat generally makes food cook faster because the increased temperature speeds up the chemical reactions involved in cooking. Similarly, in chemical reactions, a rise in temperature typically leads to a faster rate of reaction.