Problem 18 - Chemistry in 30 days

Question

Which of the following statements is correct for the activation energy of a reaction? (a) It increases with increase in temperature. (b) When the activation energy is zero the rate constant is temperature- dependent. (c) It decreases with decrease in temperature. (d) It is nearly independent of temperature, over a wide range.

Step-by-Step Solution

Answer

The correct statement is (d): Activation energy is nearly independent of temperature, over a wide range.

Step 1: Understanding Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. It's the energy barrier that the reactants must overcome to form products.

Step 2: Analyzing Statement (a)

Statement (a) claims that activation energy increases with an increase in temperature. However, activation energy is a property dependent on the reaction mechanism, not on temperature. Temperature affects the rate of reaction, not the activation energy itself.

Step 3: Evaluating Statement (b)

Statement (b) states that when the activation energy is zero, the rate constant is temperature-dependent. This is incorrect because with zero activation energy, the reaction rate would not be dependent on the temperature as there is no energy barrier to overcome.

Step 4: Examining Statement (c)

Statement (c) suggests that activation energy decreases with a decrease in temperature. Similar to statement (a), this is incorrect since activation energy remains constant regardless of temperature changes.

Step 5: Considering Statement (d)

Statement (d) asserts that activation energy is nearly independent of temperature over a wide range. This is a correct statement, as activation energy remains constant for a given reaction unless altered by a catalyst.

Step 6: Conclusion

Considering all the statements, only statement (d) correctly describes the behavior of activation energy. Activation energy is intrinsic to the reaction mechanism and does not depend on temperature.

Key Concepts

Chemical Reaction

A chemical reaction happens when molecules interact with each other to form new compounds. This process involves breaking bonds in the reactants and forming new bonds in the products. The starting materials of a chemical reaction are known as reactants, while the substances formed are indicated as products.
Often, chemical reactions require energy to break the existing bonds before new ones can form. This energy is termed activation energy, which is the minimum energy needed to initiate a chemical change. For a chemical reaction to occur, the reacting molecules must possess enough energy to overcome this energy barrier. If the molecules have less energy, the reaction won't happen.
Chemical reactions can be exothermic or endothermic:

• Exothermic reactions: Release energy, often in the form of heat. For instance, burning wood releases heat energy.
• Endothermic reactions: Absorb energy. An example is the process of photosynthesis in plants, where energy from sunlight is absorbed to make sugar from carbon dioxide and water.

Temperature Dependence

Temperature plays a crucial role in the rate at which chemical reactions occur. It doesn't affect the activation energy of a reaction but does impact how quickly the reaction proceeds. Generally, an increase in temperature results in a rise in the reaction rate.
This happens because higher temperatures provide reacting molecules with more kinetic energy. As a result, a larger number of molecules have enough energy to surpass the activation energy barrier, leading to more frequent successful collisions.
According to the Arrhenius equation: \[ k = A e^{-\frac{E_a}{RT}} \]where:

• \(k\): Reaction rate constant
• \(A\): Pre-exponential factor or frequency factor
• \(E_a\): Activation energy
• \(R\): Universal gas constant
• \(T\): Temperature in Kelvin

This equation shows that as the temperature \((T)\) increases, the exponential factor decreases, leading to a higher rate constant \((k)\), thus speeding up the reaction. Temperature is an essential factor affecting reaction rates but not the intrinsic activation energy of the reaction itself.

Reaction Mechanism

The reaction mechanism describes the step-by-step sequence of elementary processes that lead to a chemical reaction. It provides insight into how reactants are converted into products and gives a detailed picture of which bonds break and which new bonds form during the reaction.
Each step in a reaction mechanism involves a simple reaction called an elementary step. These elementary steps can involve rearrangement of atoms or formation of intermediates that are not seen in the final reaction equation.
Reaction mechanisms help chemists understand, predict, and control reactions by:

• Identifying the molecularity of each elementary step, which can be unimolecular or bimolecular.
• Informing the role of intermediates, which are often short-lived and not present in the overall stoichiometric equation.
• Providing detailed insight into which bonds break and which new bonds form in each step.

Catalyst

A catalyst is a substance that speeds up a chemical reaction without itself being consumed or altered in the process. It does not initiate the reaction but rather makes it faster by providing an alternative pathway with a lower activation energy.
Catalysts have specific characteristics:

• They participate in the reaction by forming temporary bonds with reactants, creating an intermediate compound that aids in lowering the activation barrier.
• They remain unchanged at the end of the reaction, so even small amounts can drive significant changes in reaction rates.
• A catalyst can be highly specific, only affecting certain reactions without influencing others.

Common examples include enzymes, which are biological catalysts that speed up metabolic reactions essential for life.
The role of a catalyst is crucial in many industrial processes where increased efficiency and reduced production time are beneficial. By lowering the activation energy, catalysts reduce the thermal energy needed to achieve the reaction, leading to faster and more cost-effective processes.