Question

On increasing the temperature, the rate of a reaction : (a) always increases (b) always decreases (c) first increases and then decreases (d) may increase or decrease depending upon the nature of the reaction

Short answer

The rate of a reaction may either increase or decrease with an increase in temperature, depending on the nature of the reaction and involved mechanisms.

Step-by-step solution

Understanding the Effect of Temperature on Reaction Rate

The rate of a chemical reaction is affected by temperature changes. According to the Collision Theory, as the temperature increases, particles gain kinetic energy, move faster, and collide more often and with greater energy, which generally increases the rate of a reaction.

Considering Activation Energy

Every reaction has a characteristic activation energy (Ea). If the temperature is raised, particles with energy equal to or greater than Ea will increase in number, typically leading to an increased reaction rate.

Evaluating Exceptions

However, there are exceptions such as reactions that are already extremely fast at a given temperature or reactions where temperature changes could denature a catalyst or reactant, thereby actually decreasing the rate of reaction.

Understanding the Dependency

The change in reaction rate with temperature is dependent on the nature of the reacting substances and their specific reaction mechanisms.

Selecting the Correct Option

By considering the above points, we conclude that the rate of a reaction may increase or decrease with an increase in temperature depending on the nature of the reaction. Therefore, the correct option is (d) may increase or decrease depending upon the nature of the reaction.

Key concepts

Collision Theory

Understanding how the reactions happen at a molecular level is crucial for grasping why temperature affects reaction rates. Collision Theory lays the foundation for this understanding. It posits that for a reaction to occur, reacting particles must collide with sufficient energy and proper orientation.

As temperature increases, particles gain kinetic energy and, hence, move more rapidly. This results in a higher number of collisions per unit of time. However, not all collisions result in a reaction; only those that meet or exceed the required energy threshold, known as 'activation energy', lead to a chemical change.

Thus, while an increase in temperature generally causes an increase in the reaction rate due to more frequent collisions, the outcome also heavily relies on other factors, such as the activation energy and reaction mechanism involved.

Activation Energy

Activation energy, often denoted as \(E_a\), is a critical concept when discussing how temperature affects chemical reactions. It is the minimum energy needed for reactants to transform into products during a chemical reaction.

Imagine activation energy as a barrier that particles must overcome for a reaction to take place. When the temperature rises, a greater fraction of particles gain enough kinetic energy to surpass this barrier. This increase in energy is often visualized on a potential energy diagram, showing how particles must reach a higher energy state during the transition phase of a reaction.

It's important to note that while a higher temperature typically reduces the relative size of this barrier (making it easier for particles to reach the activation energy), specifics such as the presence of catalysts, nature of reactants, and overall reaction mechanism can alter this effect.

Chemical Kinetics

Chemical Kinetics is the branch of chemistry that studies the rates of chemical reactions and the factors influencing those rates. It seeks to understand the steps or sequences of a reaction, known as the reaction mechanism, and how these steps are affected by various conditions, such as temperature.

A key principle of kinetics is the rate law, an equation that relates the rate of a reaction to the concentrations of the reactants and the temperature. When you increase the temperature, for reactions with higher activation energies, the rate constant typically increases, suggesting that the reaction rate will also increase.

Kinetic studies also look at the half-life of reactions, how catalysts affect reaction rates, and the effect of changing concentrations. Such knowledge is vital for optimizing reactions in industrial processes, understanding enzyme activity in biology, and even for developing and preserving food products.

Reaction Mechanisms

A reaction mechanism is like a detailed story of a chemical reaction, describing the step-by-step sequence of elementary reactions that lead to the overall transformation. These mechanisms provide insights into the intricacies of how reactants become products, including details such as the formation and breakage of bonds, the role of intermediates, and the influence of catalysts.

Temperature can affect each of these elementary steps differently, sometimes accelerating certain steps while slowing down others. For this reason, even if the temperature does usually increase reaction rates due to higher energy collisions, the exact effect on the overall rate of a reaction can vary depending on its specific mechanism.

Understanding these mechanisms is key in fields like drug design, where precise interactions can make or break a medication's efficacy, and in environmental science, where they can predict how pollutants will react under varying conditions.