Problem 85 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Rate of which type of elementary reaction increases with increase in temperature? (a) Thermal (b) Exothermic (c) Endothermic (d) All

Step-by-Step Solution

Answer

The rate of endothermic reactions increases with an increase in temperature because these reactions absorb heat.

Step 1: Understanding Reaction Types

First, recognize the different types of reactions mentioned. A thermal reaction generally involves heat. Exothermic reactions release heat, meaning the products have less energy than reactants. Endothermic reactions absorb heat, meaning the products have more energy than reactants.

Step 2: Effect of Temperature on Reactions

An increase in temperature provides more thermal energy to the reactants, increasing the speed of molecules and the number of effective collisions. This causes a rate increase in reactions which absorb heat, as higher temperature shifts the equilibrium to favor the products in endothermic reactions.

Step 3: Identifying the Correct Option

Based on the understanding of temperature effects, rate of endothermic reactions increases with temperature because they absorb energy from the surroundings. Exothermic reaction rates can actually decrease with temperature as they release heat. Therefore, endothermic reactions are most affected by an increase in temperature.

Key Concepts

Temperature Effect on Reaction Rate

The temperature of a system is a critical factor that plays a pivotal role in the rate of chemical reactions. When you increase the temperature, you're essentially pumping energy into the reactants, allowing molecules to move more rapidly. This heightened movement amplifies the likelihood of more frequent and more forceful collisions between reactants, which is a necessity for a reaction to occur.

Think of it as a dance floor: if the music is slow, people are less likely to bump into each other. Crank up the tempo, and suddenly there's a higher chance of dancers colliding. In the realm of chemistry, these collisions correspond to the reactant molecules - the faster and the harder they collide, the greater the chance of a reaction.

Furthermore, a spike in temperature can also equip the reactant molecules with more energy to overcome what is known as the activation energy barrier - the minimum energy needed for a reaction to proceed. By surpassing this threshold more readily at higher temperatures, reactions can occur at faster rates. This idea is encapsulated in the Arrhenius equation, which mathematically relates the rate of reaction to temperature.

Exothermic vs Endothermic Reactions

Diving into the heat of the moment, two major reaction types are exothermic and endothermic reactions, both of which have distinctive characteristics based on energy transfer.

An exothermic reaction is like a furnace, blazing out energy in the form of heat as the reaction proceeds. It's the chemical version of a warm embrace, leaving the surroundings cozier than before. The hallmark of an exothermic process is that the potential energy of the products is lower than that of the reactants, signifying that the reaction has released energy into the environment.

In contrast, endothermic reactions are the universe's ice packs, absorbing energy from their surroundings to move forward. These are akin to an uphill climb where products stockpile more energy than the reactants started with, mimicking a scenario where products reach a higher energy level.

The intriguing aspect here is how these reactions behave with temperature changes. With more heat, endothermic reactions quicken their pace as they lap up the extra energy, akin to athletes energized by an enthusiastic crowd. Exothermic reactions, however, might slow down as they essentially 'feel' less compelled to emit energy when it's already warm outside.

Elementary Reactions

Elementary reactions represent the simplest form of chemical reactions, involving a direct and single step process. They are the building blocks in the intricate architecture of complex reactions. In the world of chemistry, these are the 'one-move wonders' - chemical changes that don't necessitate a series of intermediary actions to yield products.

Let's draw a parallel with a straightforward, one-on-one handshake between two people. This simplicity is characteristic of elementary reactions, which can occur in the form of a unimolecular reaction, involving a single reactant molecule undergoing a transformation, or a bimolecular reaction, akin to a handshake, where two reactant molecules collide to form products.

When it comes to temperature dependence, an increase in temperature can dramatically enhance the rate of elementary reactions, particularly endothermic ones. Since these reactions take energy cues directly from their environment without intermediaries, the effect of temperature on them is markedly pronounced, like a sprinter feeling the direct boost from a tailwind.