Question

If the transition state is product-like, then nature of reaction is (A) Exothermic (B) Never possible (C) Endothermic (D) None of these

Short answer

The correct answer is (C) Endothermic as the transition state being product-like suggests that the products have a higher energy level than the reactants, which is characteristic of an endothermic reaction.

Step-by-step solution

Understand transition states and energy profiles

A transition state is the highest energy state of a reaction. It exists in between the reactants and products in an energy reaction profile, representing the least stable, highest-energy state that the reaction must pass through to convert the reactants to products. The transition state plays an essential role in determining the activation energy, which is the minimum energy required for a reaction to occur. An energy profile shows the change in energy during a reaction, which can be either endothermic or exothermic. An endothermic reaction is one in which energy is absorbed, and the products have higher energy than the reactants. Conversely, an exothermic reaction is one in which energy is released, and the products have lower energy than the reactants.

Comparing the transition state to the reactants and products

If the transition state is product-like, it means that it is closer in energy and structure to the products than the reactants. In an energy profile, the transition state would be closer to the products' energy level.

Determine the nature of the reaction

To determine whether the reaction is endothermic or exothermic, we need to compare the energy levels of the reactants and products. If a reaction is endothermic, the products have a higher energy level than the reactants, and if a reaction is exothermic, the products have a lower energy level than the reactants. Since the transition state is product-like, it implies that the energy level of the products is closer to the transition state. This suggests that the energy difference between the reactants and products is relatively small. The nature of an endothermic reaction is such that the products have a higher energy than the reactants, and thus a higher energy than the transition state. In this case, due to the transition state being product-like, we can conclude that the reaction is endothermic. So, the correct answer is: (C) Endothermic

Key concepts

Endothermic Reaction

In an endothermic reaction, energy is absorbed from the surroundings. This absorption of energy means that the products have a higher energy level than the reactants.
Imagine you're building a tower where you need to put in extra effort to reach the top. Similarly, an endothermic reaction requires energy input to proceed.

• Energy Absorption: Energy is taken in, indicated by an increase in the surroundings' temperature.
• Higher Product Energy: End products contain more energy than the starting substances.
• Energy Profile: The curve on an energy diagram for endothermic reactions slopes upwards.

Recognizing endothermic reactions can be easier once you understand they often absorb heat. A simple everyday example is the melting of ice, where heat must be absorbed to convert solid ice to liquid water.

Energy Profile

An energy profile is a graphical representation showing the energy changes during a chemical reaction. It traces the path from reactants to products, clearly marking the transition state at the peak.
Think of it as a mountainous terrain where each point represents a different energy level along the reaction pathway.

• y-Axis: Represents energy levels.
• x-Axis: Depicts the progress of the reaction.
• Transition State: Found at the highest point on the curve, indicating the highest energy state.

The energy profile helps to visualize whether a reaction is endothermic or exothermic. For endothermic reactions, the graph ascends because the products are at a higher energy level than the reactants. Understanding these graphs can guide you in determining the energy requirement of a reaction and predicting its behavior.

Activation Energy

Activation energy is the minimum amount of energy required to start a reaction. It's like the initial push needed to get a ball rolling uphill. Without this necessary energy "kick", a reaction cannot proceed.
The concept of activation energy explains why certain reactions are slow and might require additional energy inputs, like heat, to get going.

• Energy Barrier: Activation energy is an obstacle reactants must overcome to transform into products.
• Depicted on Energy Profiles: Represented as the energy difference from the reactants to the transition state peak.
• Catalysts: These substances lower the activation energy, making it easier for reactions to occur.

Recognition of activation energy is crucial because it allows chemists to manipulate reactions for desired outcomes. For instance, in a laboratory setting, a catalyst might be used to lower the activation energy and accelerate a reaction that would otherwise take longer or not occur under normal conditions.