Question

\(\Delta H\) for a reaction is negative. Compare the energy of the products and the reactants. Is the reaction endothermic or exothermic? (Chapter 15\()\)

Short answer

The given reaction has a negative \(\Delta H\), which means that the reactants have more energy than the products. Since the enthalpy change is negative, the reaction is exothermic.

Step-by-step solution

Review the definition of enthalpy change and the relationship between reactants and products' energy

Enthalpy change (\(\Delta H\)) is the difference between the enthalpy (or total energy) of the products and the reactants in a chemical reaction. Mathematically, this is expressed as: \[\Delta H = H_{products} - H_{reactants}\] A positive value of \(\Delta H\) implies that the products have more energy than the reactants, while a negative value implies that the reactants have more energy than the products.

Define endothermic and exothermic reactions

An endothermic reaction is a chemical reaction in which energy (usually in the form of heat) is absorbed from the surroundings. In such reactions, the enthalpy change (\(\Delta H\)) is positive, meaning the products have higher energy than the reactants. An exothermic reaction, on the other hand, is a chemical reaction in which energy (usually in the form of heat) is released to the surroundings. For exothermic reactions, the enthalpy change (\(\Delta H\)) is negative, meaning the reactants have higher energy than the products.

Determine if the reaction is endothermic or exothermic based on the given value of \(\Delta H\)

We are given that \(\Delta H\) for the reaction is negative, which means that the reactants have more energy than the products. From our understanding of endothermic and exothermic reactions, we can now determine that the reaction is an exothermic reaction, as it has a negative enthalpy change (\(\Delta H\)).

Key concepts

Enthalpy Change

Enthalpy change is a concept that revolves around the transfer of energy during a chemical reaction. It is represented by the symbol \( \Delta H \), which indicates the difference between the total energy of the products and that of the reactants.
In any chemical process, energy has to be either absorbed or released because atoms need energy to break bonds and form new ones. When calculating enthalpy change:

• If \( \Delta H \) is negative, this means the final energy of the products is lower than the initial energy of the reactants.
• If \( \Delta H \) is positive, this signifies that the products have greater energy compared to the reactants.

This change in enthalpy helps in categorizing reactions into two main types: exothermic or endothermic, providing us a deeper understanding of energy flow in reactions.
It's important for students to grasp this concept, as it is foundational in understanding how energy influences the direction and behavior of chemical reactions.

Energy of Reactants and Products

The energy content of reactants and products plays a crucial role in determining the nature of a chemical reaction. To visualize this:

• Reactants start at a certain energy level before the reaction occurs.
• Products end up at another energy level after the reaction happens.

The difference in these energy levels is what we call the enthalpy change \( \Delta H \). If the energy of the reactants is higher than the energy of the products, this indicates that energy has been released into the surroundings. This energy difference:

• Explains why some reactions release heat, making them exothermic.
• In contrast, if the products have more energy than the reactants, heat is absorbed, making the reaction endothermic.

Understanding the energy of reactants and products allows chemists to predict the direction of the reaction and the energy changes involved. This insight is vital for applications where controlling reaction conditions is crucial, such as in industrial processes and energy production.

Endothermic Reaction

An endothermic reaction is characterized by its ability to absorb energy from its surroundings. This absorption often occurs in the form of heat, which is required to drive the reaction forward.
In such reactions, the energy consumed is used to break bonds in the reactants, leading to a positive \( \Delta H \). This means that the products formed store more energy than the reactants started with. As a result:

• Endothermic reactions feel cold to the touch, as they draw heat away from their environment.
• Common examples include photosynthesis, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

Recognizing endothermic reactions is crucial because they often require specific conditions to occur automatically. Understanding the requirement for external energy helps in the design of chemical processes that harness or control these reactions.