Question

Explain why ?H for an exothermic reaction always has a negative value.

Short answer

In an exothermic reaction, energy is released, making the products have lower energy than the reactants. The enthalpy change, $\mathrm{\Delta }H=H_{\text{products}}-H_{\text{reactants}}$, will be negative since H_products < H_reactants. Thus, $\mathrm{\Delta }H$ for an exothermic reaction always has a negative value, indicating more energy is released than absorbed.

Step-by-step solution

Define an Exothermic Reaction

An exothermic reaction is a chemical reaction that releases energy to its surroundings in the form of heat. In other words, the products of an exothermic reaction contain less energy than the reactants, hence heat is released.

Discuss Enthalpy Change (?H)

Enthalpy change, denoted as ?H, represents the difference in the enthalpy (total energy content) of a system between the initial reactants and the final products. Mathematically, it can be expressed as: $$\mathrm{\Delta }H=H_{\text{products}}-H_{\text{reactants}}$$

Explain Entropy Changes in Exothermic Reactions

In an exothermic reaction, the products have lower energy than the reactants. This is because the energy is released in the form of heat to the surroundings. So, H_products will be less than H_reactants. Using the enthalpy change formula, we have: $$\mathrm{\Delta }H=H_{\text{products}}-H_{\text{reactants}}$$ Since H_products < H_reactants, ?H will always be negative. In other words, ?H for an exothermic reaction will always have a negative value because more energy is being released than is being absorbed by the reaction.

Key concepts

Enthalpy Change

In the realm of chemistry, enthalpy change is a critical concept to grasp. This term, often represented as $\mathrm{\Delta }H$, captures the overall energy change in a reaction. Simply put, it's the difference in energy between reactants and products. Imagine starting with a certain amount of energy with your reactants. As they transform into products, you either absorb or release energy. This shift is what we call enthalpy change.
Understanding how this change is calculated is fundamental. The mathematical formula for enthalpy change is:$$\mathrm{\Delta }H=H_{\text{products}}-H_{\text{reactants}}$$In this equation:

• $H_{\text{products}}$ is the enthalpy (energy content) of the products after the reaction.
• $H_{\text{reactants}}$ is the enthalpy of the initial reactants.

This calculation gives insight into whether energy is gained or lost during the chemical transformation.

Energy Release

Energy release is synonymous with many chemical reactions, especially when we talk about exothermic processes. In such reactions, energy is often expelled as heat, transferring this energy notably to the surroundings. You can think of it like warming up your hands by a fire—the warmth is the energy being released.
Exothermic reactions are found everywhere in daily life, from burning wood to metabolic processes in our bodies. These reactions are integral in understanding how reactions can transfer energy. If you imagine a container where a reaction happens:

• The reaction zone releases energy in the form of heat.
• The surrounding environment absorbs this energy.

This transfer explains why exothermic reactions are pivotal in generating heat and maintaining energy balance in various systems.

Negative Enthalpy

The notion of negative enthalpy is tightly linked to exothermic reactions. A negative enthalpy value, reflected as $\mathrm{\Delta }H<0$, is indicative of energy being released rather than absorbed. But why does this happen?

• In exothermic reactions, products have lower energy than reactants start with.
• The excess energy not needed by products is expelled as heat.
• This results in minor energy content within product molecules compared to initial reactants.

Therefore, by the definition based on enthalpy calculations, a negative enthalpy signals that there's a net loss of energy from the system. This is what makes exothermic processes so fascinating—it's a glimpse of molecules settling into lower energy states by sharing their surplus with the environment.

Chemical Thermodynamics

Chemical thermodynamics provides a deeper insight into how energy transforms during reactions. It doesn't just study the energy itself, but also the direction and feasibility of chemical reactions. A cornerstone of this field is understanding why reactions happen the way they do.
In chemical thermodynamics:

• It describes how energy transfer is linked to changes in temperature, pressure, and volume.
• It helps in predicting which reactions are spontaneous based on energy considerations.
• It utilizes principles like enthalpy and entropy to create models for understanding reaction behaviors.

Chemical thermodynamics is pivotal for engineers and chemists. It allows them to design processes that harness energy efficiently. Whether you're developing a new fuel source or creating a pharmaceutical drug, mastering these principles is key to innovation and practical application in industry.