Question

Explain the difference between an exothermic and an endother- mic reaction. Give the sign of \(\Delta H\) for each type of reaction.

Short answer

\(\Delta H\) is negative for exothermic reactions as they release heat, and positive for endothermic reactions as they absorb heat.

Step-by-step solution

Define Exothermic Reaction

An exothermic reaction is a chemical reaction that releases energy by light or heat. It is characterized by the transfer of energy to the surroundings and a negative enthalpy change, meaning that the total enthalpy of the products is less than that of the reactants.

Define Endothermic Reaction

An endothermic reaction, on the other hand, is a chemical reaction that absorbs energy from the surroundings. This is indicated by a positive enthalpy change because the total enthalpy of the products is greater than that of the reactants.

Indicate the Sign of \(\Delta H\) for Exothermic Reaction

For an exothermic reaction, the sign of \(\Delta H\) is negative because energy is being released and the products have less stored energy than the reactants.

Indicate the Sign of \(\Delta H\) for Endothermic Reaction

For an endothermic reaction, the sign of \(\Delta H\) is positive because energy is absorbed, meaning the products have more stored energy than the reactants.

Key concepts

Enthalpy Change

Enthalpy change, symbolized as \( \Delta H \), is a measure of heat energy released or absorbed when a chemical reaction occurs at constant pressure. It's an important concept in thermodynamics that helps us understand how much energy is exchanged during a reaction. If \( \Delta H \) is negative, it means that the reaction releases heat to the surroundings, and we call it an exothermic reaction. Conversely, if \( \Delta H \) is positive, the reaction absorbs heat from the surroundings, and it's known as an endothermic reaction.

In exothermic reactions, the products are more stable and have less stored energy than the reactants, resulting in the release of excess energy - usually in the form of heat or light. In endothermic reactions, additional energy is needed to break the bonds of the reactants, leading to higher energy products. This energy is typically provided by heat absorption, making the reaction mixture feel cold.

Chemical Reactions

Chemical reactions involve the transformation of one set of chemical substances to another. These transformations can be observed through changes in color, temperature, the formation of a gas, or the appearance of a solid precipitate. During these reactions, the bonds between atoms are broken and new bonds are formed, which involves energy changes. Depending on whether energy is released or absorbed, chemical reactions are classified as exothermic or endothermic.

Exothermic reactions, such as combustion or neutralization, typically feel hot to the touch as they release energy. These reactions are spontaneous and often occur naturally. Endothermic reactions, like photosynthesis or the dissolution of ammonium nitrate in water, absorb energy, making their surroundings cooler.

Energy Transfer in Reactions

The transfer of energy in chemical reactions is a core aspect of chemistry, as it determines the reaction's behavior and practical applications. Energy is required to break chemical bonds, and energy is released when new bonds form. In exothermic reactions, more energy is released in the formation of products than is consumed during the breaking of reactant bonds, resulting in a net release of energy. In contrast, endothermic reactions require additional energy to proceed because they consume more energy to break reactant bonds than is released when new products are formed.

This transfer of energy can be observed through temperature changes in the reaction environment - in exothermic reactions, the environment heats up, while in endothermic reactions, it cools down. Understanding these energy transfers is vital for controlling reaction conditions in industrial processes, biological systems, and safety protocols for reactions that release or consume large amounts of energy.

\(\Delta H\) Sign

The \(\Delta H\) sign is a simple yet revealing indicator of the nature of a chemical reaction. When we say that \(\Delta H\) is negative, denoted as \(\Delta H < 0\), we're dealing with an exothermic reaction where the system loses heat to the surroundings. The negative sign corresponds to a loss of energy from the system.

In contrast, a positive \(\Delta H\), or \(\Delta H > 0\), signifies an endothermic reaction, where heat is absorbed from the surroundings into the system. This principle is not only critical for understanding reaction energetics in a laboratory but also plays a huge role in industrial chemistry, where controlling the heat of reaction is essential for safety and efficiency.