Question

In the exothermic reaction the enthalpy of reaction is always: (a) Zero (b) Positive (c) Negative (d) None of these

Short answer

The enthalpy of reaction in an exothermic process is negative.

Step-by-step solution

Understanding Exothermic Reactions

Exothermic reactions are chemical processes that release energy into their surroundings, usually in the form of heat. This means the products have less energy than the reactants.

Defining Enthalpy of Reaction

The enthalpy of reaction ( H) is defined as the change in enthalpy when reactants are transformed into products. It is calculated as:  H = H_products - H_reactants.

Analyzing Enthalpy Change for Exothermic Reactions

Since exothermic reactions release energy, the products have less enthalpy than the reactants. Therefore, H_products < H_reactants, leading to a  H that is negative ( H < 0).

Selecting the Correct Option

Given that the enthalpy of reaction for an exothermic process is negative, the correct answer is (c) Negative.

Key concepts

Enthalpy of Reaction

Enthalpy of reaction is a key concept in understanding chemical reactions. Essentially, it measures the heat change when reactants are converted into products. It is expressed as \( \Delta H \), where \( H \) represents the enthalpy of a substance. The enthalpy change provides important insights into the nature of a reaction, indicating whether it absorbs or releases energy. In simple terms, if the products of a reaction have lower enthalpy than the reactants, the reaction releases heat, which is referred to as an exothermic reaction. Conversely, an endothermic reaction absorbs heat, meaning the products have a higher enthalpy than the reactants. Calculating \( \Delta H \) is as simple as subtracting the enthalpy of the reactants from that of the products. Understanding these distinctions helps to predict how a reaction will influence its environment, such as increasing or decreasing temperature.

Chemical Thermodynamics

Chemical thermodynamics is the branch of chemistry that deals with the flow and exchange of energy, particularly heat, during chemical reactions. This field helps us understand how energy changes determine reaction spontaneity and direction. Central to this is the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed. During an exothermic reaction, chemical energy is transformed into heat energy, which is released to the surroundings. Thermodynamics also helps to measure the efficiency of a reaction. It integrates concepts such as entropy, enthalpy, and Gibbs free energy to assess the feasibility of a reaction and how energy exchanges occur. By studying chemical thermodynamics, one can make informed predictions about the outcomes of chemical processes.

Energy Release

Energy release is a defining feature of exothermic reactions. These reactions release energy because the products are more stable (lower in energy) than the reactants. This release primarily occurs as heat energy, contributing to temperature increases in the surroundings. For instance, combustion of fuels is a common example of an exothermic reaction, where the heat generated can be used to power engines or heat homes. Energy release can be quantified by measuring the change in enthalpy \( \Delta H \). In practice, an exothermic reaction with a negative \( \Delta H \) indicates that energy has been released. Understanding this concept is crucial for both practical applications and theoretical chemistry, as it affects everything from industrial processes to natural phenomena. In everyday life, energy release by exothermic reactions is utilized in a variety of technologies, from hand warmers to power stations, highlighting its importance in both chemistry and real-world applications.