Energy Transfer in a Chemical Reaction

View Assign Share Link Report

Energy Transfer in a Chemical Reaction

This lesson aligns with NGSS PS1.B

Introduction

Chemical reactions are governed by the principles of thermodynamics. Chemical reactions involve the breaking and forming of bonds between atoms and molecules. As these transformations occur, energy is either absorbed or released. The concept of energy is central to the study of chemical reactions, and it is essential to recognize the two main categories of energy changes: endothermic and exothermic. In this article, we will learn about energy transfer in chemical reactions, exploring the concepts of endothermic and exothermic reactions, internal energy, activation energy, and the role of catalysts.

Endothermic Reactions:

In an endothermic reaction, the system absorbs energy from its surroundings. This means that the products of the reaction have higher energy content than the reactants. One common example of an endothermic reaction is the process of photosynthesis in plants. During photosynthesis, carbon dioxide and water react in the presence of sunlight to produce glucose and oxygen. The energy from the sun is absorbed by the plants to facilitate this reaction, making it endothermic.

The general representation of an endothermic reaction can be expressed as follows:

A+B(reactants)+energy→ C+D(products)

Exothermic Reactions:

Conversely, in an exothermic reaction, the system releases energy to its surroundings. This implies that the reactants have higher energy content than the products. Combustion reactions, such as the burning of wood or the reaction between hydrogen and oxygen to form water, are classic examples of exothermic reactions. The heat and light emitted during these processes signify the release of energy.

The general representation of an exothermic reaction can be expressed as follows:

A+B(reactants)→C+D(products)+energy

Internal Energy of a System

The internal energy of a system involves the sum of all microscopic forms of energy within it. This includes the kinetic energy associated with the motion of particles (translational, rotational, and vibrational), as well as potential energy arising from forces between particles.

First Law of Thermodynamics:

The first law of thermodynamics, often referred to as the law of conservation of energy, dictates that the total energy of an isolated system remains constant. In simpler terms, energy can neither be created nor destroyed; it can only be transferred or transformed from one form to another.

ΔU=Q−W

Here,ΔU represents the change in internal energy,

Q denotes heat transfer, and

W signifies work done on or by the system.

Energy Transfer in Different Thermodynamic Processes:

Isobaric Process:

In an isobaric process, the pressure of the system remains constant. During such a process, energy transfer occurs through both heat and work. The first law of thermodynamics can be expressed as

ΔU=Q−P⋅ΔV

where

P is the constant pressure.

Isochoric Process:

In an isochoric process, the volume of the system remains constant. As the volume change (ΔV) is zero, the work done (W) is also zero, and the first law simplifies to

ΔU=Q

Isothermal Process:

An isothermal process occurs at constant temperature. For an ideal gas undergoing isothermal expansion, the heat absorbed (Q) is equal to the work done (W), resulting in

ΔU=0

Adiabatic Process:

An adiabatic process is characterized by the absence of heat transfer (Q=0). In this case, the change in internal energy (ΔU) is solely attributed to work done (W). The first law simplifies to

ΔU=−W

Activation Energy

Activation energy is the minimum amount of energy required for a reaction to occur. Even in exothermic reactions where the products have lower energy than the reactants, there is an initial energy barrier that must be overcome for the reaction to proceed.

Imagine a ball on top of a hill. To roll the ball down the hill, you need to give it a push to overcome the potential energy barrier at the top. Similarly, in a chemical reaction, reactant molecules must collide with sufficient energy to overcome the activation energy barrier and transform into products. The difference in potential energy between the reactants and the activated complex is the activation energy.

Catalysts

Catalysts are substances that alter the rate of a chemical reaction without being consumed in the process. They achieve this by providing an alternative reaction pathway with a lower activation energy. By lowering the activation energy, a catalyst facilitates a faster and more efficient reaction.

The presence of a catalyst does not change the overall energy change in a reaction; it only influences the activation energy required. This is depicted in the energy profile diagram for a catalyzed reaction, where the peak of the activation energy is lower compared to the uncatalyzed reaction. Enzymes, for example, are biological catalysts that play a crucial role in various biochemical reactions in living organisms.

Summary

In an endothermic reaction, the system absorbs energy from its surroundings. This means that the products of the reaction have higher energy content than the reactants.
Conversely, in an exothermic reaction, the system releases energy to its surroundings. This implies that the reactants have higher energy content than the products.
Activation energy is the minimum amount of energy required for a reaction to occur.
Catalysts are substances that alter the rate of a chemical reaction without being consumed in the process.
By lowering the activation energy, a catalyst facilitates a faster and more efficient reaction.

Related Worksheets: