The Chemical Reaction, An Introduction

Hello,

This post will focus on the Chemical Reaction; what it is, why it happens, and the signs that it is happening.

Characteristics of Chemical Reactions

The word "characteristics" relates to defining properties of a person, place, object, or process.  For our purposes, "characteristics" refer to a process, the chemical reaction.  Chemical reactions are everywhere: They are, quite literally, the very thing that sets this habitat we call earth in motion.

What is a Chemical Reaction?

A chemical reaction is a process that changes the chemical composition of a material or mixture of materials.  These materials can be elements and/or compounds.  A Chemist knows that a chemical reaction can be thought of as consisting of parts; reactant(s), intermediate(s), and product(s).  The reactants are what we have before a chemical reaction takes place.  The products are those materials formed from a chemical reaction.  The intermediate is a very unstable, short-lived material which quickly changes to form the products.

Why Does a Chemical Reaction Happen?

Chemical reactions usually happen when possible products are energetically favored.  This generally means that the products of a chemical reaction are less likely to react than the original reactants from which they are formed.  A chemical reaction can be forced to occur or can, once started, continue on its own unattended.  Every chemical reaction process results in a change in free energy.  Free energy is the total amount of energy that can be extracted from a material or mixture of materials.  A forced reaction results in an increase in free energy.  A reaction which continues unattended will result in a decrease in free energy.  A decrease in free energy means that less energy can be extracted from the products than can be extracted from the reactants.  Free energy itself consists of two different types of energies: (1) Enthalpy, aka. Heat of Reaction and (2) Entropy, aka. Energy Associated With Disorder.  Enthalpy is the heat energy change of a reaction associated with a change in total bond energy.  Whenever we talk about bond energy, we mean the potential, i.e. stored-up, energy contained in a material or mixture of materials.  We generally speak of molar bond energy, which is the number of kilojoules heat energy stored within one mole of an element or compound.  The enthalpy of a reaction is calculated by subtracting the total reactant bond energy from the total product bond energy.  If the result is negative, then heat is released and we have an exothermic reaction.  If heat energy is absorbed, the enthalpy is positive and we say that we have an endothermic reaction.  The entropy increases when the products are in lower density states than the reactants (such as the formation of a liquid from a solid) and when the total number of product molecules is greater than the total number of reactant molecules.

Signs That a Chemical Reaction Has Occurred

For all the technicalities that a person could explore regarding chemical reactions, the evidence of their happening is quite simple.  For most reactions, the common signs that they have occurred are change in temperature, change in color, change in state, formation of a solid, disappearance of a solid, change in pH, and production of a gas (e.g. formation of bubbles).

That's all for this post.  Thanks for Reading!

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