The Chemical Reaction, Part 2 - Free Energy

Greetings,

This post explains the concept of free energy, which also involves reaction enthalpy and entropy.  An understanding of free energy requires some knowledge of the enthalpy and entropy concepts, so we'll start with those.

Enthalpy

Enthalpy, a.k.a., Heat of Reaction, is a measure of the total bond energy associated with elements and/or compounds involved in a chemical change.  During the course of a chemical reaction, the change in bond energy, as reactants are converted to products, manifests as a release or "taking up" of heat energy.  Heat energy is either released to the surroundings or taken up from the surroundings as a chemical reaction progresses.  Enthalpy is normally expressed as kilojoules per mole, kJ/mol.  Since a chemical reaction is a transformation of chemical species from one form to another, we are interested in change in enthalpy, "delta H".  When it comes to a change in enthalpy for a reaction, a common question arises: What is a mole for a chemical reaction?  A "mole of a chemical reaction" consists of molar amounts of reactants and products equal to the coefficients of the corresponding balanced chemical equation.

Measuring the Enthalpy Change of a Reaction

A device called a calorimeter is used to measure the total enthalpy change of a chemical reaction.  The calorimeter is calibrated so that a certain Celsius temperature change equates to a particular amount of heat energy transferred to or from the calorimeter.  Then, a known amount of reactants are introduced into the calorimeter and the temperature of the "reaction system" is monitored until the reaction has stopped.  The change in temperature is then determined and used to obtain the total heat energy transferred to or from the calorimeter.  

Standard Heat of Reaction

It can be useful to determine Heat of Reaction at standard conditions: 0.1 MPa (Mega-Pascals) partial pressure for gases, 1 M (moles solute per liter solution) concentration for aqueous solutions and a temperature of 25 degrees Celsius (298 K).  Standard Heats of Formation, Hof, have been published for very many compounds (and elements in "un-natural states").  The Standard Heat of Formation of the naturally occurring state of an element, such as diatomic oxygen gas, is considered to be 0 (zero).  A Standard Heat of Reaction value less than zero indicates an exothermic chemical change while a positive value means that the actual reaction should be endothermic.

 Entropy

Entropy is an absolute amount of energy for a specific compound or naturally occurring element associated with the degree of intermolecular attractions present.  Entropy is also a state function because its value depends on properties inherent to a particular phase of matter.  Generally speaking, entropy values are higher for chemical systems consisting of lower energy intermolecular attractions.  In a sense, a positive entropy change for a chemical reaction indicates products in a higher state of "disorder" as compared to the reactants.  On the other hand, reactions resulting in products in a more condensed state tend to have decreasing entropy values.  Entropy can also be thought of as the energy "cost" of forming products in an overall greater level of disarray than the reactants.  The total entropy change of a reaction also depends on absolute temperature (Kelvin scale), so this must be taken into account for the determination of Free Energy Change.  Entropy is represented with an uppercase 'S' to indicate its dependence on the state of a 'system'.

Free Energy

The Free Energy Change of a chemical reaction system is essentially the Enthalpy Change adjusted for the effect of Changing Entropy.  The available "Free Energy" of a chemical system represents the maximum amount of heat energy which could be utilized to do work.  The Free Energy Change value also indicates whether or not a chemical reaction is spontaneous (self-sustaining upon initiation).  Free Energy is indicated with an uppercase 'G' in honor of Josiah Gibbs, the great scientist who contributed greatly to our understanding of Chemical Thermodynamics. The following diagram illustrates Standard Free Energy Change calculations for methanol combustion.

In the case of an exothermic reaction, entropy will tend to increase resulting in the release of more heat energy than the effect of enthalpy change alone.

That's all for now.  As always, thank you for reading!

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