Chemical Reactions and Associated Energy Changes

Greetings,

A subject explored at the outset of this label, changes in matter and energy, will now be discussed in more detail, specifically for chemical changes.  This post is a focus on potential energy change within a chemical system.  Potential energy changes which accompany chemical changes dictate the amount and direction (to or away from the environment) of heat flow.  The sum-total of potential energy and kinetic (heat) energy must remain constant during a chemical change, but the ratio between them will change.  Potential energy is stored up heat and kinetic energy is heat associated with molecular motion.  The following diagram summarizes the heat-flow phenomenon.

An Analogy for The Reaction Progress Chart

A chemical reaction can be studied to obtain the amount of potential energy contained as the reaction progresses.  A plot of that data produces a Reaction Progress Chart.  A person walking up and over and the related potential energy changes can be used as a model for understanding the reaction progress chart.  The force of attraction for a person climbing a hill is gravity while molecular systems involve electrostatic attractions between electrons and nucleii.  For a reaction to occur a certain maximum energy must be attained: This energy is the Activation Energy, Ea.  The total energy released for an exothermic reaction depends on the heat of reaction and entropy change.  The heat of reaction is a measure of the change in potential energy of a chemical system.  Entropy is energy associated with increasing disorder.  A reaction starting with a solid reactant and forming gas products is an example of increasing entropy.  The total effect of an entropy increase is dramatically increased with a greater increase in temperature.  Generally speaking, an exothermic reaction has a negative enthalpy change and a positive entropy change.  A negative enthalpy change means that the potential energy of reaction products is lower than the potential energy of reactants.  The next diagram shows a potential energy change analogy and a typical reaction progress chart for an exothermic system.

Notice that the change in potential energy of a reaction is the difference between the PE "height" of the products and the PE "height" of the reactants.  The analogy of a man walking over a hill looks similar and is a lot closer to our "normal" reality!

That's all for this post.

Thank you for Reading!

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