Greetings,
This post covers the concept of reaction enthalpy and the related topic of activation energy. The Enthalpy of Reaction is also known as "Heat of Reaction", therefore the symbol "H" is used.
Reactants and products are represented using simple geometric shapes. The details of molcular structures and corresponding bond-energy-summations are not necessary for understanding the fundamental concepts of reaction enthalpy.
It is important to understand that knowledge of certain thermodynamic quantities, such as "molar enthalpy" of a particular reaction are not indicative of the actual absolute rate of a chemical reaction. There are many reactions with low activation energy values which are very slow (e.g., the formation of rust; aka Iron(III) Oxide). Reaction Rate Theory will be discussed in a future post.
That's all for now. As always, thank you for reading!
A Publication of http://ExcellenceInLearning.biz
This post covers the concept of reaction enthalpy and the related topic of activation energy. The Enthalpy of Reaction is also known as "Heat of Reaction", therefore the symbol "H" is used.
Heat of Reaction
The Heat of Reaction measures the total bond energy change when reactants are completely converted into products (in the case of a non-equilibrium complete reaction). If the products have a total bond energy (aka potential energy) lower than the total bond energy of the reactants, then potential energy is lost and must manifest as some form, commonly heat. In the case of lowering potential energy, we say that the reaction is "exothermic". Some reactions produce products higher in potential energy than the reactants and we say they are "endothermic".
The Reaction Progress Chart
The progress of a chemical reaction can be shown by graphing heat energy vs. reaction time. This chart demonstrates two important quantities: 1) Change in Reaction Enthalpy and 2) Activation Energy. The Reaction Progress Chart can be thought of as a hill or barrier which must be surmounted before reactants can begin to convert into products. At the height of the energy barrier there exists a hypothetical entity called the Activated Complex. The activated complex is thought to be an intermediate structure "midway" between reactants and products. The activated complex is a very unstable, short-lived structure and, therefore cannot be isolated (although very high-speed analytical instrumentation may indicate its existence).
The following image depicts a sample reaction progress chart.
It is important to understand that knowledge of certain thermodynamic quantities, such as "molar enthalpy" of a particular reaction are not indicative of the actual absolute rate of a chemical reaction. There are many reactions with low activation energy values which are very slow (e.g., the formation of rust; aka Iron(III) Oxide). Reaction Rate Theory will be discussed in a future post.
That's all for now. As always, thank you for reading!
A Publication of http://ExcellenceInLearning.biz