An exothermic reaction: a brief theoretical introduction
Without a doubt, all students who are studying chemistry know perfectly that there exist many different classifications of diverse chemical processes according to their distinctive characteristics that are of interest to chemists, such as types of reaction mechanisms, types of products and reagents, changes in energy balance, etc. In addition, every branch of modern chemistry has its own type of classification according to the concrete peculiarities of studying processes. With an eye to demonstrating students the most obvious characteristics of different chemical reactions and teaching them the essential fundamentals of modern chemistry, moving from the simple and demonstrative educational material to the much more sophisticated themes, it is advisable to start their acquaintance with modern classifications of chemical reactions, using an exothermic reaction as a comprehensible and, undoubtedly, interesting example. Therefore, let us start this topic by considering the main attributes of a standard chemical exothermic reaction.
We can divide all types of reactions into two great groups: exo- and endothermic processes according to the transformations that originate in the energy balance of a standard chemical equation. An exothermic reaction is a standard chemical process, which releases energy in a form of light waves, heat or in both forms. A chemical endothermic reaction is the complete opposite to an exothermic reaction because during this process a system of reagents absorbs energy from its surroundings. Therefore, during a chemical exothermic reaction we can observe a release of energy, whereas an endothermic reaction requires a supply of energy for its existence. Doubtlessly, it is a quite complicated assignment to measure an absolute amount of energy in an isolated chemical system as well as various changes in it. Thereby, in order to considerably simplify our calculations and eschew unnecessary complications in our measurements, we should work with a specific indicator called the enthalpy change. It allows researchers to measure all significant data about each specific chemical reaction correctly, regardless of its type and avoid oversophisticated computations, which lead to uninformative conclusions. The enthalpy change (∆H) is specific index, which is equal to the shift of internal energy of a system together with the work, which has to be performed with an eye to changing a volume of a system against constant pressure. Usually, in practice, we determine the enthalpy change of a combustion exothermic reaction; thereby, the main device used to measure this index is a standard bomb calorimeter. In fact, the enthalpy change of a chemical exothermic reaction always has a negative value, according to the common definition of the enthalpy, whereas the same index of an endothermic reaction has a positive value. In truth, students often confuse indicators of the energy change for exo- and endothermic reactions. Surprisingly, it is one of the most ubiquitous and, simultaneously, characteristic mistakes, therefore, one should try to realize and remember this fact form the very beginning of an educational course with an eye to eschewing inappropriate inaccuracies in calculations.
Doubtlessly, the understanding of the very conception that diverse examples of chemical exothermic reactions surround us everywhere and anytime allow students to understand the primal principles of these processes. For example, one can easily name a few types of exothermic reactions, such as neutralization reactions between acids and various groups of metals (alkalis), a standard reaction between a strong acid and water and, obviously, an old-fashioned combustion reaction (a simple burning process). Different types of melting reactions, electrolysis, etc. appertain to the class of endothermic reactions. Of course, students have to understand that there exist unimaginably great number of much more complicated chemical transformations, which also can be classified according to this scheme, and that we can employ these terms not only to describe single chemical processes, but also various cascades of chemical transformations. For example, all students are aware of the photosynthesis reaction, which supplies all living creatures on Earth with oxygen that is necessary for oxidation processes in all aerobes. In truth, photosynthesis is a common term that characterizes a wide range of chemical transformations, during which sunlight energy accumulates in organic molecules. However, the photosynthetic process can be regarded as a classical endothermic reaction because it requires a considerable input of energy from the environment. Each type of the combustion reaction has to be classified as an exothermic reaction because it transfers energy to its environment. Furthermore, a process of breathing as a continual series of chemical transformations, during which organisms store energy in the molecules of ATP (adenosine triphosphate), also has to be regarded as a standard exothermic reaction.
A standard laboratory report on an exothermic reaction: a practical guideline
With an eye to studying the most significant characteristics of a classical exothermic reaction it is recommended to perform a series of experiments with chemical reagents that are available in virtually all laboratories in modern colleges, universities and specialized educational centers. Of course, one should always remember that all exothermic reactions always occur with the large release of energy or, approximately speaking, with heating. Therefore, in addition to a standard set of rules of behavior in a chemistry laboratory, one should examine all the fire safety regulations, methods of individual safety and First Aid procedures. Remember that some of the reagents used in this experiment can be not only flammable, but also toxic. Thereby, don’t remove your goggles until the experiment will be finished and follow all safety precautions with an eye to keeping yourself healthy and unharmed.
In fact, the best educational strategy is to begin with a relatively simple and safe experiment. For example, one of the most effective models, which is used for study of all demonstrative peculiarities of exothermic reactions, is a classical experiment with sodium bicarbonate and acetic acid. In addition, it has a considerable range of advantages over other chemical experiments aimed at the same purpose. All reagents that are necessary for this chemical experiment are undoubtedly ready at hand in every modern chemical laboratory and they are not toxic or flammable. Additionally the chemical reaction between these two reagents takes only a very small period of time, which allows students to conduct a series of experiments in order to understand all the nuances of this chemical transformation. All these factors permit us to state that this experiment is ideal for study of the most informative and demonstrative characteristics of an exothermic reaction. Therefore, here is a list of actions, which must to be performed during this chemical experiment:
- Firstly, one should add approximately 10 ml of acetic acid to a test flask.
- Secondly, one has to register the temperature of the initial mixture.
- Thirdly, one should add a small scope of sodium bicarbonate to the mixture.
- In fact, this part of the experiment if the most significant for all the calculations, thereby, one should be maximally careful and attentive while performing it. Wait until sodium bicarbonate will be completely dissolved. Then observe and record all observable changes. Register the temperature of the mix.
- In order to attain the highest possible accuracy in calculations one has to repeat all previous stages 4-5 times.
- Then one should clean all used laboratory equipment and report to the teacher.
- Finally, one has to register all the computations in the laboratory report and calculate the average value.
In addition, it is also has to be emphasized that the thermometer should stay in the initial mixture until the experiment will be completely finished. In fact, this point is crucial for success of all computations. According to the primal principles of thermodynamics, after the start of the tests the thermometer becomes a part of the initial thermodynamic system. Energy released during our exothermic reaction also heats the thermometer. Therefore, it would be a serious mistake to remove it before the experiment will be finished. Additionally, one has to keep in mind that the readings of the initial and final temperature of the mixture may insignificantly vary from exterminator to exterminator. Hence, a little differences between records of different students cannot be regarded as a mistake.
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