Energy in chemical changes

Each beat of the human heart uses about 1 J of energy, and to raise this book (of mass close to 1.5 kg) from the floor to a tabletop about 0.97 m above the floor requires about 14 J (Fig. A.6). Because energy changes in chemical reactions tend to be of the order of thousands of joules for the amounts usually studied, it is more common in chemistry to use the kilojoule (kj, where 1 kj = 103 J). 

Most of us associate temperature (7) with the concepts of hot and cold. More accurately, however, temperature is the property of an object that determines the direction of heat flow. Heat flows naturally from a warm object to a cool object, from higher temperature to lower temperature. Heat is a form of energy, and because energy changes in chemical systems have important consequences, chemists are interested in temperature changes that occur during chemical transformations. 

If the standard state is chosen consistently for all species, the arbitrary choice of P° will cancel out of AGrxn, AGf°[compound], and related free energy changes in chemical reactions. 

Refer to the "Causes of Change" chapter for a discussion of energy changes in chemical reactions. 

You may have noticed that chemical equations are sometimes written with more information than at other times. For example, I don t always include the subscripts (for example, (s) or (g)), which indicate the state of the matter at the time of the reaction. Also, I don t always indicate whether energy is present as a reactant or as a product. When you use these notations depends upon the type of information that you want to convey at a given time. In general, it is always okay to show more information than you need to. In the next lesson you will learn how to show energy changes in chemical equations. 

I See the Saunders Interactive General Chemistry CD-ROM, Screen 6.12, Energy Changes in Chemical Processes. 

This is a consequence of the fact that no thermodynamically exact method is known that allows the determination of individual ion activities. Thermodynamics is the systematic study of energy changes in chemical and physical systems, and as such is independent of models and theories, including the molecular theory of matter. Acknowledging the existence of dissociated ions in solution is necessary for many purposes, but is not necessary to the definition of thermodynamically exact quantities. ... 

ENERGY CHANGES IN CHEMICAL REACTIONS Review Questions... 

Thus valence bond theory gives a clearer picture of chemical bond formation than the Lewis theory does. Valence bond theory states that a stable molecule forms from reacting atoms when the potential energy of the system has decreased to a minimum the Lewis theory ignores energy changes in chemical bond formation. 

Two concepts play important roles in determining the extent and speed of a chemical reaction thermodynamics, which deals with energy changes in chemical reactions, and kinetics, which describes the rate or speed of a chemical reaction. 

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