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5.9: Classifying Heat Transfers in Chemical Reactions

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  • Learning Objectives
    • Explain the Law of Conservation of Energy.
    • Define endothermic.
    • Define exothermic.

    The primary objective of this chapter is to quantify the amount of heat that is transferred during physical and chemical changes.  The previous two sections of this chapter have discussed and quantified the relationships between heat transfer and temperature changes and phase changes, respectively, which are both classified as physical changes.  Heat transfers also occur during chemical changes, as exemplified in the combustion reactions that were introduced in Chapter 4.  As a result of the molecular-level transformations that occur during chemical changes, heat energy must be either absorbed in, or generated by, nearly all chemical reactions, as will be discussed in the current and following sections.

    Bond Energy & The Law of Conservation of Energy

    Recall that, with the exception of the noble gases, elements are not stable in their atomic forms because they do not contain fully-satisfied valence shells.  As a result, bonding occurs through either ionic or covalent interactions, in which the unpaired electrons in these atoms are transferred or shared, respectively, in order to achieve octet configurations.  The particles that exist in the resultant compounds are more stable, meaning that they have less energy, relative to the atoms that were present prior to the bonding process.  Recall that the Law of Conservation of Matter states that particles cannot be created or destroyed in the course of a chemical reaction.  Because matter and energy are related through Albert Einstein's mass-energy equivalence principle, which is represented in the well-known equation, E = mc2, the Law of Conservation of Energy states that energy also cannot be created or destroyed during a chemical change.  Therefore, the excess energy that is present in atoms prior to the bonding process cannot simply dissipate and, instead, is stored as potential energy in the resultant bonds that are formed.  The specific amount of energy that is stored in a bond is dependent on the identities of the elements that are associated with that bond, which, in turn, directly influence whether the bond is formed through ionic or covalent interactions.  Furthermore, single, double, and triple covalent bonds each store different amounts of energy.

    Additionally, remember that the identity of a chemical must be transformed during a chemical reaction.  At a molecular level, the composition of a chemical is defined by the types of atoms that it contains, as well as on the ratio in which those atoms are present.  Therefore, in order to alter the identity of a substance, one or more atoms that were present in the initial substance must be removed or new atoms must be introduced.  In order for the composition of a chemical to be changed in one or both of these ways, the bonds that exist between these atoms must also be altered.  Pre-existing bonds must be broken, so that the associated atoms can separate and rearrange, and new bonds are ultimately created.  As a result, the overall combination of bonds and, therefore, the total amounts of stored energy that are present before and after a chemical reaction are not identical.  In other words, the combined energy of the reactants that undergo a chemical change is not equal to the total energy of the products that are generated from that reaction.  However, just as energy cannot be created or destroyed when compounds and molecules are formed from individual particles, the Law of Conservation of Energy mandates that this difference in reactant and product energies must be offset by a corresponding heat transfer between the reaction system and its surroundings.  If the combined energy of the reactants is greater than the total energy of the products, the excess energy is released as heat, causing any objects that are in contact with the reaction to become warmer.  These reactions, in which heat is generated as a product, are classified as exothermic.  In contrast, if the reactants have less energy, overall, than the products, heat must be absorbed as a reactant to initiate the corresponding chemical change.  As energy is transferred into an endothermic reaction system, the surroundings must lose an equivalent amount of energy and become cooler.

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