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CHEMISTRY 1062 | University of the West Indies | Dr. John-Thomas | www.professorjohn-thomas.com Endothermic Versus Exothermic Reactions To understand the difference between these two types of reactions, we need to explore a couple of other concepts. In addition to kinetic energy (vibrational, rotational and translational motion), molecules also have potential energy. Potential energy is energy due to position and composition. It is stored in molecular bonds that exist within molecules (intramolecular) and also between different molecules (intermolecular). In water for example, there is energy stored in the bonds between oxygen and the two hydrogen atoms in each molecule, and also between the oxygen atom of one molecule and one of the hydrogen atoms of another molecule. Intramolecular bonds Intermolecular bonds The sum of all kinetic and potential energies of a substance is known as enthalpy (H). If in a reaction molecule A becomes molecules B and C, and if molecule A has more energy that both B and C combined, then the excess energy will be released into the environment. The environment becomes hotter; we have an exothermic reaction: A Æ B + C + energy Reactant products On a graph exothermic reactions are represented as follows: Energy Progress of reaction O H H O H H reactant product CHEMISTRY 1062 | University of the West Indies | Dr. John-Thomas | www.professorjohn-thomas.com If we examine the graph more closely, we will notice that exothermic reactions have a negative change in enthalpy. A change in enthalpy, ∆H, is defined as the enthalpy of products – heat of reactants: ∆H = Hp - Hr What is that little hill labeled, Ae? Ae = activation energy. This is the energy that reactants must absorb in order to form products, even if the products will not need the energy to store within their bonds. So Ae = Hmaximum - Hreactants Examples of exothermic reactions: • Digestion of food releases energy • All combustion reactions (fires) C + O2 Æ CO2 + energy • Adding an alkali metal to water 2 Na + 2 H2O Æ 2 NaOH + H2 + energy • Condensation of water • Explosion of bombs Endothermic Reactions If substance A must take energy away from the environment in order to form product D, then the reaction is said to be endothermic, and the victimized environment will feel colder after the reaction. ∆H = (+) for endothermic reactions and their profile looks like the following: Examples of endothermic reactions: CHEMISTRY 1062 | University of the West Indies | Dr. John-Thomas | www.professorjohn-thomas.com • Melting of ice absorbs energy • Dissolving ammonium nitrate in water( the essence of commercial cold packs) NH4 NO3(s) + energyÆ NH4 NO3(aq) • N2 + O2 + energyÆ NO
