6-1 Experiment 6 THERMOCHEMISTRY I. Learning Objectives… ♦ To introduce the concept of thermochemistry. ♦ To illustrate the additivity of heats of reaction ( Hess’s Law ). ♦ To define and investigate exothermic and endothermic reactions. II. Background Information Thermochemistry is the study of the heat energy involved in chemical reactions and changes of physical state . Heat energy ( thermal energy) is always spontaneously transferred from hotter to colder matter. The First Law of Thermodynamics (Law of Energy Conservation) states that the total energy of the universe must remain constant. Therefore, all energy transferred between a system and its surroundings must be accounted for as heat or work . The standard S.I. unit for heat energy is the joule ( J ). It takes 4.184 joules (1 calorie) to raise the temperature of one gram of water by 1 ° C. The kilojoule (kJ) is commonly used (1000 joule = 1 kilojoule) in many applications. Exothermic and Endothermic Reactions When a chemical reaction takes place at roughly constant temperature and pressure (bench-top conditions), the system defined by the reactants and products either absorbs or releases heat energy. If the reacting system releases heat energy to its surroundings, a concurrent increase in surroundings temperature is observed, and the reaction is exothermic . If the system absorbs heat energy from its surroundings, a decrease in the surroundings temperature is observed, and the reaction is endothermic. A measure of the amount of heat given off or absorbed in any chemical reaction is called the enthalpy change or heat of reaction, and is given the symbol Δ H . The relationship of enthalpies of
1 f .
6-3 Δ H ° rxn = ∑ [n Δ H ° f (products)] - ∑ [n Δ H ° f (reactants)] Tables containing the standard heats of formation for a number of compounds are available in the appendices of any general chemistry textbook.
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