Enthalpy and Calorimetry
Enthalpy
Enthalpy is a property of a system and is equal to E + PV, where E is the internal energy of the system, P is the pressure, and V is the volume. In a chemical reaction, the enthalpy change is equal to the total enthalpy of the products minus the enthalpy of the reactants. The following equation is used to determine the change in enthalpy in a chemical reaction.
When the products have a greater enthalpy than the reactants, delta H ends up positive. In these situations, the higher enthalpy in the products results in the absorbtion of heat by the system, thus an endothermic reaction. Also, if delta H is negative, that means that the reactants have more enthalpy, and must "release" it on its way to becoming products. How do the reactants "release" their enthalpy? They generate heat, and the reaction is exothermic. So, when delta H is positive, the reaction is endothermic, and when delta H is negative, the reaction is exothermic.
Calorimetry
Calorimetry is used to determine the heat released or absorbed in a chemical reaction. The calorimeters shown here can determine the heat of a solution reaction at constant (atmospheric) pressure. The calorimeter is a double styrofoam cup fitted with a plastic top in which there is a hole for a thermometer. (It's crude, but very effective!) Key techniques for obtaining accurate results are starting with a dry calorimeter, measuring solution volumes precisely, and determining change in temperature accurately.
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Hess's Law
