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Unformatted text preview: 1 Experiment 2: Enthalpy of Chemical Reactions - Hesss Law PURPOSE In this two-part experiment, you will work with a partner to determine the change of enthalpy of a neutralization reaction and determine the enthalpy of formation of magnesium oxide by calorimetry. Both parts of the experiment will require you to measure the temperature change of several chemical reactions with a coffee cup a calorimeter, to determine the heat energy given off by the reactions, to sum the reactions and determine the change in enthalpy of an additional reaction by using the principles of Hesss law. OBJECTIVES 1. Use Hesss law to determine the enthalpy change of the reaction between aqueous ammonia and aqueous hydrochloric acid. 2. Use Hesss law to determine the enthalpy of formation of magnesium oxide. 3. Use prior knowledge about the summation of reaction heats. 4. Compare your calculated enthalpy change with the accepted experimental results. PRINCIPLES Thermochemistry is the study of the change of energy through observation, measurement, and prediction of energy changes that occur in both physical and chemical processes. thermo = heat energy THE FIRST LAW OF THERMODYNAMICS Energy is the capacity to do work or transfer heat. In thermodynamics, we categorize the energy changes that occur during a change in a chemical or physical process as heat and work. Energy can be further classified into two general types: kinetic and potential. 1. Kinetic energy (KE) is disruptive forces energy of motion. E kinetic = mv 2 2. Potential energy (PE) is cohesive forces energy possessed by a system through its position or composition. (Electrostatic forces of the atoms in a substance) PE + KE = E total . The total amount of energy in the universe is constant. (1 st Law of Thermodynamics) Processes that release heat energy in their surrounding are called exothermic . E.g. water freezing (a physical change) or combustion of methane (a chemical reaction). Processes that absorb heat energy from their surrounding are called endothermic. E.g. water melting (a physical change) or the reaction of solid hydrated Ba(OH) 2 8H 2 O (barium hydroxide octahydrate) with excess solid NH 4 NO 3 (ammonium nitrate). The latter chemical reaction absorbs heat and will freeze water 2 vapor in its immediate surroundings. Therefore, when the reaction takes place in a beaker on a wet surface, it will freeze and attach to the surface. THERMODYNAMIC TERMS System the part of the universe we want to study. Surroundings everything else that may or may not be capable of reacting with the system. Universe system + surroundings. State of a system a complete description of the system (temp, pressure, physical state, and number of moles and identity of each component in the system) State function (a.k.a. state variable) the properties of a system, i.e., temperature, volume, pressure....
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