Piedmont Hills High School AP Chemistry 2018-2019 Laboratory Experiment 3 – Calorimetry Experiment 11/6/18 Purpose: 1. Determine the specific heat of the unknown metal. 2. Determine and record the enthalpy change of the dissolution of 1.00 mole of sodium hydroxide 3. Determine the enthalpy change of the reaction of magnesium metal and oxygen gas using Hess’s law. Introduction: Hess’s Law states that the total enthalpy change during the complete course of chemical reaction is the same whether the reaction is made in one step or in several steps. The law allows for the calculation of the enthalpy changes of reactions that cannot be observed due to equipment capability limitations. Using other reactions to calculate the enthalpy changes of the various steps of a reaction, Hess’s Law makes it possible to add all the enthalpy changes together to obtain the total enthalpy change of the reaction. Thus, reactions such as that of magnesium metal and oxygen gas can be calculated for their enthalpy change even if they cannot be observed within a calorimeter. Calorimetry is the act of measuring changes in state functions of a material for the purpose of calculating the transfer of heat due to changes of the material’s state. For example, changes include chemical reactions, physical changes, or phase transitions. Calorimetry is performed using a calorimeter. A thermometer is placed inside the calorimeter through a hole to read the changes in temperature. Calorimetry will be used in this experiment to calculate the specific heat of certain materials and the enthalpy change of chemical reactions. The standard enthalpy of formation is the total change in enthalpy during the formation of 1 mole of a substance from its constituent elements. All substances involved in the formation are in their standard states. When there is more than one reaction involved in the formation of a compound, Hess’s Law can be used to calculate the standard enthalpy of formation. The specific heat of a substance is the amount of heat needed to raise the temperature of the unit mass of the substance by 1 . By comparing the amount of heat transferred from an ℃ unknown substance to another substance with a known specific heat, the specific heat of the unknown can be calculated using the known substance’s values. The experiment will also use a water bath to heat the unknown metal. The water bath will be heated to boiling and the metal will be placed in a test tube that is inside the water. The heat from the water will heat the metal while still keeping the metal dry to prevent any heated water getting into the calorimeter. The technique will prevent inaccuracies in calculating the metal’s specific heat. Materials:
1. 1 Styrofoam calorimeter 2. Wireless temperature probe 3. Chromebook 4. 1.0M HCl solution 5. Sodium hydroxide beads 6. Magnesium oxide (s) 7. Magnesium metal 8. 1 test tube 9. 1 hot plate 10. 2 400 ml beakers 11. 1 stirring rod Procedure: A. Determining the Specific Heat of the Unknown Metal.
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