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Q1: Why is polystyrene used to construct a calorimeter?
Polystyrene is an excellent insulator that minimizes heat exchange between the calorimeter's interior and external environment. This property ensures that the heat measured during reactions comes primarily from the chemical reaction itself rather than from heat loss to surroundings, making temperature measurements more accurate and reliable for calculating enthalpy changes.
Q2: What is the purpose of calibrating a calorimeter before measuring reaction heat?
Calorimeter calibration determines how much heat the calorimeter itself absorbs during temperature changes. By adding hot water to cold water and measuring the temperature change, you calculate the calorimeter's heat capacity. This value is then used to account for heat absorbed by the calorimeter during chemical reactions, ensuring accurate enthalpy measurements.
Q3: How does polishing magnesium ribbon affect the reaction with hydrochloric acid?
Polishing removes the outer layer of magnesium oxide that forms when magnesium metal is exposed to air. This reveals fresh, reactive magnesium metal underneath, ensuring the reaction proceeds efficiently and completely. Without polishing, the oxide layer would slow or inhibit contact between the magnesium and hydrochloric acid.
Q4: Why is stirring important during calorimetry experiments?
Stirring ensures uniform mixing of reactants and products throughout the solution, allowing the temperature probe to accurately measure the true temperature of the entire mixture. Vigorous stirring also helps dissolve reactants completely and distributes heat evenly, preventing localized hot or cold spots that would compromise temperature readings and enthalpy calculations.
Q5: What assumptions allow enthalpy change to equal internal energy change in these experiments?
The experiments assume constant pressure, negligible volume change as the solution heats, and no energy transfer between the solution and surroundings. Under these conditions, the enthalpy change equals the heat released or absorbed, which approximately equals the change in internal energy. These assumptions simplify calculations and make the measured temperature changes directly proportional to reaction enthalpy.
Q6: How do you calculate the enthalpy of formation of magnesium oxide from two separate reactions?
You measure the enthalpy of the magnesium-hydrochloric acid reaction and the magnesium oxide-hydrochloric acid reaction separately. Then you reverse the second reaction and combine it with the first reaction and the known enthalpy of formation of water using Hess's Law. This combination yields the enthalpy of formation of magnesium oxide, which you compare to literature values to determine percent error.
Q7: Why must magnesium oxide and hydrochloric acid reaction data be collected after the magnesium metal reaction?
The second reaction is performed after the first to allow time for data processing and to ensure the calorimeter is thoroughly cleaned and dried between experiments. Additionally, the experimental design uses both reactions together with Hess's Law to calculate the enthalpy of formation of magnesium oxide, requiring both datasets to be complete before final calculations can be performed.