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Heat of Solution

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The molar heat of solution (Hsoln) of a substance is the change in the chemical energy when one mole of the substance is dissolved in a solvent. In this experiment you will measure 50 mL of room temperature water. Pour the water into a Styrofoam cup (calorimeter). Place the calorimeter in a 250 mL beaker to stabilize it. Weigh out 10.0 g Sodium thiosulfate pentahydrate (Na2S2O3 · 5H2O). Record the actual mass used to three decimal places. Record the mass and the temperature of the water in the cup to the nearest 0.1 ºC. Add the solid to the water and stir until it is completely dissolved. (Use a data table to display your data.) Measure the temperature again, and record it to the nearest 0.1 ºC. Calculate the thermal energy change (H) and the chemical energy change (-H). Calculate the amount (moles) of sodium thiosulfate pentahydrate which dissolved. Then calculate the heat of solution (J/g) and Hsoln (kJ/mol) for thiosulfate pentahydrate. Now, repeat the experiment by dissolving approximately 4.0 g of calcium chloride (CaCl2) in 50 mL of water. Record the exact amounts used. Repeat the calculations to obtain the heat of solution for calcium chloride. Heat of Solution ­ Write your report on a separate sheet or sheets of paper. A. Purpose: B. Safety: C. Data: (Place in a table) Na2S2O3·5H2O CaCl2 Mass of solid _____g _____g Moles of solid _____mol _____mol Volume of water _____mL _____ mL Mass of water (assume DH2O=1.00 g/mL) _____ g _____ g ti (initial temp.) _____ºC _____ºC tf (final temp.) _____ºC _____ºC tf-ti (change in temp.) _____ºC _____ºC thermal energy change H (qsurr) _____kJ _____kJ chemical energy change ­H (qsys) _____kJ _____kJ Gram heat of solution _____kJ/g _____kJ/g Molar heat of solution (Hsoln) _____kJ/mol _____kJ/mol Additional Observations: D. Calculations: Show ALL calculations for both substances. Be sure to label all calculations. 1. qsurroundings: Hsurroundings = (msoln)(cH2O)(TH2O) 2. qsystem 3. Gram heat of solution = -H/g of substance dissolved 4. Molar heat of solution Hsoln= -H/mol of substance dissolved E. Conclusions: F. Questions: 1. Which reaction is endothermic? What is the energy being used for? Where is the energy coming from? 2. Draw the potential energy diagram showing the heat flow for the endothermic process. Label all parts including the axis, heat of dissociation, heat of hydration, heat of solution, reactants or products for each section. For a hint, see pages 415 and 553 in your textbook. 3. Which reaction is exothermic? Where is the energy coming from? Where is the energy going? 4. Draw the potential energy diagram showing the heat flow for the exothermic process. Label all parts including the axis, heat of dissociation, heat of hydration, heat of solution, reactants or products for each section. 5. Calculate the thermal and chemical energy changes when 5.85 g of sodium chloride are dissolved in 400.0 mL of water in an insulated calorimeter. The temperature drops by 0.32 ºC. Draw sequential pictures of the process described at the molecular/atomic level. 6. Calculate the heat of solution (kJ/mol) of NaCl from the information in question 5. 7. If Hsoln CaCl2 is -82.8 kJ/mol, analyze your experimental results (include % error) and determine at least two possible sources of error. Describe how each of those sources of error contributed to the final answer (did the error result in an experimentally calculated value that was too large or too small? Why?

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Heat of Solution

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