1. What measurements did you make during this experiment? 2. Use your average temperature change to calculate the amount of heat energy, q , produced in the reaction. In determining the mass, m , of the solution use 1.11 g/mL for the density. Use 4.18 J/(g • °C) as the specific heat, c p , of the solution. 3. Use the heat energy that you calculated in question 2 above to determine the enthalpy change, ∆ H neut , for the reaction in units of kJ/mol of phosphoric acid. This is your experimental value of ∆ H neut . 4. Write the balanced net ionic equation for the complete neutralization of H 3 PO 4 with NaOH. 5. Use a table of standard thermodynamic data below to calculate the ∆ H neut of neutralization for the net ionic equation of phosphoric acid reacting completely with sodium hydroxide. Consider this the accepted value of ∆ H neut . Standard Enthalpy of Formation Δ H f ° (kJ/mol) OH − ( aq ) −230 H + ( aq ) 0 H 2 O ( ) −286 6. Calculate the percent discrepancy between the calculated (accepted) value of the ∆ H of neutralization of H 3 PO 4 and your experimental value. 7. A student conducts two trials of this experiment, but neglects to rinse the calorimeter cup between trials. What effect, if any, would this error have on the calculated value of the ∆Hof neutralization?
1. This experiment conforms to the guidelines for the thirteenth laboratory experiment listed in the College Board AP Chemistry guide (the Acorn book). 2. Prepare the 0.60 M H 3 PO 4 by adding 40.5 mL of concentrated (o-phosphoric acid, 14.8 M ) H 3 PO 4 to sufficient water to make 1 L of solution. 3. Prepare the 1.85 M NaOH solution by adding 74.0 g solid NaOH to sufficient water to make 1 L of solution. 4. You may wish to insert the temperature probe into a 1-hole stopper prior to clamping it. If the probe does not slide in easily, then use a utility knife to split the stopper rather than damage the probe. Also note that the illustration shows only one cup, but the procedure calls for two to enhance results. You may also opt to fashion a lid from a piece of cardboard with a hole in the center to accommodate the temperature probe. HAZARD ALERTS Phosphoric Acid: Skin and eye irritant; slightly toxic by ingestion and inhalation; corrosive; burns tissue. Hazard Code: A—Extremely hazardous. Sodium Hydroxide: Corrosive solid; skin burns are possible; much heat evolves when added to water; very dangerous to eyes; wear face and eye protection when using this substance. Wear gloves. Hazard Code: B—Hazardous. The hazard information reference is: Flinn Scientific, Inc., Chemical and Biological Catalog Reference Manual, P.O. Box 219, Batavia, IL 60510, (800) 452-1261, SAMPLE DATA TABLE Adapted from Advanced Chemistry with Vernier & Laboratory Experiments for Advanced Placement Chemistry by Sally Ann Vonderbrink, Ph. D.