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Calorimetry of Chemical Changes Chemical reactions that occur in aqueous solutions can also be studied using a poly- styrene calorimeter. The chemical system usually involves aqueous reactant solutions that are considered to be equivalent to water. The assumptions and formulas applied are iden- tical to those used in the analysis of energy changes during state changes and dissolving. When aqueous solutions of acids and bases react, they undergo a neutralization reac- tion. For example, potassium hydroxide and hydrobromic acid solutions react to form water and aqueous potassium bromide: KOH Caq) HBr, Caq)' KBr, Caq) The molar enthalpy of reaction for systems such as this is sometimes called the heat of neutralization, or enthalpy of neutralization. PI Practice Understanding Concepts 6. List three assumptions made in student investigations involving simple calorimeters. 7. The energy involved in the process r^O^ -> H 2 0 ffl could be described as a molar enthalpy of condensation. Describe the type of molar enthalpy that would be associ- ated with each of the following reactions: Br 2ffl (a] Cb) Cc) LiBrt C0 2fs) ts5 Bri (eD C 3 H 8(g) NaOH 50 2Cg) taq) 3 C0 2(ffl (aq) HCI Caq5 2 NaCI Caq) 4 H 2 0 0) H 2 0 m 9. Applying Inquiry Skills 8. In a calorimetry experiment in which you are measuring mass and temperature using equipment available to you in your school lab, which measurements limit the certainty of the experimental result? Explain. [a] A laboratory technician adds 43.1 ml of concentrated, 11.6 mol/L hydrochloric acid to water to form 500.0 ml of dilute solution. The temperature of the solution changes from 19.2°C to 21.8°C. Calculate the molar enthalpy of dilution of hydrochloric acid. What effect would there be on the calculated value forthe molar enthalpy of dilu- tion if the technician accidentally used too much water so that the total volume was actually more than 500.0 ml? Explain. The dissolving of an acid in water is a very exothermic process. Dilute acid solu- tions should always be made by adding acid to water. Explain why adding water to acid is very dangerous. 10. In a laboratory investigation into the reaction (b) (cj Ba[N0 3 ) 2Cs3 + K 2 S0 4C Caq) 2 KN0 3(aq5 a researcher adds a 261 -g sample of barium nitrate to 2.0 L of potassium sulfate solu- tion in a polystyrene calorimeter. Evidence As the barium nitrate dissolves, a precipitate is immediately formed. 7", = 26.0°C T 2 = 29.1°C Analysis (a) Calculate the molar enthalpy of reaction of barium nitrate. Section 5.2 INVESTIGATION 5.2.1 Molar Enthalpy of a Chemical Change (p. 348) How are molar enthalpies deter- mined? Use the equations and gen- eralizations you've learned to determine a value forthe molar enthalpy of neutralization of sodium hydroxide by sulfuric acid. Answers 9. (a) -10.9kJ/mol 10. Ca] -26 kJ/mol NEL Thermochemistry 311
