Calorimetry

Experiment (12) Calorimetry Objectives: This experiment is designed to measure the Heats of Reaction of a series of chemical reactions. There are several kinds of "heats of Reactions", as an example: heat of solution, heat of neutralization, heat of combustion, heat of formation, etc. However, we will study only the firt two of these: 1- Heat of Solution: It is the heat flow during the process of solution; as an example when NaOH dissolves in water, hydrated ions are formed an heat is released the solution absorbs this heat and, therefore increases in temperature. Where the heat of solution in KJ is . 2- Combined heat of solution and heat of neutralization: in a second reaction, solid sodium hydroxide is reacted with an aqueous solution of hydrogen chloride. During this reaction, water and a dilute aqueous solution of sodium ions and chloride ions are formed. This time both the heat of solution and the heat of neutralization are released: 3- Heat of neutralization: it is the heat evolved at constant pressure when acid are neutralized with a base. It can be obtained from the two steps above. The difference between the heat of reaction (obtained by dissolving solid NaOH in aqueous HCl solution (step 2) and the heat of solution (obtained by dissolving solid NaOH in water step 1). 4- In another reaction (which we shall not perform in the lab.), we could add distilled water to aqueous solution of sodium chloride. The heat released in the heat of dilution. This effect was present in the reactions of step 1, 2 and 3 above; but we shall neglect this heat of dilution in our calculation because it is smaller than the experimental errors expected in the measurement of the other heats. 5- The final experiment of this laboratory class will be to determine the heat of solution of an unknown salt, as in step 1 above. Example (1): 2.00g of solid CsOH is dissolved in 200.0mL of water in a calorimeter. The temperature of the water was raised from 22.3 to 23.4o. Calculate the heat of solution in KJ/mol? (Assume the specific heat of solution to be 4.184 j/goC and the density of solution to be 1.00 g/mL). a- Temperature change, b- Total mass of solution, m = (200.0 mL water) (1.00 g/mL) + 2.00g CsOH = 202.0g. c- Heat absorbed by the solution = heat evolved by CsOH = mass of solution x specific heat of solution x . d- Moles of CsOH = mass / Molar mass = 2.00 g/149.9 mol/g = 0.01334mol. e- Heat evolved per mole of CsOH = 930J/0.01334mol = 7.0 x 104 J/mol. f- Heat of reaction in KJ/mol = -7.0 x 104 J/mo x (1 KJ/103J) = -70 KJ/mol. Note: the minus sign shows that the reaction is exothermic; i.e heat is given up (evolved) by the system. Example (2): 2.00g of solid CsOH reacted with 200.0 mL of an aqueous solution of hydrogen chloride in a calorimeter. The temperature of the solution was increased from 22.3 to 24oC. Calculate the heat of reaction in KJ/mol? (Assume the specific heat of solution to be 4.184 J/g. oC. And the density of solution to be 1.00 g/mL). a- Temperature change, . b- Total mass of solution – 202.0g. c- Heat absorbed by the solution = 1.7 x 103 J. d- Moles of CsOH = 0.01334 mol. e- Heat evolved per mole of CsOH = 1.3 x 105 J/mol. f- Heat of reaction in KJ/mol = -1.3 x 102 KJ/mol. Note: The difference in the heat of reaction (Example 2) and the heat of solution (Example 1) is the heat of neutralization. So the heat of neutralization of NaOH with HCl is -60KJ/mol. Procedure: Assume that the density of all solution is 1.00 g/mL and their specific heat is 4.184oC. A- Determination of the heat of solution of NaOH (ΔH1): 1- Add 100.0 ml of water to the calorimeter. 2- Close the calorimeter and stir the water until a constant temperature is reached. Measure the initial temperature of water. 3- Weigh 2.0 g of NaOH. You should weight the NaOH fast, since it is very Hygroscobic. 4- Add the solid NaOH to the HCl solution in the calorimeter and stir gently until the maximum temperature reached. B- Determination of the heat of reaction (ΔH2): 1- Add 100.0 ml of 0.500 M HCL solution to the calorimeter. 2- Stir the solution as in procedure 1 above, and record the initial temperature. 3- Weigh 2.0 g of NaOH. You should weight the NaOH fast, since it is very Hygroscobic. 4- Add the solid NaOH to the HCL solution in the calorimeter and stir gently until the maximum temperature is reached. Measure the maximum (Final) temperature. C- Determination of the heat of solution of an unknown salt: 1- Pick up an unknown salt from the instructor and record the unknown number on the report sheet. You will be told the molar mass of the salt. 2- Repeat the procedures in part A, using 3.0g of the unknown salt in place of NaOH. Data and Results: Trail 1 Trial 2 Part A. Determination of the heat of solution of NaOH (ΔH 2) Volume of water ……………. ……………. Mass of water ……………. ……………. Mass of NaOH ……………. ……………. Molar mass of NaOH 40.0g/mol Moles of NaOH ……………. ……………. Mass of solution ……………. ……………. Initial temperature ……………. ……………. Final temperature ……………. ……………. ΔH1 of solution ……………. ……………. Average ΔH1 ……………. ……………. Part B. Determination of the heat of reaction of NaOH (ΔH2) and the heat of neutralization: Trail 1 Trial 2 Volume of HCl ……………. ……………. Mass of HCl ……………. ……………. Mass of NaOH ……………. ……………. Molar mass of NaOH ……………. ……………. Mass of solution ……………. ……………. Initial temperature ……………. ……………. Final temperature ……………. ……………. ΔH2 of solution ……………. ……………. Average ΔH2 ……………. ……………. Heat of neutralization (ΔH2 - ΔH1) ……………. ……………. Average heat of neutralization ……………. …………….