Thermochemistry Questions Lab

AP Questions: Thermochemistry 1984 B Standard Heat of Formation,Hfo Substance in kJ mol-1 ---------------------------------------------C(s) 0.00 CO2(g) -393.5 H2(g) 0.00 H2O(l) -285.85 O2(g) 0.00 C3H7COOH(l) ? ------------------------ The enthalpy change for the combustion of butyric acid at 25 oC, Hcomb, is -2,183.5 kilojoules per mole. The combustion reaction is C3H7COOH(l) + 5 O2(g)  4 CO2(g) + 4 H2O(l) (a) From the above data, calculate the standard heat of formation, Hfo, for butyric acid. (b) Write a correctly balanced equation for the formation of butyric acid from its elements. 1988 B Substance C(s) H2(g) C2H5OH(l) H2O(l) (a) (b) Enthalpy of Combustion, H (kiloJoules/mol) -393.5 -285.8 -1366.7 -- Write a separate, balanced chemical equation for the combustion of each of the following: C(s), H2(g), and C2H5OH(l). Consider the only products to be CO2 and/or H2O(l). In principle, ethanol can be prepared by the following reaction: 2 C(s) + 2 H2(g) + H2O(l)  C2H5OH(l) Calculate the standard enthalpy change, H, for the preparation of ethanol, as shown in the reaction above. 1988 D An experiment is to be performed to determine the standard molar enthalpy of neutralization of a strong acid by a strong base. Standard school laboratory equipment and a supply of standardized 1.00 molar HCl and standardized 1.00 molar NaOH are available. (a) What equipment would be needed? (b) What measurements should be taken? (c) Without performing calculations, describe how the resulting data should be used to obtain the standard molar enthalpy of neutralization. (d) When a class of students performed this experiment, the average of the results was -55.0 kilojoules per mole. The accepted value for the standard molar enthalpy of neutralization of a strong acid by a strong base is -57.7 kilojoules per mole. Propose two likely sources of experimental error that could account for the result obtained by the class. 1995 B Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking. (a) Write a balanced equation for the complete combustion of propane gas, which yields CO2(g) and H2O(l). (b) Calculate the volume of air at 30°C and 1.00 atmosphere that is needed to burn completely 10.0 grams of propane. Assume that air is 21.0 percent O2 by volume. (c) The heat of combustion of propane is -2,220.1 kJ/mol. Calculate the heat of formation,,Hfo, of propane given that ,Hfo of H2O(l) = -285.3 kJ/mol and ,Hfo of CO2(g) = -393.5 kJ/mol . (d) Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00 kilograms of water (specific heat = 4.18 J/g.K), calculate the increase in temperature of water. 1998 B C6H5OH(s) + 7 O2(g)  6 CO2(g) + 3 H2O(l) When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow. Substance C(graphite) CO2(g) H2(g) H2O(l) O2(g) C6H5OH(s) (a) (b) (c) Standard Heat of Formation, Hfo; at 25oC (kJ/mol) 0.00 -393.5 0.00 -285.85 0.00 ? Calculate the molar heat of combustion of phenol in kilojoules per mole at 25 oC. Calculate the standard heat of formation , Hfo, of phenol in kilojoules per mole at 25oC. If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110oC. (Assume no oxygen remains unreacted and that all products are gaseous.) 2001 B 2 NO(g) + O2(g)  2 NO2(g) H°= -114.1 kJ, S°= -146.5 J K-1 The reaction represented above is one that contributes significantly to the formation of photochemical smog. (a) Calculate the quantity of heat released when 73.1 g of NO(g) is converted to NO2(g). (b) For the reaction at 25C, the value of the standard free-energy change, G, is -70.4 kJ. (i) Calculate the value of the equilibrium constant, Keq, for the reaction at 25C. (ii) Indicate whether the value of G would become more negative, less negative, or remain unchanged as the temperature is increased. Justify your answer. (c) Use the data in the table below to calculate the value of the standard molar entropy, S, for O2(g) at 25C. Standard Molar Entropy, S (J K-1 mol-1) (d) NO(g) 210.8 NO2(g) 240.1 Use the data in the table below to calculate the bond energy, in kJ mol -1, of the nitrogen-oxygen bond in NO2 . Assume that the bonds in the NO2 molecule are equivalent (i.e., they have the same energy). Bond Energy (kJ mol-1) Nitrogen-oxygen bond in NO 607 Oxygen-oxygen bond in O2 495 Nitrogen-oxygen bond in NO2 ?