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Problem 69
For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up ΔH°f for each substance in Appendix C. (a) NO2(g) (b) SO3(g) (c) NaBr(s) (d) Pb(NO3)2(s).
Problem 70a
Write balanced equations that describe the formation of the following compounds from elements in their standard states, and then look up the standard enthalpy of formation for each substance in Appendix C: (a) CH3OH(l)
- The following is known as the thermite reaction: 2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(s). This highly exothermic reaction is used for welding massive units, such as propellers for large ships. Using standard enthalpies of formation in Appendix C, calculate _x001F_H ° for this reaction.
Problem 71
Problem 72
Acetylene (C2H2(g)) is used for welding because oxyacetylene is the hottest burning common fuel gas. Using standard enthalpies of formation, calculate the quantity of heat produced when 10 g of acetylene is completely combusted in air under standard conditions.
- Using values from Appendix C, calculate the standard enthalpy change for each of the following reactions: (a) 2 SO2(g) + O2(g) → 2 SO3(g) (b) Mg(OH)2(s) → MgO(s) + H2O(l) (c) N2O4(g) + 4 H2(g) → N2(g) + 4 H2O(g) (d) SiCl4(l) + 2 H2O(l) → SiO2(s) + 4 HCl(g)
Problem 73
Problem 74
Using values from Appendix C, calculate the value of H for each of the following reactions: (a) CaO(s) + 2 HF(g) → CaF2(s) + H2O(g) (b) Fe2O3(s) + 3 C(s) → 2 Fe(s) + 3CO(g) (c) 2 CO(g) + 2 NO(g) → N2(s) + 2 CO2(g) (d) 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2Og)
Problem 75
Complete combustion of 1 mol of acetone (C3H6O) liberates 1790 kJ: C3H6O(l) + 4 O2(g) → 3 CO2(g) + 3 H2O(l) ΔH° = -1790 kJ Using this information together with the standard enthalpies of formation of O2(g), CO2(g), and H2O(l) from Appendix C, calculate the standard enthalpy of formation of acetone.
Problem 76
Calcium carbide (CaC2) reacts with water to form acetylene (C2H2) and Ca(OH)2. From the following enthalpy of reaction data and data in Appendix C, calculate H°f for CaC2(s): CaC2(s) + 2 H2O(l) → Ca(OH2)(s) + C2H2(g) ΔH° = -127.2 kJ
Problem 77
Gasoline is composed primarily of hydrocarbons, including many with eight carbon atoms, called octanes. One of the cleanest–burning octanes is a compound called 2,3,4- trimethylpentane, which has the following structural formula: The complete combustion of one mole of this compound to CO2(g) and H2O(g) leads to ΔH° = -5064.9 kJ. (b) By using the information in this problem and data in Table 5.3, calculate H°f for 2,3,4-trimethylpentane.
Problem 79b
Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (b) Calculate the standard enthalpy change for the reaction, assuming H2O(g) as a product.
Problem 79c
Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (c) Calculate the heat produced per liter of ethanol by combustion of ethanol under constant pressure. Ethanol has a density of 0.789 g/mL.
Problem 79d
Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (d) Calculate the mass of CO2 produced per kJ of heat emitted.
Problem 80b
Methanol (CH3OH) is used as a fuel in race cars. (b) Calculate the standard enthalpy change for the reaction, assuming H2O(g) as a product.
Problem 80c
Methanol (CH3OH) is used as a fuel in race cars. (c) Calculate the heat produced by combustion per liter of methanol. Methanol has a density of 0.791 g/mL.
Problem 80d
Methanol (CH3OH) is used as a fuel in race cars. (d) Calculate the mass of CO2 produced per kJ of heat emitted.
- Without doing any calculations, predict the sign of H for each of the following reactions: (a) NaCl1s2¡Na+ 1g2 + Cl-1g2
Problem 81
- Without doing any calculations, predict the sign of _x001F_H for each of the following reactions: (a) 2 NO2(g) → N2O4(g) (b) 2 F(g) → F2(g) (c) Mg2+(g) + 2 Cl-(g) → MgCl2(s) (d) HBr(g) → H(g) + Br(g)
Problem 82
Problem 84a
Use bond enthalpies in Table 5.4 to estimate H for each of the following reactions: (a)
- (a) Use enthalpies of formation given in Appendix C to calculate _x001F_H for the reaction Br2(g) → 2 Br(g), and use this value to estimate the bond enthalpy D(Br–Br). (b) How large is the difference between the value calculated in part (a) and the value given in Table 5.4?
Problem 85
Problem 86
(a) The nitrogen atoms in an N2 molecule are held together by a triple bond; use enthalpies of formation in Appendix C to estimate the enthalpy of this bond, D(N‚N). (b) Consider the reaction between hydrazine and hydrogen to produce ammonia, N2H41g2 + H21g2¡2 NH31g2. Use enthalpies of formation and bond enthalpies to estimate the enthalpy of the nitrogen– nitrogen bond in N2H4. (c) Based on your answers to parts (a) and (b), would you predict that the nitrogen–nitrogen bond in hydrazine is weaker than, similar to, or stronger than the bond in N2 ?
- Consider the reaction 2 H2(g) + O2(g) → 2 H2O(l). (b) Without doing a calculation, predict whether your estimate in part (a) is more negative or less negative than the true reaction enthalpy.
Problem 87
Problem 87a
Consider the reaction 2 H2(g) + O2(g) → 2 H2O(l). (a) Use the bond enthalpies in Table 5.4 to estimate H for this reaction, ignoring the fact that water is in the liquid state.
- Consider the reaction H2(g) + Br2(l) → 2 HBr(g). (b) Without doing a calculation, predict whether your estimate in part (a) is more negative or less negative than the true reaction enthalpy.
Problem 88
- (a) What is meant by the term fuel value? (b) Which is a greater source of energy as food, 5 g of fat or 9 g of carbohydrate?
Problem 89
Problem 90b
(b) A particular chip snack food is composed of 12% protein, 14% fat, and the rest carbohydrate. What percentage of the calorie content of this food is fat?
- According to its nutrition label, the same meal also contains 140 mg of potassium ions. Do you think the potassium contributes to the caloric content of the food?
Problem 91
Problem 91a
(a) A serving of a particular ready-to-serve brown & wild rice meal contains 4.5 g fat, 42 g carbohydrate, and 4.0 g protein. Estimate the number of calories in a serving.
- A hamburger contains 28 g fat, 46 g carbohydrate, and 25 g protein. What is the fuel value in kJ in one hamburger? How many calories does it provide?
Problem 92
Problem 93
The heat of combustion of fructose, C6H12O6, is -2812 kJ/mol. If a fresh golden delicious apple weighing 120 g contains 16.0 g of fructose, what caloric content does the fructose contribute to the apple?
Problem 94
The heat of combustion of ethanol, C2H5OH(l), is -1367 kJ/mol. A bottle of stout (dark beer) contains up to 6.0% ethanol by mass. Assuming the density of the beer to be 1.0 g/mL, what is the caloric content due to the alcohol (ethanol) in a bottle of beer (500 mL)?