### Practice Problems

 Question: Consider the reaction at 25°, 1 atm: C(s) + O2(g) -> CO2(g), ΔH = -393.5 kJ. Which of the following are correct interpretations of the thermochemical equation? A. per gram of C reaction, 393.5/12.0 kJ is evolved to the surroundings B. the enthalpy of formation of CO2(g) is -393.5 kJ/mol C. whether or not CO is formed as an intermediate in the combustion, ΔH is still -393.5 kJ/mol for the combustion of C to CO2 D. all of the above Answer: D

 Question: Solve the problems below: A. How much energy is necessary to boil 5.00 grams of water (the heat of vaporization of water is 2260 J/g)? B. How many joules are given off if 100 milliliters of water freeze? C. What is the heat of vaporization of a substance if 26.00 grams are boiled, using 198.9 kilojoules? D. How many joules are required to heat 13.4 grams of water from -50 °C to 200 °C (the specific heat of ice is 2.0 J/g * K and that of steam is 2.1 J/g * K)? Answers: A. 5.00 g * 2260 J/g = 11300 joules, or 11.3 kilojoules. B. 100 g * 333 J/g = 33300 joules, or 33.3 kilojoules. C. 198900 J / 26.00 g = 7650 J/g. D. Start by finding the amount of energy needed to heat the ice from -50 °C to 0 °C: (2.0 J/g * K) * (13.4 g) * (50 K) = 1608 joules. Next, find the energy needed to melt the ice: (333 J/g) * (13.4 g) = 4462.2 joules. Then, find the energy needed to heat the water from 0 °C to 100 °C: (4.184 J/g * K) * (13.4 g) * (100 K) = 5606.56 joules. Next, find the joules necessary to boil the water: (2260 J/g) * (13.4 g) = 30284 joules. Finally, find the energy needed to heat the steam from 100 °C to 200 °C: (2.1 J/g * K) * (13.4 g) * (100 K) = 2814 joules. Finally, add up all the results to get a figure of 44774.76 joules. Significant figures have been ignored in this problem.

 Question: The gas phase reaction HCl + Br Cl + HBr is endothermic. This implies that A. an H-Br bond is stronger then an H-Cl bond B. an H-Cl bond is stronger then an H-Br bond C. ΔH for the reaction is negative D. none of the above Answer: B

 Question: The gas phase reaction HCl + Br Cl + HBr is endothermic. This implies that A. ΔH for the reaction is negative B. ΔH for the reaction is negligable C. ΔH for the reaction is positive Answer: C

 Question: Solve the problems below: A. How many degrees will the temperature of a 63-gram sample of a substance with a specific heat of 1.672 J/g * K rise if 9000 joules are applied? B. What is the specific heat of an element if 10 kilojoules heats a 214-gram sample by 24 Kelvins? C. How much water is in a beaker if 130 kilojoules heats the water by 76 Kelvins? D. How many joules are required to heat a 1-kilogram bar of aluminum (specific heat = 0.902 J/g * K) by 250 Kelvin? Answers: A. 9000 J / ((1.672 J/g * K) * 63 g) = 85.44 Kelvins. B. Specific heat = 10000 J / (214 g * 24 K) = 1.947 J/g * K. C. 4.184 J/g * K = 130000 J / (X grams * 76 K) => mass = 408.826 grams. D. 0.902 J/g * K = X joules / (1000 g * 250 K) => joules = 225500 joules, or 225.5 kilojoules.

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