Chem 161-2009 Homework 7th week

Chem 161-2009 Homework 7th week - MasteringPhysics...

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Chem 161-2009 Homework 7 th Week and part of 8 th Week Chapter 6: 21,23,25,27,33,37,39,41,43,45,49,51,55,59,63,65,67,69,71,73,75,79,88,90,97,98,101 First Law of Thermodynamics 21. How much heat, in joules, must be involved in a process in which a system does 123 cal of work if the internal energy of the system is to remain unchanged? 1 cal = 4.184J ∆U = q + w 0 = q - 123cal q = +123cal +123 cal x (4.184J/cal) = +515J 23. Can a system do work and absorb heat at the same time? Can it do so while maintaining constant internal energy? Yes, a system can do work and absorb heat at the same time. For example, if a system forms gas then, since its gas is expanding, the surrounding’s gas is being compressed, and therefore work is being done on the surroundings. If this reaction is endothermic, then the system is absorbing heat, while doing work on the surroundings. It can do so while maintaining a constant internal energy. If the amount of joules absorbed in the form of heat is exactly equal to the amount of joules lost in the form of work on the surroundings, then the internal energy will not have changed. Enthalpy and Enthalpy Changes 25. At high temperatures, water is decomposed to hydrogen and oxygen. H 2 O(g) → H 2 (g) + ½ O 2 (g) Decomposition of 67.3 g H 2 O at constant pressure requires that 902 kJ of heat be absorbed by the system. Is the reaction endothermic or exothermic? What is the value of q for the reaction, per mole of water? Is the value of q equal to ∆U or ∆H? Explain. mol = g/MW 67.3g/(18.02gmol -1 ) = 3.735 mol If heat is absorbed by the system the reaction is endothermic. 902 kJ/3.735 mol = 242 kJ/mol of water. At constant volume, ∆U = q v . At constant pressure, ∆H = q p Since this reaction is being done at constant pressure, then q = ∆H. 27. When 0.1500 mol of solid calcium oxide is mixed with 0.1500 mol of liquid water, solid calcium hydroxide is formed and 9.78 kJ of heat is released. Write a chemical equation for the reaction producing 1 mol of calcium hydroxide, including the physical states of all the substances and the value of ∆H. 0.1500CaO(s) + 0.1500H 2 O(l) 0.1500Ca(OH) 2 (s) ∆H = -9.78J 1CaO(s) + 1H 2 O(l) 1Ca(OH) 2 (s) ∆H = -65.2J Heats of Reaction and Reaction Stoichiometry 1
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33. Calcium oxide (lime) reacts with carbon dioxide to form calcium carbonate (chalk). CaO(s) + CO 2 (g) → CaCO 3 (s) ∆H = -178.4 kJ How many kilojoules of heat are evolved in the reaction of 0.500 kg CaO(s) with an excess of carbon dioxide? ∆H of -178.4 kJ is for the reaction of 1 mol of CaO. MWcao = 56.08g/mol 500g/56.08gmol -1 = 8.916 mol CaO CaO(s) + CO 2 (g) → CaCO 3 (s) ∆H = -178.4kJ x 8.916 mol = -1590 kJ 8.916mol 37. How many liters of ethane, measured at 17 o C and 714 Torr, must be burned to give off 2.75 x 10 4 kJ of heat? 2C
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This document was uploaded on 11/02/2011 for the course GEN CHEM 162 at Rutgers.

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Chem 161-2009 Homework 7th week - MasteringPhysics...

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