Chpt 6 Solutions

Chpt 6 Solutions - CHAPTER 6 THERMOCHEMISTRY Questions 11....

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182 CHAPTER 6 THERMOCHEMISTRY Questions 11. Path-dependent functions for a trip from Chicago to Denver are those quantities that depend on the route taken. One can fly directly from Chicago to Denver, or one could fly from Chicago to Atlanta to Los Angeles and then to Denver. Some path-dependent quantities are miles traveled, fuel consumption of the airplane, time traveling, airplane snacks eaten, etc. State functions are path-independent; they only depend on the initial and final states. Some state functions for an airplane trip from Chicago to Denver would be longitude change, latitude change, elevation change, and overall time zone change. 12. Products have a lower potential energy than reactants when the bonds in the products are stronger (on average) than in the reactants. This occurs generally in exothermic processes. Products have a higher potential energy than reactants when the reactants have the stronger bonds (on average). This is typified by endothermic reactions. 13. 2 C 8 H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2 O(g); all combustion reactions are exothermic; they all release heat to the surroundings, so q is negative. To determine the sign of w, con- centrate on the moles of gaseous reactants versus the moles of gaseous products. In this combustion reaction, we go from 25 moles of reactant gas molecules to 16 + 18 = 34 moles of product gas molecules. As reactants are converted to products, an expansion will occur. When a gas expands, the system does work on the surroundings, and w is negative. 14. H = E + P V at constant P; from the definition of enthalpy, the difference between H and E at constant P is the quantity P V. Thus, when a system at constant P can do pressure- volume work, then H E. When the system cannot do PV work, then H = E at constant pressure. An important way to differentiate H from E is to concentrate on q, the heat flow; the heat flow by a system at constant pressure equals H, and the heat flow by a system at constant volume equals E. 15. a. The H value for a reaction is specific to the coefficients in the balanced equation. Be- cause the coefficient in front of H 2 O is a 2, 891 kJ of heat is released when 2 mol of H 2 O is produced. For 1 mol of H 2 O formed, 891/2 = 446 kJ of heat is released. b. 891/2 = 446 kJ of heat released for each mol of O 2 reacted. 16. Use the coefficients in the balanced rection to determine the heat required for the various quantities. a . 1 m o l H g × Hg mol kJ 7 . 90 = 90.7 kJ required
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CHAPTER 6 THERMOCHEMISTRY 183 b. 1 mol O 2 × 2 O mol 2 / 1 kJ 7 . 90 = 181.4 kJ required c. When an equation is reversed, H new = −∆ H old . When an equation is multiplied by some integer n, then H new = n ( H old ). Hg(l) + 1/2 O 2 (g) HgO(s) H = 90.7 kJ 2Hg(l) + O 2 (g) 2HgO(s) H = 2( 90.7 kJ) = 181.4 kJ 17. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) H = ! 891 kJ C H 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(g) H = ! 803 kJ H 2 O(l) + 1/2 CO 2 (g) 1/2 CH 4 (g) + O 2 (g) H 1 = ! 1/2( ! 891 kJ) 1/2 CH 4 (g) + 2 O 2 (g) 1/2 CO 2 (g) + H 2 O(g) H 2 = 1/2( ! 803 kJ) ______________________________________________________________________________________ H 2 O(l) H 2 O ( g ) H = H 1 + H 2 = 44 kJ The enthalpy of vaporization of water is 44 kJ/mol.
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This note was uploaded on 10/27/2010 for the course CHEM 102 taught by Professor Peterpastos during the Spring '08 term at CUNY Hunter.

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Chpt 6 Solutions - CHAPTER 6 THERMOCHEMISTRY Questions 11....

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