WA07_CHE-121-mar14

# WA07_CHE-121-mar14 - Written Assignment 7 Energy and...

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Written Assignment 7: Energy and Thermochemistry Answer all assigned questions and problems, and show all work. 1. Consider this reaction: 2CH 3 OH( l ) + 3O 2 ( g ) 4H 2 O( l ) + 2CO 2 ( g ) ∆ H = –1452.8 kJ/mol a. Is this reaction endothermic or exothermic? (2 points) Exothermic. b. What is the value of ∆ H if the equation is multiplied throughout by 2? (2 points) -1452.8 kJ/mol x 2 = 2905.6 kJ/mol c. What is the value of ∆ H if the direction of the reaction is reversed so that the products become the reactants and vice versa? (2 points) 1452.8 kJ/mol d. What is the value of ∆ H if water vapor instead of liquid water is formed as the product? (2 points) (Reference: Chang 6.24) 4H 2 O - 4H 2 O (4 x -241.8 kJ/mol) – (4 x -285.8 kJ/mol) = 176 kJ/mol -1452.8 kJ/mol + 176 kJ/mol = -1276.8 kJ/mol 2. The first step in the industrial recovery of zinc from the zinc sulfide ore is roasting, that is, the conversion of ZnS to ZnO by heating: 2ZnS( s ) + 3O 2 ( g ) 3ZnO( s ) + 2SO 2 ( g ) H = –879 kJ/mol Calculate the heat evolved (in kJ) per gram of ZnS roasted. (5 points) (Reference: Chang 6.25) ZnS = 97.44 g/mol -879 kJ/mol/ 97.44 g/mol = -9.021 kJ/g 3. A 6.22 kg piece of copper metal is heated from 20.5 °C to 324.3 °C. Calculate the heat absorbed (in kilojoules) by the metal. Specific heat Cu = 0.385 J/g°C. (5 points) (Reference: Chang 6.33) C = 0.385 J/g°C T = 324.3 °C – 20.5 °C = 303.8 °C 6,220g x 0.385 J/g°C x 303.8 °C = 728,000 J / 1,000 = 728 kJ 4.

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