SilberbergChap6ISM

# SilberbergChap6ISM - CHAPTER 6 THERMOCHEMISTRY ENERGY FLOW...

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CHAPTER 6 THERMOCHEMISTRY: ENERGY FLOW AND CHEMICAL CHANGE 6.1 No , an increase in temperature means that heat has been transferred to the surroundings, which makes q positive. 6.2 Δ E = q + w = w, since q = 0. Thus, the change in work equals the change in internal energy. 6.3 a) electric heater b) sound amplifier c) light bulb d) automobile alternator e) battery (voltaic) 6.4 Heat energy; sound energy (impact) Kinetic energy (falling text) Potential energy (raised text) Mechanical energy (raising of text) Chemical energy (biological process to move muscles) 6.5 The change in a system’s energy is Δ E = q + w . If the system receives heat, then its q final is greater than q initial so q is positive. Since the system performs work, its w final < w initial so w is negative. The change in energy is (+425 J) + (–425 J) = 0 J . 6.6 Δ E = q + w = –255 cal + (–428 cal) = –683 cal 6.7 C( s ) + O 2 ( g ) CO 2 ( g ) + 6.6 x 10 10 J (2.0 ton) a) Δ E (kJ) = (6.6 x 10 10 J) 3 1kJ 10 J ⎛⎞ ⎝⎠ = 6.6 x 10 7 kJ b) Δ E (kcal) = (6.6 x 10 10 J) 3 1 cal 1 kcal 4.184 J 10 cal ⎜⎟ = 1.577 x 10 7 = 1.6 x 10 7 kcal c) Δ E (Btu) = (6.6 x 10 10 J) 1Btu 1055 J = 6.256 x 10 7 = 6.3 x 10 7 Btu 6.8 CaCO 3 ( s ) + 9.0 x 10 6 kJ CaO( s ) + CO 2 ( g ) (5.0 ton) a) Δ E (J) = (9.0 x 10 6 kJ) 3 10 J = 9.0 x 10 9 J b) Δ E (cal) = (9.0 x 10 6 kJ) 3 10 J 1 cal 4.184J = 2.15105 x 10 9 = 2.2 x 10 9 cal c) Δ E (Btu) = (9.0 x 10 6 kJ) 3 10 J 1 Btu 1 kJ 1055 J = 8.5308 x 10 6 = 8.5 x 10 6 Btu 6-1

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6.9 Δ E (J) = (4.1 x 10 3 Calorie) 3 10 cal 4.184 J 1 Calorie 1 cal ⎛⎞ ⎜⎟ ⎝⎠ = 1.7154 x 10 7 = 1.7 x 10 7 J Δ E (J) = (4.1 x 10 3 Calorie) 3 3 10 cal 4.184 J 1 kJ 1 Calorie 1 cal 10 J = 1.7154 x 10 4 = 1.7 x 10 4 kJ 6.10 a) Exothermic , the system (water) is releasing heat in changing from liquid to solid. b) Endothermic , the system (water) is absorbing heat in changing from liquid to gas. c) Exothermic , the process of digestion breaks down food and releases energy. d) Exothermic , heat is released as a person runs and muscles perform work. e) Endothermic , heat is absorbed as food calories are converted to body tissue. f) Endothermic , the wood being chopped absorbs heat (and work). g) Exothermic , the furnace releases heat from fuel combustion. Alternatively, if the system is defined as the air in the house, the change is endothermic since the air’s temperature is increasing by the input of heat energy from the furnace. 6.11 An exothermic reaction releases heat, so the reactants have greater H ( H initial ) than the products ( H final ). Δ H = H final H initial < 0. I n c r e a s i g , H R e a c t a n t s P r o d u c t s Δ H = ( ), (exothermic) 6.12 R e a c t a n t s P r o d u c t s Δ H = (+), (endothermic) 6.13 a) Combustion of hydrocarbons and related compounds require oxygen (and a heat catalyst) to yield carbon dioxide gas, water vapor, and heat. Combustion of ethane: 2 C 2 H 6 ( g ) + 7 O 2 ( g ) 4 CO 2 ( g ) + 6 H 2 O( g ) + heat 2 C 2 H 6 + 7 O 2 (initial) 4 CO 2 + 6 H 2 O (final) Δ H = ( ), (exothermic) 6-2
b) Freezing of water: H 2 O( l ) H 2 O( s ) + heat I n c r e a s i g , H H 2 O( l ) (initial) H 2 O( s ) (final) Δ H = ( ), (exothermic) 6.14 a) Na( s ) + ½ Cl 2 ( g ) NaCl( s ) + heat Na( s ) + 1/2 Cl 2 ( g ) NaCl( s ) Δ Η = (−), ( exothermic ) b) C 6 H 6 ( l ) + heat C 6 H 6 ( g ) Increasing, C 6 H 6 ( g )

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SilberbergChap6ISM - CHAPTER 6 THERMOCHEMISTRY ENERGY FLOW...

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