CHEM151Chapter_8

Chemistry (MasteringChemistry Series)

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Unformatted text preview: 12/4/2008 1 Chapter 8 Chapter 8 Thermochemistry: Thermochemistry: 1 Thermochemistry: Thermochemistry: Chemical Energy Chemical Energy Energy Changes in Chemical Rxns Energy Changes in Chemical Rxns y Most reactions give off or absorb energy y Energy is the capacity to do work or supply heat. ◦ Heat : transfer of thermal (kinetic) energy Heat : transfer of thermal (kinetic) energy between two systems at different temperatures (from hot to cold) Metal bar in water Metal bar drilled Thermochemistry Thermochemistry y Movie: ammonium chloride and barium hydroxide y Energy in reactions helps to explain why they occur. Substances react more or less NH 4 Cl + Ba(OH) 2 vigorously depending on their inherent stability. 3 Zn + I 2 Formation of H 2 O thermite 12/4/2008 2 Types of Energy Types of Energy y Work (w) : energy transfer when forces are applied to a system y Heat (q) : energy transferred from a hot object to a cold one ◦ Radiant energy Æ heat from the sun Radiant energy Æ heat from the sun ◦ Thermal energy Æ associated with motion of particles ◦ Potential energy Æ energy associated with object’s position or substance’s chemical bonds ◦ Kinetic energy Æ energy associated with object’s motion Internal Energy Internal Energy 5 Heat versus Temperature Heat versus Temperature y Describe the difference between the two. y SI unit of energy: J Molecular heat transfer 2 2 s m kg 1 J 1 ⋅ = y 1 watt = 1 J/s, so a 100 Watt bulb uses 100 J each second y We often use the unit of kJ to refer to chemical heat exchanges in a reaction. 1 kJ = 1000 J y Energy is also reported in calories: ◦ Amount of energy needed to raise 1 gram of water by 1 o C ◦ 1 cal = 4.184 J 12/4/2008 3 Energy and Energy Conservation Energy and Energy Conservation y Heat: form of energy transferred from object at higher temperature to one at lower temperature y Thermochemistry : study of heat changes in chemical reactions, in part to predict whether or not a reaction will occur y Thermodynamics : study of heat and its transformations y First Law of Thermodynamics: Energy can be converted from one form to another but cannot be created or destroyed Conservation of Energy Conservation of Energy System and Surroundings System and Surroundings y System loses heat (negative); gains heat (positive) 12/4/2008 4 Endothermic vs Exothermic Endothermic vs Exothermic y Endothermic reaction: q is positive (q > 0) ◦ Reaction (system) absorbs heat ◦ Surroundings feel cooler y Exothermic reaction: q is negative (q < 0) Exothermic reaction: q is negative (q 0) ◦ Reaction releases heat ◦ Surroundings feel warmer Applications of heat Applications of heat emission/absorption emission/absorption 11 11 Specific Heat and Heat Capacity Specific Heat and Heat Capacity y Specific heat (sp. ht.) : amount of heat required to raise 1 gram of substance by 1 o C y Use mass, specific heat, and Δ T to calculate the amount of heat gained or lost: y q = ms Δ T Æ ms = C Æ q = C Δ T ◦ Heat capacity (C)...
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CHEM151Chapter_8 - 12/4/2008 1 Chapter 8 Chapter 8...

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