Ch 7 (Thermochemistry, Heat and Work, Enthalpy)

Ch 7 (Thermochemistry, Heat and Work, Enthalpy) - Chapter 7...

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1 Chapter 7: Thermochemistry Energy & Thermal Chemistry Themes: Energy – Fueling our Bodies Energy - Fueling our World 1 Things We Burn Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant . – Rapid Combustion: accompanied by the production of heat or both heat and light in the form of either a glow or flames. 2 » Slow Combustion: takes place at low temperatures. Respiration is an example of slow combustion. Global Energy Sources Oil Coal Gas Fossil Fuels ra-Watts 3 In 2004, the worldwide energy consumption of the human race was on average 15 TW (= 1.5 x 10 13 W) with 86.5% from burning fossil fuels. Nuclear Hydro Ter Years (1965-2005) Refer to Ch. 7-9, pg 257-263 The Energy We Get from Fuels • Comparing Heats of Combustion: Fuel kJ/g Hydrogen -150.0 Natural Gas -49.5 Gasoline -47.8 Fossil Fuels 4 • The higher the heat of combustion, the better the fuel! • What is Heat of Combustion? • Where does the energy stored in these fuels come from? Coal -28.3 Methanol -22.7 Cellulose -17.5 Municipal waste -12.7 Capturing Energy from the Sun • Photosynthesis is an ENDOTHERMIC process • Sunlight (energy) is used to 5 build glucose and other simple sugars 6CO 2 + 6H 2 O C 6 H 12 O 6 (s) + 6O 2 (g) Chlorophyll Sunlight ⎯⎯⎯⎯→ 3 2.8 10 / o H kJ mol Δ= “A leaf is an amazingly cheap and efficient solar cell.” Utilizing the Captured Energy • Plants – as food – fueling respiration 6 • Energy is extracted by combustion of the plant material • Recall: 6CO 2 + 6H 2 O C 6 H 12 O 6 (s) + 6O 2 (g) • As the forward process is ENDOTHERMIC , Δ H > 0 the reverse reaction is EXOTHERMIC , Δ H < 0 •Fossil Fuels –fueling homes & cars Chlorophyll Sunlight 3 2.8 10 / o H kJ mol
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2 Issues with Burning Fossil Fuels • Non-renewable – Plant Matter, aged >300 million years! • Contributions to Global Warming –Ne t CO 2 increase in atmosphere 7 Burning Fossil Fuels – CO 2 Emissions Per Capita 8 Tonnes System versus Surroundings Open System: material & energy exchange Closed System: only Energy exchange 9 Isolated system: Neither material nor energy exchange Refer to Ch. 7-1, pg 226-227 Energy • Energy = the capacity to do work • Potential Energy: 10 • Kinetic Energy: • Thermal Energy? Refer to Ch. 7-1, pg 226-227 Heat • Thermodynamics: The study of heat and its transformations • Thermochemistry: The branch of thermodynamics, as it relates to 11 thermodynamics, as it relates to chemical reactions • Heat (q): energy transferred between a system and its surroundings as a result of a temperature gradient Refer to Ch. 7-2, pg 228-231 Heat Capacity • Heat Capacity (C): the amount of heat required to change the temperature of a system by one degree (J. o C -1 ) or (J. K -1 ) /g 12 Refer to Ch. 7-2, pg 228-231 Specific Heat in J/
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3 Defining a System’s Capacity to Store Heat • Heat Capacity (C) = the quantity of heat (q) required to change the temperature of a q = m x specific heat x Δ T = C x Δ T 13 “system” by one degree • Specific Heat Capacity = “system” is one gram of material
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Ch 7 (Thermochemistry, Heat and Work, Enthalpy) - Chapter 7...

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