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Chemistry CH 7 - Chapter 7 Thermochemistry Why nature gas...

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Chapter 7 Thermochemistry Why nature gas is a better fuel than coal and why the energy value of fats is greater than that of carbohydrates and proteins?
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7-1 Getting Started: Some Thermnology 7-2 Heat 7-3 Heats of Reaction and Calorimetry 7-4 Work 7-5 The First Law of Thermodynamics 7-6 Heats of Reaction: Δ U and Δ H 7-7 Indirect Determination of Δ H : Hess’s aw 7-8 Standard Enthalpies of Formation 7-9 Fuels as Sources of Energy Contents
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7-1 Getting Started: Some Therminology Systems and their surroundings The study of thermochemistry requires the definition of system and surroundings . The system is the particular sample of matter under investigation, and the surroundings include everything else outside the system.
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Open system Heat Heat Closed system Heat Matter (water vapor) Heat Isolated system a b c Vacuum flask a b c Systems and their Surroundings
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a. Open system. The beaker of hot coffee transfers energy to the surroundings-it loses heat as it cools. Matter is also transfers in the form of water vapor b. Closed system . The flask of hot coffee transfers energy (heat) To the surroundings as it cools. Because the flask is stoppered, no water vapor escapes and no matter is transferred. c. Isolated system. Hot coffee in an insulated flask approximates an isolated system. No water vapor escapes, and for a time at least, little heat is transferred to the surroundings.
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System No energy in or out No matter in or out Boundary Isolated System: No interaction across the system boundary. (a system that does not interact with its surroundings) System is the part of the universe chosen for study Isolated System
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Energy: The capacity to do work Kinetic energy: The energy of moving object Potential energy: The Energy has the potential to do work Work: done when a force acts through a distance Wok = force × distance = [ m (kg) × a( m s-2)] × d(m) Kinetic energy = ½ × m (kg) × [ u ( m /s)]2 (7.1) SI unit of energy: 1 joule (J ) = 1 kgm 2 s -2 1 cal = 4.184J (exactly) (7.2) 1Cal=1000cal=1kcal
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7-2 Heat Heat( q ), like work, is energy transferred between a system and its surroundings as a result of a temperature difference. We will discuss the quantity of heat following three questions: Heat capacity Specific heat Law of conservation of energy
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The quantity of heat required to change the temperature of a system by one degree is called the heat capacity of the system. If the system is a mole of substance, we can use the term molar heat capacity. If it is one gram of substance, we call it specific heat capacity, or more commonly, Specific heat(sp ht). The specific heat of water depends some what on temperature, but, over the range from 0 to 100 ºC, its value is about: 1.00cal/g ºC =1cal g -1 ºC -1 = 4.18J/ g ºC = 4.18 g -1 ºC –1 (7.3)
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Example for calculating a quantity of heat How much heat is required to raise the temperature of 7.35g of water From 21.0 to 98.0 ºC? (Assume the specific heat of water is 4.18J g -1 ºC -1 throughout this temperature range. ) Solution The specific heat is the heat capacity of 1.00g water: 4.18J/g water ºC.
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