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Chapter 7

Lecture and Textbook Collaborated Notes - Chapter 7 - CHEM 1A03

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Aadil Merali Juma

Chem 1A03 Chapter 7: Thermochemistry 7.1 Getting Started: Some Terminology 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 of flames  Slaw Combustion – takes place at low temperatures o Eg/ Respiration Global Energy Sources The Energy We Get From Fuels  Comparing Heats of Combustion: o The higher the heat of combustion, the better the fuel Capturing and Using Energy from the Sun  Photosynthesis is an endothermic process  Sunlight (energy) is used to build glucose and other simple sugars chlorophyll  6 CO + 6H O  C H O + 6O ΔH = +2.8x10 kJ/mol 2 sunlight 12 6(s) 2(g)  Energy is extracted by combustion of the plant material  As the forward process is Endothermic, ΔH>0  The reverse reaction is exothermic, ΔH<0 Issues with Burning Fossil Fuels  Non-renewable o Plant matter, aged> 300 million years  Contributions to Global Warming o Net CO increase in atmosphere 2 Chem 1A03 System vs. Surroundings  System – part of the universe chosen for study  Surrounding – the part of the universe outside the system with which the system interacts  Open System – material and energy change  Closed System – only energy exchange  Isolated System – neither material nor energy exchange Energy  Energy – the capacity to do work o Potential Energy – stored energy o Kinetic Energy – energy of a moving object o Thermal Energy  Proportional to the temperature of a system  Depends on the number of particles present 7.2 Heat  Heat (q) – energy transferred between a system and its surroundings as a result of a temperature gradient o Thermal energy is transferred until the average molecular kinetic energies of the two bodies become the same, until the temperatures become equal Heat Capacity  Heat Capacity (C) - the amount of heat required to change the temperature of a system by one degree (J°C ) or -1 (JK ) Origin of Heat Capacity  Thermal energy is expressed as molecules internal motions  Molecular-level complexity correlated to heat capacity  Energy available from these internal degrees of freedom contributes to a substances specific heat capacity Defining a Systems Capacity to Store Heat  q = m x specific heat x ΔT = CΔT  Heat Capacity (C) = the quantity of heat (q) required to change the temperature of a “system” by one degree  Specific Heat Capacity = “system” is 1 g of material  Molar Heat Capacity = “system” is 1 mol of material  qsystem surroundings law of conservation of energy – in interactions between a system and its surroundings the total energy remains constant – energy is neither created nor destroyed  q system- qsurroundings 7.3 Heat of Reaction and Calorimetry Heat of Reaction - q rxn  qrxn the quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system at constant temperature o qrxn< 0 exothermic reaction (Heat Produced) o q > 0 endothermic reaction (Heat Required) rxn o qsystem 0 isolated system – thermal energy transferred between components of the system o Eg/ qrxn+ qcalorim0  Calorimeter – a device for measuring quantities of heat; experimentally determines heat of reaction Chem 1A03 Bomb Calorimetry (Constant Volume): q rxnor Combustion of Sucrose  Combustion of 1.010g sucrose C12 O22 112.3 g/mol T = 24.92°C initial Heat Capacity of cal. = 4.90 kJ/°C q = C x ΔT  if calorimeter is assembled same way each time calorim calorim qcalorim 4.90kJ/°C x (28.33-24.92)°C = 16.17 kJ qsys= qrxn+ qcalorim0 qrxn= -qcalorim -16.71 kJ 3 qrxn= -16.71 kJ x 342.3 g = -5.65x10 kJ/mol 1.010g mol Calorimeters at Constant Pressure  Can use an ice-calorimeter to measure the enthalpy change in a redox or acid base reaction  Coffee Cup Calorimeter – like ice-calorimeter  constant pressure device o qsystem 0 Enthalpy Change, ΔH, and Phase Change  Heat is required for phase transitions (which occur at constant T)  latent (hidden) heat of fusion  Molar Enthalpy of Fusion o H2O (s) O 2 (l) ΔH = 6.01 kJ (at 273.15K) Chem 1A03 7.4 Work Differentiating Work and Heat  Familiar with ΔH  endothermic or exothermic, indicating energy required, or given off during a chemical reaction  Energy can also be input or output from a system via “work” Pressure-Volume Work, w  Explosives  Gases formed on combustion of gasoline  How much work is done by expanding gas?  Work = force x distance m = massmg  w = - m x g x Δh x A g = acceleration due to A gravity DistancΔh= = - extV A = area 7.5 The First Law of Thermodynamics  Internal Energy, U – the total energK (E anP E ) in a system o Translational o Rotational o Vibrational o Chemical Bonds o Electrostatic
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