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Lecture 11


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McMaster University
David Brock

CHEM 1A03 LECTURE 11 – THERMOCHEMISTRY October 31 2012 Quiz 2 Content: Test 1, Chapter 15, 5 and 20. Test 2 Content: Fund. Skills, chapter 8,9,10 (6 marks) Chapter 15(6) 5(6) 20(6), 7(4)[up to slide 25) and Lab 2+3 Combustion Complex sequence of exothermic chemical reactions between a fuel and an oxidant (typically oxygen) • Rapid combustion: accompanied by the production of heat or both heat and light in the form of either a glow or flames. • Slow Combustion: takes place at low temperatures. Respiration is an example of slow combustion The higher the heat of combustion, the better the fuel. Heat of Combustion: Energy release by combusting materials. Solar Photosynthesis is an endothermic process. Sunlight is used to build glucose and other simple sugars. ∆H = +2.8x10 kJ/mol Energy is extracted by combustion of the plant material As the forward process is endothermic, the reverse reaction (respiration) is exothermic. Issues with Burning Fossil Fuels Non-renewable (took >300 million years to develop) Contributes to Global Warming – Net CO in2rease in atmosphere System versus Surroundings Open system: material and energy flow Closed system: only energy exchange Isolated system: neither material nor energy exchange Energy Energy: the capacity to do work Potential Energy, Kinetic Energy, Thermal Energy (KE of molecules, atoms) Heat (q): energy transferred between a system and its surroundings as a result of a temperature gradient Heat Capacity (C): The amount of heat required to change the temperature of a system by 1 o degree (J/ C) Thermal energy is expressed as a molecule’s internal motions. Molecular-level complexity is correlated to heat capacity. Energy available from these internal degrees of freedom (rotational, translational, vibrational) contributes to a substance’s specific heat capacity. Capacity to Store Heat q = mc∆T = C∆T Heat Capacity (C): The quantity of heat (q) required to change the temperature of a system by one degree. Mass times specific heat capacity compounded. Specific heat capacity: system is 1 g of material Molar heat capacity: system is 1mol of material. o Sample Problem: A 100.0 g copper sample (specific heat = 0.385J.g-1.0C-1) at 100.0 C is added to 50.0 g water, at 26.5oC. What is the final temperature? Heat goes out of one system and into another. qwater -q copper mc(T -T) = -mc(T -T) NOTE :=> T will be equal for both copper F I water F I copper F and water. Rearrange for T F. Heat of Reaction: q rxn The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system at constant temperature. qrxn< 0 exothermic reaction (heat produced) qrxn0 endothermic reaction (heat required) qsystem 0 isolated system: thermal energy transferred between components of the system. Endothermicity does not necessitate nonspontaneity  exothermic reactions are not indicative of spontaneity. Bomb calorimetry (Constant Volume) qcalC cal ∆T qcal= amount of heat transferred to surroundings by system. This is equal to amount of heat given off by reaction. qsystem 0 in bomb calorimetry; in this case, the surroundings is everything OUTSIDE of the calorimeter. qcal= -qrxn Coffee Cup Calorimetry – Constant Pressure Rather than gauging temperature change with a thermometer, we will use the phase change of o ice instead. Ice, in llbm with water is at 0 C as long as ice is still present. Therefore, temperature does not begin to change until ice is completely gone. Enthalpy Change, ∆H and Phase Change When water boils or ice melts, what is the temperatur
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