THERMOCHEMISTRY

THERMOCHEMISTRY - 1 CHEM 1000 3.0 Thermochemistry 1...

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Unformatted text preview: 1 CHEM 1000 3.0 Thermochemistry 1 Thermochemistry Petrucci, Harwood and Herring: Chapter 7 CHEM 1000 3.0 Thermochemistry 2 Thermochemistry The study of energy in chemical reactions A sub-discipline of thermodynamics Thermodynamics studies the bulk properties of matter and deduces a few general laws It does not require any knowledge/assumptions of molecules CHEM 1000 3.0 Thermochemistry 3 Definitions System The part of the universe we chose to study Surroundings The rest of the universe (normally we only worry about the immediate surroundings) Process A physical occurrence (usually involving energy flow) 2 CHEM 1000 3.0 Thermochemistry 4 Additional definitions Open system A system where energy and matter can be exchanged with the surroundings Closed system A system where energy but not matter can be exchanged with the surroundings Isolated system A system where neither energy nor matter can be exchanged with the surroundings CHEM 1000 3.0 Thermochemistry 5 CHEM 1000 3.0 Thermochemistry 6 Systems and Energy All systems will contain energy In thermodynamics we are interested in the flow of energy, particularly in the forms of heat and work . Note that heat and work occur when there is a process. They only exist when something happens. The system has energy, (often described as the capacity to do work), it does not have heat or work. 3 CHEM 1000 3.0 Thermochemistry 7 Heat Heat Energy that is transferred between a system and its surroundings as a result of temperature differences Heat transfer can change the temperature of something but it does not always do (only) that Heat transfer can melt or vaporize material CHEM 1000 3.0 Thermochemistry 8 In the case of a material that does not change phase, the increase in temperature of a system T due to the input of a given amount of heat q is given by q = c T heat (J) heat capacity temperature (J K-1 ) change(K) CHEM 1000 3.0 Thermochemistry 9 The heat capacity is a constant that depends on the system. So its not particularly useful. It is better to be able to define heat capacity in terms of a particular compound q = nC T q = mc T Number molar heat mass specific heat of moles capacity (J K-1 mol-1 ) (kg) (J K-1 kg-1 ) 4 CHEM 1000 3.0 Thermochemistry 10 Units The SI unit of heat is a Joule (since it is an energy) The older unit of heat is the calorie which is defined as the heat required to raise the temperature of 1 g of water 1 O C 1cal = 4.184 J CHEM 1000 3.0 Thermochemistry 11 Sign convention and conservation of energy When heat flows between a system and its surroundings, we define: q to be positive if heat is supplied to the system q to be negative if heat is withdrawn from the system If there are no phase changes, conservation of energy requires that q system + q surroundings = 0 or q system = -q surroundings CHEM 1000 3.0 Thermochemistry 12 Work Often when a chemical reaction occurs,...
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THERMOCHEMISTRY - 1 CHEM 1000 3.0 Thermochemistry 1...

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