2_Thermochemistry(S)_Sept.29

2_Thermochemistry(S)_Sept.29 - CHEM 1000 3.0...

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Unformatted text preview: CHEM 1000 3.0 Thermochemistry 1 Thermochemistry • Petrucci, Herring Madura and Bissonnette : Chapter 7 CHEM 1000 3.0 Thermochemistry 2 Problem set • Chapter 7 • Questions: 6, 19, 41, 47, 57, 58, 74, 91 CHEM 1000 3.0 Thermochemistry 3 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 4 Definitions • S y s t e m – The part of the universe we chose to study • Surroundings – The rest of the universe (normally we only worry about the immediate surroundings) • P r o c e s s – A physical occurrence (usually involving energy flow) CHEM 1000 3.0 Thermochemistry 5 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 6 CHEM 1000 3.0 Thermochemistry 7 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. CHEM 1000 3.0 Thermochemistry 8 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 9 CHEM 1000 3.0 Thermochemistry 10 • In the case of a material that does not change phase, the increase in temperature of a system Δ T due to the input of a given amount of heat q is given by q = c Δ T heat (J) heat capacity temperature (J K-1 ) change (K) CHEM 1000 3.0 Thermochemistry 11 • The heat capacity is a constant that depends on the system. So it’s not particularly useful. • It is better to be able to define heat capacity in terms of a particular compound q = nC Δ T q = mc Δ T Number molar heat mass specific heat of moles capacity (J K-1 mol-1 ) (kg) (J K-1 kg-1 ) CHEM 1000 3.0 Thermochemistry 12 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 13 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 3.0CHEM 1000 3....
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2_Thermochemistry(S)_Sept.29 - CHEM 1000 3.0...

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