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Unformatted text preview: Chapter 6 Thermochemistry Goals of Chapter • Assess heat transfer associated with changes in temperature and changes of state. • Apply the First Law of Thermodynamics. • Define and understand the state functions enthalpy (H) and internal energy (E). • Calculate the energy changes in chemical reactions and learn how these changes are measured; ( stoichiometry!!! ). Thermochemistry Study of the relationships between energy changes and chemical processes Next: some key definitions……………………. Types of Energy Energy: capacity to do work or to transfer heat • Kinetic Energy energy of motion; KE = ½ mv 2 • Potential Energy stored energy : fuel of motor-cars, trains, jets. It is converted into heat and then to work. position : water at the top of a water wheel. It is converted to mechanical E Joule ‘jewl’ • SI unit of energy • the energy of a 2 kg mass moving at 1 m/s • KE = ½ mv 2 = ½(2 kg)(1 m/s) 2 = 1 kg m 2 /s 2 = 1 J • 1 cal is the amount of energy required to raise the temperature of 1 g water 1°C • 1 cal = 4.184 J 1 cal = 1 calorie • 1000 cal = 1 kcal = 1 Cal 1 Cal = dietary calorie (nutritional calorie) System and Surroundings SYSTEM • The object (chemicals) under study SURROUNDINGS Everything outside the system ( water, beaker, bench… Energy is transferred between system and surroundings The system + surroundings = ? Thermodynamic State • The set of conditions that specify all properties of the system is called the thermodynamic state of a system . • For example… – The number of moles and identity of each substance. – The physical state of each substance. – The temperature of the system. – The pressure of the system. – The volume of the system. The First Law of Thermodynamics-is based on the law of conservation of energy; energy is neither created nor destroyed, it is merely transferred from one object to another, or converted from one form to another • “ The combined amount of energy in the universe is constant .” Internal Energy (E) the total energy of a system: Σ of kinetic and potential E of all particles (atoms, molecules, or ions, electrons) in the system • E is a state function; change in E does not depend on how change of state happens change in E = E; E can be measured • E = E final – E initial (final and initial states) E > 0 (+) indicates system gains energy during process (E increases) E < 0 (−) indicates system loses energy during process (E decreases) E = q + w • based on first law of thermodynamics • q = heat transferred into/out of the system • w = work done on/by the system • w > 0 (+) work is done on system by surroundings (eg. compressing gas); E of system increases • w < 0 (–) work done by system on surroundings (expanding gas); E decreases • q > 0 (+) heat flows into system; E • q < 0 (–) heat flows out of system; E • q and w are not state functions!!...
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