IX ThermodynamicsA - Thermodynamics Terms/Definitions...

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Thermodynamics
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Terms/ Definitions Thermodynamics Deals with the interconversion of heat an other forms of energy Thermochemistry Deals with heat change in chemical reactions State Function Function that depends only on the conditions (state) not on how the state was obtained
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Energy(E) Internal energy = kinetic + potential energy Kinetic energy comes from molecular motion, electron motion etc. Potential energy comes from attractive and repulsive forces in nuclei, and interactions between molecules Enthalpy(H) H = E + PV Extensive property State function Can only measure the difference in enthalpy between two states
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Heat (Q,q) Transfer of thermal energy between two bodies at different temperatures Work (W,w) Form of energy, can be mechanical or non mechanical Mechanical work is normally pressure - volume work – W = -P ex V Work done by the system on the surrounding = negative Work done by the surroundings on the system = positive
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System Specific thing we are looking at Surroundings Everything outside the system Universe System + surroundings Open system Can exchange both matter and energy with the surroundings Closed system Can exchange energy but not matter with the surroundings Isolated system No exchange of matter or energy with the surroundings Q = ? Q = 0
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Thermochemical Equation Chemical equation that includes the enthalpy change e.g. H 2 O(s) --> H 2 O(l) H = 6.01 kJ Heat flow Endothermic, Q = +, heat absorbed by the system Exothermic, Q = -, heat given off by the system Units of energy Joules(J), kilojoules(kJ) Calories(cal), kilocalories(kcal) 1 cal = 4.184 J (Liter)(atmosphere) 1 L atm = 101.3 J
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Enthalpy State function Heat energy of state Look at change between states H Extensive Property - depends on the amount 1 mole of the reaction as written 1 2 N 2 (g) + 1 2 O 2 (g) NO (g) H = + 90 .4kJ/mol N 2 (g ) + O 2 (g ) 2NO (g ) H = ? = 2 ( + 90 .4kJ /m o l)=180 .8 kJ /m o l
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Changes sign as reverse reaction: Follows Hess’ Law 1 2 N 2 (g) + 1 2 O 2 (g) NO (g) H = + 90 .4kJ/mol NO (g) 1 2 N 2 (g) + 1 2 O 2 (g) H = ? - 90 .4kJ/mo l
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Hess’ Law If a reaction can be considered to go by a series of steps, H of the reaction is the sum of H of the steps.
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