IX Thermodynamics B

# IX Thermodynamics B - ThermodynamicsB Thermodynamics...

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Thermodynamics B

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Thermodynamics Deals with the interconversion of heat an other forms of energy First Law: Energy can be converted from one form to another, it can not be created or destroyed. i.e. The energy of the universe is constant. E = q + w
Want to be able to predict if process or reaction will occur spontaneously (without outside interference) In many cases, spontaneous reactions are exothermic, but not always. CaCl 2 (s) Ca +2 (aq) + 2 Cl - (aq) H 2 O H = - 82.8 kJ NH 4 NO 3 (s) NH 4 + (aq) + NO 3 - (aq) H 2 O H = + 25.0 kJ So: can’t use H to determine spontaneity. New term: Entropy: (S) Entropy is a state function Entropy is a measure of the randomness or disorder of a system. The greater the disorder, the greater the entropy e.g. S(s) < S(l) << S(g)

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Different from enthalpy, can have standard absolute Entropy (S o ) So: S o = S o f - S o i S o always > 0 Second Law of Thermodynamics: Entropy of the universe always increases for an irreversible process or remains constant for a reversible process. S un iverse 0 S universe = S system + S surroundings S surroundings comes primarily from change in heat energy (q at constant pressure) but also depends on the Temperature of the surroundings. S surr = Q surr T = -∆ H sys T
Third Law of Thermodynamics Entropy of a pure, perfectly crystalline substance is zero at absolute zero of temperature. Why? At 0 K motion ceases, therefore, there is only one possible structure and no randomness (perfect order) S=k ln = number of arrangements k = boltzmann constant So, third law gives us Absolute Entropy

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Determine if the entropy of the system: A) increases B) decreases C) stays the same 1. Benzene(s) --> benzene (l) A 2. NaNO 3 (s) --> NaNO 3 (aq) A 3. S(s) + O 2 (g) --> SO 2 (g) C 4. MgCO 3 (s) --> MgO(s) + CO 2 (g) A 5. PCl 3 (l) + Cl 2 (g) --> PCl 5 (s) B 6. 2 HgO(s) --> 2 Hg(l) + O 2 (g) A
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