Topic: Reaction Spontaneity - PowerPoint PPT Presentation

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Topic: Reaction Spontaneity

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  1. Do Now: Topic: Reaction Spontaneity

  2. Spontaneous Processes =physical or chemical change that occurs with no outside intervention • However, some energy may be supplied to get process started = activation energy Iron rusting 4Fe(s) + 3O2(g)  2Fe2O3(s) H = -1625 kJ CombustionCH4(g) + 2O2(g)  CO2(g)+ 2H2O(l) H = -891kJ

  3. Because both reactions are spontaneous and exothermic, some 19th century scientist thought that all spontaneous reactions are exothermic…until they one day… H2O(s)  H2O(l) endothermic

  4. Some Spontaneous Processes are endothermic, some are exothermic • ΔH plays a role in determining whether a chemical process occurs spontaneously, but there is another piece to the puzzle. ENTROPY

  5. Entropy, S • measure of disorder or randomness of particles in system • Law of Disorder (aka 2nd law of thermodynamics): spontaneous processes always occur in direction that increases chaos of system

  6. Changes in Entropy,S S = Sfinal – Sinitial or Sproducts – Sreactants Spontaneous reactions need +S

  7. Nature is lazy & disorganized • For system to lower energy: must release energy • nature favors exothermic processes with -H • Nature wants to increase S, so Sfinal > Sinitial • Nature wants S to be positive • But: • spontaneous endothermic processes can occur • so something(s) else important, too Low energy (- ΔH) Chaos (+ ΔS)

  8. Affecting S • Changes in state • Sgas>Sliquid>Ssolid • Dissolving • CO2(g) CO2(aq)Sgas>SDissolved Gas • NaCl(S)NaCl(aq) SDissolved Solid>SSolid • SMixture>SPureSubstance • Entropy increases as you increase the number of gas molecules • 2SO3(g) 2SO2(g) + O2(g)S > 0 • Temperature • KE increases, Entropy increases

  9. Predicting Entropy • Which has more entropy: • 1 mole dry ice [CO2(s)] • 1 mole CO2 gas 1 mole of CO2 gas • Which has more entropy: • 2 mole NaCl(s) • 1 mole NaCl(aq) • 1 mole NaCl(aq)

  10. Predicting Spontaneity • Use Gibbs free energy expression G = H - TS • G = Free energy (available to do work) • H = enthalpy (heat) of reaction • T= temperature • S = change in entropy (disorder) • If G is (-) then rxn is spontaneous • If G is (+) then rxn is not spontaneous

  11. G = H - TS Sometimes, more likely at high temp Never spontaneous Sometimes, more likely at low temp Always spontaneous If G is (-)then rxn isspontaneous IfG is (+)then rxn isnot spontaneous

  12. Summary Nature is lazy and disorganized wants -H and +S Because the universe equals the system and the surrounding, spontaneous reactions will occur if -H and +S always

  13. Entropy = 0? pure crystal with no imperfections at 0K • every atom is where it’s supposed to be • nothing is moving