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Chemical Thermodynamics

Chemical Thermodynamics. Entropy. Mr. Shields Regents Chemistry U13 L08. Entropy. One thing you need to understand is that the Universe tends toward disorder. For example … If I drop a box of crayons organized by color in a box

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Chemical Thermodynamics

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  1. Chemical Thermodynamics Entropy Mr. Shields Regents Chemistry U13 L08

  2. Entropy One thing you need to understand is that the Universe tends toward disorder. For example … If I drop a box of crayons organized by color in a box They don’t stay organized as they fall and hit the floor. New born babies grow old as time passes as cellular defects Continue to pile up. Old people do not grow young. And chemical reactions don’t suddenly Undo what has already taken place

  3. Entropy • Here’s some more examples… • - If I drop a glass on the floor • it shatters! • - And what if I gathered up all those pieces • and re-dropped them on the floor again? Do • they recombine to form the original glass? • Of course not! These are examples of the universe tending • Towards disorder Your room ! neat one day and messy the next!

  4. This will happen But this won’t Entropy Entropy (S) is the second thermodynamic term we’ll discuss. This term is a measure of the DISORDERof a system Greater disorder Less order

  5. Entropy of states When matter changes states Entropy either increases Or Decreases Consider for example the following changes of states: ICE  Water  STEAM Has Entropy increased or decreases? Sure, entropy has increased (i.e ∆S increases, and when Entropy increases the change in entropy is positive, +∆S).

  6. Entropy of States So we now know that when we change phase Entropy increases in this order: Solids < Liquids < Gas What about the entropy of a dissolved solid ??? What has Greater entropy, the solid or the dissolved solid? The solid exists as a crystalline substance. The solvated ions have less order and thus have more entropy ∆S

  7. Entropy Values The larger the entropy value (i.e. the more positive) the greater the disorder Just as the change in Enthalpy is DH = Hproduct – H reactant The change in Entropy is DS = Sproduct – Sreactant We can however measure DH directly. For instance, the heat Lost or gained can be measured in a CALORIMETER But … DS can not be measured directly.

  8. Entropy Values We will not focus on how DS can be measured Instead we will simply focus on attributes that will enable Us to determine whether DS is + or – If DS is + - Entropy has increased - Products have more disorder - And Sproducts > Sreactants For example: Propane (l)  Propane (g) The entropy change would be positive (+DS) because liquid Propane is more ordered than Propane gas.

  9. Entropy Values If DS is (–) - Entropy has decreased - Products have less disorder - And Sproducts < Sreactants For example: Water(l)  water(s) Entropy is decreasing for this phase change since a solid is more ordered than a liquid (∆S is negative) Another example could be … I2(g)  I2 (s) why?

  10. Increasing Entropy • Summary of Positive changes in ENTROPY (+DS): • - Solid  liquid • Liquid  Gas • Solid  Solution • Liquid  Solution • Entropy also increases • if the Temp. of the • system increases • Example: Hg (l, 20 deg C)  Hg (l, 40 deg C) Higher ∆S … Why?

  11. Positive Entropy • Entropy also increasesif there aremore moleculesof • product than reactant (+∆S) • examples: 2NH3(g)  N2(g) + 3H2(g) • HgO  Hg + O2 (+∆S) Problem: What’s the sign of DS in the following reaction NH3(g) + HCl(g)  NH4CL (s) Negative (-DS) since order is increasing because we are Going from more particles to fewer particles

  12. Spontaneity Since the Universe favors disorder… In any SPONTANEOUS process, the overall ENTROPY of the Universe will always increases (+DS) Therefore, if the DS of the rxn is + Then the process will tend to be spontaneous

  13. Spontaneity • We’ve discussed in this unit two thermodynamic terms • Enthalpy and Entropy; Keep in mind… • - A change in Enthalpy (DHrxn) can be (+) or (-) • - If it is (-) the reaction is exothermic • - Exothermic reactions tend to be spontaneous • A change in Entropy (DSrxn) can be (+) or (-) • - If DSrxn is (+) the rxn tends towards greater disorder • - Increasing disorder favors rxn spontaneity

  14. Know case 1 & 2 Case DH Rxn DS Rxn Spontaneous Rxn? 1 (-) favorable (+) favorable YES 2 (+) unfavorable (-) unfavorable NO 3 (-) favorable (-) unfavorable ? 4 (+) unfavorable (+) favorable ? Predicting Spontaneity So how can we predict if a reaction is spontaneous or not? Case 1: Spontaneous reaction Case 2: Non-spontaneous reaction Case 3: ? Depends upon the value of DS (is it smaller than DH) Case 4: ? Depends upon the value of DH (is it smaller than DS)

  15. PROBLEM: What is the sign of ∆H and ∆S for each reaction and Predict if the rxn is Spontaneous, not spontaneous or not predictable: a) C6H12O6(s) + 6O2(g)  6CO2(g)+ 6H2O(g) + Heat b) 2CO(g) + O2(g)  CO2(g) + Heat c) 2C(s) + 2H2(g) + heat  C2H4(g) • -∆H, +∆S, Spontaneous • -∆H, -∆S, Not Predictable • +∆H, -∆S, Not Spontaneous

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