1 Entropy Mr. ShieldsRegents 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 -H-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 Entropy neat one day and messy the next! Your room !

4 Entropy Entropy (S) is the second thermodynamic term we’ll discuss. This term is a measure of the DISORDER of a system This will happen But this won’t 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 D H = H product – H reactant The change in Entropy is D S = S product – S reactant We can however measure D H directly. For instance, the heat Lost or gained can be measured in a CALORIMETER But … D S can not be measured directly.

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

9 If D S is (–) - Entropy has decreased - Products have less disorder - And S products < S reactants 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 … I 2 (g)  I 2 (s) why?

10 Increasing Entropy Summary of Positive changes in ENTROPY (+ D S): - Solid  liquid Liquid  Gas Solid  Solution Liquid  Solution - E- 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 increases if there are more molecules of product than reactant (+∆S) examples: 2NH 3 (g)  N 2 (g) + 3H 2 (g) HgO  Hg + O 2 Problem: What’s the sign of D S in the following reaction NH 3 (g) + HCl(g)  NH 4 CL (s) Negative (- D S) since order is increasing because we are Going from more particles to fewer particles (+∆S)

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

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

14 Predicting Spontaneity So how can we predict if a reaction is spontaneous or not? ?(+)favorable(+)unfavorable4 ?(-)unfavorable(-)favorable3 NO(-)unfavorable(+)unfavorable2 YES(+)favorable(-)favorable1 Spontaneous Rxn? D S Rxn D H RxnCase Know case 1 & 2 Case 1: Spontaneous reaction Case 2: Non-spontaneous reaction Case 3: ? Depends upon the value of D S (is it smaller than D H ) Case 4: ? Depends upon the value of D H (is it smaller than D S)

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) C 6 H 12 O 6 (s) + 6O 2 (g)  6CO 2 (g)+ 6H 2 O(g) + Heat b) 2CO(g) + O 2 (g)  CO 2 (g) + Heat c) 2C(s) + 2H 2 (g) + heat  C 2 H 4 (g) a)-∆H, +∆S, Spontaneous b)-∆H, -∆S, Not Predictable c)+∆H, -∆S, Not Spontaneous