Chern 12 Notes

II.3 Enthalpy and Entropy

Goals:

1. Define enthalpy and entropy.

2. Determine whether enthalpy and entropy is increasing or decreasing in a reaction.

3. Predict if a reaction will be spontaneous (compare enthalpy and entropy)

Enthalpy

We know that enthalpy (H) refers to the energy of a substance.We have talked about All (which is the change in enthalpy of a reaction)

---. -- ---- -----.-.-. ----C--A-,n-e-xo-t-he-r-m-ic--re-ac-ti-on~)

----_. --_.--_._---

Enthalpy l-\' i{ Q. f.A. L:r

Reactants

llH -E- The "EnthalpyChange"

1-----....-;... . .

. Products i. \ "'" \- __ ..L..___ Mf1f'.JC\

Progress of Reaction

In an Exothermic Reaction (AH is negative), the _PX-O o\u..L:\-.S(heat term is on the LEFT, product side)~. Re.ctG\- ---7 ~~ A

In an Endothermic Reaction (&I is positive), the Cf Ct C-\-Grot s ~t\V ~(heat term is on the RIGHT, reactant side)

\\ e,,-v L \ t)V}ty eY\~h4l0f

-t ~t.~t

Pagel

Ef'6\o E~o+ ---?>

Look at the following examples: L1. A + B ~ C + D L1H = -24 kJ is E)(D. Enthapy is 6elv~Ct~:r:92. X + y «=" Z ~H ~ 87 kJ' S is ENOO. Enthapy is ifll( (uS, ~

3. E + D ~ F +.45 kJ L is E~o .Enthapy is u\-eG 0 .v4. G + J + 36 kJ ~ L + M S is 5t-J~D.Enthapy is '\'

What is the tendeny in regards to enthalpy?

Q: Do you think reactions tend towards minimum or maximum enthalpy?

Ex: If a rock is pushed off the top of a mountain, will it@

.,a)b)

stay where it is?fall up? FEfall down '0

Why?

" The rock falls down because of a natural tendency to achieve a position ofLovJ~'(" jf C\,,\+~\ \'()a <A \ PE ! "

In Chemistry, we are interested in chemical potential energy (or ENTHALPY!)

Chemical systems will tend toward a state of 0'\\' f\\iY\\J\v(\ enthalpy if sufficientactivation energy is available and no other (actors are considered. .-IL

Another way of stating this is:

A chemical reaction will favour the <3 \ DE (reactants or products) with\. /\' minimum enthalpy if no other factors are considered.11 \ C~ ~ *The "side" with the heat term is favoured.

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-------------- --- -,-, .::=-===--::..:..:-=-=--====:: ._-- -(An exothermic reaction)

Enthalpy

Reactants

Progress of Reaction are favoured

( An endothermi c reaction)

Enthalpy

Products

•••

1~ ~H is Positive)

t.c===---~- ------------------ -----------'\.m\.t' \-\

Progress of Reaction (.\e.~t.\-, are favoured

E \\ A~ \::.)(1)

-t ~Let's try some more ..../----~

a, (~C15(g~ Ch(g) + PCb(g)--~-_...-/..e(\0\ 0 thermic and the (e_RQ;\-o. rr\ 5 have minimum enthalpy.

bO Ci.~'+ 92.4 kJ E) N2(g) + 3H2(g) rEnd D thermic and the ~. e..llL, c\-O' G\. (\ \-5. have minimum enthalpy.

cO CO(g) + 3H2(g) ~ c24~;:H~~3 kJ ~

-e::.....'60 thermic and the \>'{U du.t..\'S have minimum enthalpy-

d. C12(g) <=" c§;) dH ~ -25 k.J ~

E.'j..0 the~i; and the t> \-0 rll.lC.:-\'S -have minimum enthalpy.

~H= 92.5 kJ

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:;> ------ _____Ex 0 ' le_ If the reaction: CO(g) + 3H2(g) E )I cCH4~L~_~~ + 49.3 kJ ?I~J~hC-"v'L

..1 J r.. I \Q t'i'\; fl r\was proceeding to the right, the enthalpy would be (Ael v .tCl~ \ \'\;J--ing. Is this a

favourable change? ~E SG:r-- -- >

If the reaction:\fCI5~E)I Ch(g) + PCI3(g)f. i1H = 92.5 kJ

was proceeding to the right, the enthalpy would be

favourable change? _N'-"'O""'-- __

The other factor to consider: ENTROPY (RANDOMNESS}

Ex: Melting of water

G;;i>--\£--'- '-!f!=-U.::.c-_c-..c:\--"C'-'-'"-=-(\-'---'\--=S'---_ have minimum enthalpy. The (t Vl15Lreaction isfavoured.

heat p H20 (l)

Ef\G\otz~&Lt+ Yt~\\J.e M; ('\ \-\-

This would mean that all of the water in the universe should exist only as a solid! (Itwould not be favourable for water to exist as a liquid!) We would all be frozen solid!!!!

Obviously there is something wrong with this reasoning! We know that there is liquidwater in the universe, so what gives?

The answer relates to the other factor .Entropy"

Entropy simply means disorder. or lack of order (randomness)

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Phases of matter:

h',3h~O(6.{{ta @"".•....

-"'-'"

~.~.•...•...".....~.,-........."' .•..'"......

gasesSolid -~ liq ~ aqueous ~

---------------------------------------------------------------------------------------»Increasing ENTROPY (randomness)

There is a natural tendency for a reaction to go to the side with \'V\A- 'A \ M It ~ENTROPY. This is the side with the,most particl~ of the MOST RANDOM phase.

Examples:

In the following examples, the entropy is increasing (or the products have greaterentropy):

1. There is a gas (or gases) on the right, when there are no gases on the left of the. ~~~zequation: 'l'0o\ 0, ctS:= M ~O";;L..-/

CaC03(s) + 2 HCl(aq) EZ CaCl2(aq) +\C02(g) + H20(l)

o mo\ 0 \ ~ ClS \ m ..0 •.Q . Pro(~\.LL-\-5J h C\.v ~ r<\CA)( {(\1meJ

2. When there are gases on both sides, the side with the greater number of moles ofgas has the greatest entropy (add up coefficients of gases on left and right.):

4NH3(g) + 502(g) ~

L-\ m.o'3+ 5m·o:j

~\~ fX\o'j]

- \ au"( a\--S\J\. \0$ (}(~'pt-.$ > • • .' ,

Here are few exercises for you: _ S \0\ {. U5 MOS1- rOl(\clom ?a(\-I vl£~\'\tt SmlJ\'~ ~/f\ 1vD? \1

1. For each of the following, decide whether the reactants or the products have greaterentropy:

a) h(s) (> ~ The VW c\ lAC..\- S have greater entropy.

b) 2NH3(g)~lN2(g) + JH2(g)~YY1.o'j 'I-\-m'~The ?vn otu..C\ S

c) ~~ NH3(aq)

The '( e.~(,=O\C\'t-S

have greater entropy.

have greater entropy.

d) -.tCO(g) +J....(::12(g) ~ _LCOCb(g)~'~'O'5 \ ~.Oj· .

The '- ,(' eo [:A (' '\: a 0 \--s have greater entropy.

e) MgC03(s) + 2HCl(aq) E > MgC12(aq) + H20(1) tLC02(g)\. N\. o.j

The ~\(D c,.\u'C\- S ' have greater entropy.

SUMMARY

In nature, there is a tendency toward minimum enthalpy and maximum entropy.

• When products are favoured (enthalpy and entropy), the reaction will~O to to I\\\?\(=t \ D \\

• When both tendencies oppose each other (one favours reactants, one favoursproducts), the reacti~::mwill 1 'L, '-3 D t'O -eC~~A' \\ o: \\A (Y\

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Examples:

a) PCI3(g) + Cb(g) E) PCIS(g); llH = -92.5 kJJ. rxv. oj \'{'(,\)j

The pro aU (\-S has/have minimum enthalpy.

The 'Ce{JI. t\-- ()C\--\-S has/have maximum entropy.

If PCb and Cb are put together, what should happen?(go to completion!reacha stateof"'" ===-equilibrium/notoccurat all)~,- ...--..

b)V

2N02(g) E) N204(g) + energyd.. fl"\':~ \ 'f(\O jThe -f\d""--"--\h)--'-"---J.""-'-"'-=S has/have minimum enthalpy.

The ----\{~'''--e.=Ct.-'----''c'--'-\--'O..L:\'-'-(\--'---''-~'~=('-·---- has/have maximum entropy.

IfN02 was put in a flask, what should happen?(go to completion!reacha stateofequilibrium/notoccurat all)

c) 6H2(g) + 37 kJ E) 4PH3(g)io j\,",Oj l.\ tY' OJ

The {'----'"e--'O"-'!A ....•.("""\:-----'"'lA-'-'(\'-'-\~~=_- has/have minimum enthalpy.

The '.L--,=€.-==-r"""'-.:c,,,-' -'.L" -'a>-LLl('--'\'--'\-=·S=---, has/have maximum entropy.

If P4(s) and 6H2(g}was put in a flask, what should happen?(go to completion!reacha stateofequilibrium!~occur at';J]I:) \

-----...-.. (,0, OLCCvl .

Do Hebden Q's p. 48 #14 - 16Do worksheet 2-1

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