4.4 how far - entropy

7/18/2019 4.4 How Far - Entropy

1/43

1 of 43 Boardworks Ltd 2010


2/43



3/43


Enthalpy changes

The enthalpy changefor a process

is the heat energy exchanged withthe surroundings at constant

pressure.

Enthalpy changes are frequently

measured under standard conditions,

!e! 298Kand100kPa. "f an enthalpychange s measured under standard

condtons, the sym#ol s used n

superscrpt, !H!

Enthalpy change s g$en the sym#ol !H!

%he unts are k&mol'1!


4/43


Enthalpy changes


5/43

( of 43 Boardworks Ltd 2010

Exa"ples of enthalpy changes


6/43

) of 43 Boardworks Ltd 2010

#ond enthalpies

*hen a chemcal reacton takes place, #onds are #roken n

the reactants and #onds are formed n the products! Breakng

#ondss an endother"icprocess! +akng #onds s an

exother"icprocess!

%he "ean $ond enthalpys the a$erage mean-

$ond dissociation enthalpyfor a partcular #ond n

a range of dfferent compounds!

.recsely, t s the a$erage enthalpy change for

#reakng 1 mole of a partcular #ond n a range of

dfferent compounds n the gas phase!

%he enthalpy change for a reacton can #e calculated #y

workng out the enthalpy changes for #onds made and #onds#roken durng the reacton usng mean #ond enthalpes!


7/43

/ of 43 Boardworks Ltd 2010

%alculations using $ond enthalpies


8/43

of 43 Boardworks Ltd 2010


9/43 of 43 Boardworks Ltd 2010

&hat is a #orn'(a$er cycle)



%onstructing a #orn'(a$er cycle



#orn'(a$er cycle for *g%l2

*g+s,- %l2+g,

*g+g,- %l2+g,

Ha

+*g, -1/0

*g+g,- 2%l+g,

2

Ha+%l,

2 +-121,

*g-+g,- 2%l+g,- e'

Hi+1st,+*g, -3

Hi+2nd,+*g, -14/0

*g2-+g,- 2%l+g,- 2e'

*g2-+g,- 2%l'

+g,

2

Hea+%l,

2 +'34,

*g%l2+s,

Hl+*g%l2, '249

Hf+*g%l2,



%an you construct a #orn'(a$er cycle)



#orn'(a$er cycle 5uestions



6attice for"ation enthalpies

*hen an onc lattce s formed, the oppostely charged ons

are attracted to each other! %he stronger the attracton, the

hgher the lattce formaton enthalpy!

%wo factors ncrease the attracton and therefore the lattce

formaton enthalpy

hgh charge

small se!

7'2''

6i-7a-K-

increasing lattice for"ation enthalpy


15/431( of 43 Boardworks Ltd 2010

Polari:ation and lattice enthalpy


16/431) of 43 Boardworks Ltd 2010

ill in the "issing words; lattice enthalpy


17/431/ of 43 Boardworks Ltd 2010



Enthalpies of solution and hydration

The standard enthalpy of solution

+Hsol, is the enthalpy changewhen one "ole of an ionic

co"pound is dissol



%alculating enthalpies of solution



Enthalpy of solution calculations



actors affecting enthalpy of hydration

%he se of the enthalpy of hydraton depends on

ncreas

ng

s3e

%he charge on the on! %he larger the charge on the on,

the larger the enthalpy of hydraton!

%he se of the on! %he smaller the on, the larger theenthalpy of hydraton!

L

5a

6

'(1

'40)

'322

7'

8l'

Br'

'(0)

'3)4

'33(

=on Hhyd+k>"ol'1, =on Hhyd+k>"ol

'1,

=on Hhyd+k>"ol'1, =on Hhyd+k>"ol

'1,

7e2 7e3'1(0 '4430



&hat is a spontaneous reaction)



Entropy

Entropy is a "easure of disorder? and is gi


25/432( of 43 Boardworks Ltd 2010

Entropy change for reactions


26/432) of 43 Boardworks Ltd 2010

Predicting entropy changes


27/432/ of 43 Boardworks Ltd 2010

%alculating entropy changes

9tandard entropy changes for any chemcal reacton or

physcal change can #e calculated usng the followng smple

e:presson

;emem#er the followng ponts

entropes of elements are not ero lke Hf$alues, so they

should #e ncluded n calculatons!

the unts of entropy, S, are &6'1mol'1

S Sproducts' S

reactants

% l l i h


28/43


%alculating entropy changes

E t h l l ti


29/43


Entropy change calculations

E t h i th di


30/43


Entropy changes in the surroundings

E t h i th di


31/43


Entropy changes in the surroundings

@i$$ f


32/43


@i$$s free energy

*hether a reacton s spontaneous depends on

%he change n a quantty called the @i$$s free energy

pro$des a measure of whether a reacton s spontaneous!

%he


33/43


(ow to calculate !G

% l l ti !G


34/43


%alculating !G

i$ilit f ti


35/43

3( of 43 Boardworks Ltd 2010

easi$ility of reactions

E$en f Gs post$e at room temperature, there may #e a

hgher temperature at whch a reacton #ecomes feas#le!

"f Ss post$e, there may #e a pont at whch TSs #g

enough to outwegh the enthalpy factor!

G H' TS

post$e

negat$epost$e

negat$e

post$e

post$enegat$e

negat$e

makes TS> H

makes Gmore negat$eno effect Galways post$e

unlkely to make TS > H

yes, a#o$e a

certan temp!

alwaysne$er

usually

H S Bs te"p. increasesC easi$le)

inding the te"perature


36/43

3) of 43 Boardworks Ltd 2010

inding the te"perature

8onsder the reducton of alumnum o:de wth car#on

"f G? 0, then T? H@ S

%he temperature at whch ths reacton #ecomes feas#le

can #e calculated! %hs wll #e when G? 0!

=s #othH

andS

are post$e,G

wll #ecome negat$e fTS > H!

H -13 k>"ol'1

Bl2+s,- %+s, 2Bl+s,- %+g,

S -/81>K'1"ol'1

T 2299K

T? 133) @ (1@1000-

&hen is a reaction feasi$le)


37/43

3/ of 43 Boardworks Ltd 2010

&hen is a reaction feasi$le)

Dolu$ility


38/43


Dolu$ility

"n the same way that reactons are only feas#le f GA 0, a

su#stance wll #e solu#le n water at a specfc temperature

f GsolA 0!

Gsol Hsol' TSsol

post$e

negat$e

post$e

negat$e

post$e

post$e

negat$e

negat$e

Hsol Ssol G feasi$ilityfeas#le f Ts large enough to make

Gnegat$e

always feas#le

ne$er feas#le

usually feas#le Ts unlkely to #e

large enough to make Gpost$e-

Ther"odyna"ics


39/43


Ther"odyna"ics "ol'1

%+s,- 2+g, %2+g,

8onsder the reacton n$ol$ng the com#uston of car#on


40/43


@lossary


41/43


@lossary

&hatFs the keyword)


42/43


&hat s the keyword)

*ultiple choice 5ui:


43/43

*ultipleGchoice 5ui:

4.4 how far - entropy

Documents