ib enthalpy kh (1)

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NTHALPY

NTHALPY

CHANG SHANG S

A guide for IB level studentsA guide for IB level students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING



ENTHALPY CHANGESENTHALPY CHANGES

CONTENTS• Thermodynamis

• Enthal!y hanges

• Standard enthal!y values

• Standard enthal!y of formation

• Standard enthal!y of om"ustion

• Enthal!y of neutralisation

• Bond dissoiation enthal!y

• Hess#s la$

• Calulating enthal!y hanges using "ond enthal!ies

• Calulating enthal!y hanges using enthal!y of formation values

• Calulating enthal!y hanges using enthal!y of om"ustion values

• Pratial measurement

• Che% list



Before you start it $ould "e hel!ful to&

• "e a"le to "alane e'uations

• "e onfident $ith sim!le arithmetial o!erations

ENTHALPY CHANGESENTHALPY CHANGES



THE()*+YNA)ICSTHE()*+YNA)ICS

,irst La$ Energy an "e neither reated nor destroyed "ut It an "e onverted from one form to another

Energyhanges all hemial reations are aom!anied "y some form of energy hange

hanges an "e very o"vious -e.g. oal "urning/ "ut an go unnotied

E0othermi Energy is given out

Endothermi Energy is a"sor"ed

E0am!les E0othermi om"ustion of fuels

res!iration -o0idation of ar"ohydrates/

Endothermi !hotosynthesis

thermal deom!osition of alium ar"onate



THE()*+YNA)ICS 1THE()*+YNA)ICS 1 ENTHALPY CHANGESENTHALPY CHANGES

Enthal!y a measure of the heat ontent of a su"stane at onstant !ressure

you annot measure the atual enthal!y of a su"stane

you an measure an enthal!y CHANGE $ritten as the sym"ol ∆Η 2 3delta H 4

Enthal!y hange -∆Η/ 5 Enthal!y of !roduts 1 Enthal!y of reatants







Enthal!y of reatants 6 !roduts

∆Η 5 1 ve

E7*THE()IC Heat given out

(EACTI*N C*1*(+INATE

E N T H A L P Y








Enthal!y of reatants 6 !roduts Enthal!y of reatants 8 !roduts

∆Η 5 1 ve ∆Η 5 9 ve

E7*THE()IC Heat given out EN+*THE()IC Heat a"sor"ed


E N T H A L P Y


E N T H A L P Y




:hy astandard; enthal!y values vary aording to the onditions

a su"stane under these onditions is said to "e in its standard state ...

Pressure<1 =>> %Pa -= atmos!here/

A stated tem!erature usually ?@ -?DC/

as a guide2 Fust thin% of ho$ a su"stane $ould "e under normal la" onditions assign the orret su"sri!t -g/2 -l/ or -s/ to indiate $hih state it is in

any solutions are of onentration = mol dm1

to tell if standard onditions are used $e modify the sym"ol for ∆Η .

Enthal!y Change Standard Enthal!y Change-at ?@/

STAN+A(+ ENTHALPY CHANGESSTAN+A(+ ENTHALPY CHANGES



STAN+A(+ ENTHALPY *, ,*()ATI*NSTAN+A(+ ENTHALPY *, ,*()ATI*N

+efinition The enthal!y hange $hen *NE )*LE of a om!ound is formed in its standard state from its elements in their standard states.

Sym"ol ∆ΗDf

Jalues Ksually2 "ut not e0lusively2 e0othermi

E0am!le-s/ C-gra!hite/ 9 *?-g/ 6 C*?-g/

H?-g/ 9 M*?-g/ 6 H?*-l/

?C-gra!hite/ 9 M*?-g/ 9 H?-g/ 6 C?H*H-l/

Notes *nly *NE )*LE of !rodut on the (HS of the e'uation

Elements In their standard states have ero enthal!y of formation.

Car"on is usually ta%en as the gra!hite allotro!e.



+efinition The enthal!y hange $hen *NE )*LE of a su"stane undergoes om!lete om"ustion under standard onditions. All reatants and !roduts are in

their standard states.

Sym"ol ∆ΗD

Jalues Al$ays e0othermi

E0am!le-s/ C-gra!hite/ 9 *?-g/ 6 C*?-g/

H?-g/ 9 M*?-g/ 6 H?*-l/

C?H*H-l/ 9 *?-g/ 6 ?C*?-g/ 9 H?*-l/

Notes Al$ays only *NE )*LE of $hat you are "urning on the LHS of the e'uation

To aid "alaning the e'uation2 remem"er...you get one ar"on dio0ide moleule for every ar"on atom in the originaland one $ater moleule for every t$o hydrogen atoms

:hen you have done this2 go "a% and "alane the o0ygen.

STAN+A(+ ENTHALPY *, C*)BKSTI*NSTAN+A(+ ENTHALPY *, C*)BKSTI*N



+efinition The enthal!y hange $hen *NE )*LE of $ater is formed from its ions in dilute solution.

Jalues E0othermi

E'uation H9-a'/ 9 *HO -a'/ 6 H?*-l/

Notes A value of 1%Q mol1= is o"tained $hen strong aids reat $ith strong al%alis.

See later slides for !ratial details of measurement

ENTHALPY *, NEKT(ALISATI*NENTHALPY *, NEKT(ALISATI*N



+efinition Energy re'uired to "rea% *NE )*LE of gaseous "onds to form gaseous atoms.

Jalues Endothermi 1 Energy must "e !ut in to "rea% any hemial "ond

E0am!les Cl?-g/ 6 ?Cl-g/ *1H-g/ 6 *-g/ 9 H-g/

Notes strength of "onds also de!ends on environmentR )EAN values 'uoted

ma%ing "onds is e0othermi as it is the o!!osite of "rea%ing a "ond for diatomi gases2 "ond enthal!y 5 ? 0 enthal!y of atomisation

smaller "ond enthal!y 5 $ea%er "ond 5 easier to "rea%

B*N+ +ISS*CIATI*N ENTHALPYB*N+ +ISS*CIATI*N ENTHALPY



+efinition Energy re'uired to "rea% *NE )*LE of gaseous "onds to form gaseous atoms.

Jalues Endothermi 1 Energy must "e !ut in to "rea% any hemial "ond

E0am!les Cl?-g/ 6 ?Cl-g/ *1H-g/ 6 *-g/ 9 H-g/

Notes strength of "onds also de!ends on environmentR )EAN values 'uoted

ma%ing "onds is e0othermi as it is the o!!osite of "rea%ing a "ond for diatomi gases2 "ond enthal!y 5 ? 0 enthal!y of atomisation

smaller "ond enthal!y 5 $ea%er "ond 5 easier to "rea%

)ean Jalues H1H H1, ? N1N =

C1C H1Cl = N5N >@

C5C == H1Br N≡N @

C≡C H1I ?@@ P1P =?

C1* > H1N ,1, = C5* H1* Cl1Cl ??

C1H = H1S Br1Br =@

C1N > H1Si = I1I ==

C1, P1H ?? S1S ?

C1Cl *1* = Si1Si =

B*N+ +ISS*CIATI*N ENTHALPYB*N+ +ISS*CIATI*N ENTHALPY

Average -mean/values are 'uoted"eause the atual

value de!ends onthe environment ofthe "ond i.e. $hereit is in the moleule

KNITS 5 %Q mol1=



3The enthal!y hange is inde!endent of the !ath ta%en4

Ho$ The enthal!y hange going fromA to B an "e found "y adding thevalues of the enthal!y hanges for

the reations A to 72 7 to Y and Y to B.

∆Η r 5 ∆Η

1

9 ∆Η? 9 ∆Η

If you go in the o!!osite diretion of an arro$2 you su"trat the value of

the enthal!y hange.

e.g.∆Η? 5 1 ∆Η

1

9 ∆Η r 1 ∆Η

The values of ∆Η1

and

∆Η have "een su"trated "eause the route

involves going in the o!!osite diretion to their definition.

HESS#S LA:HESS#S LA:



3The enthal!y hange is inde!endent of the !ath ta%en4

Kse a!!lying Hess#s La$ ena"les one to alulate enthal!y hanges fromother data2 suh as...

hanges $hih annot "e measured diretly e.g. Lattie Enthal!y

enthal!y hange of reation from "ond enthal!y

enthal!y hange of reation from ∆ΗD

enthal!y hange of formation from ∆ΗDf

HESS#S LA:HESS#S LA:



Theory Imagine that2 during a reation2 all the "onds of reating s!eies are "ro%enand the individual atoms Foin u! again "ut in the form of !roduts. Theoverall energy hange $ill de!end on the differene "et$een the energy

re'uired to "rea% the "onds and that released as "onds are made.

energy released ma%ing "onds 6 energy used to "rea% "onds ... E7*THE()IC

energy used to "rea% "onds 6 energy released ma%ing "onds ... EN+*THE()IC

Enthal!y of reation from "ond enthal!iesEnthal!y of reation from "ond enthal!ies

Ste! = Energy is !ut in to "rea% "onds to form se!arate2 gaseous atoms

Ste! ? The gaseous atoms then om"ine to form "onds and energy is releasedits value $ill "e e'ual and o!!osite to that of "rea%ing the "onds

A!!lying Hess#s La$ ∆Η r 5 Ste! = 9 Ste! ?




∆Η 5 Σ "ond enthal!ies U Σ "ond enthal!ies of reatants of !roduts

∆Η 5 Σ "ond enthal!ies U Σ "ond enthal!ies of reatants of !roduts

Alternative vie$

Ste! = Energy is !ut in to "rea% "onds

to form se!arate2 gaseous atoms.

Ste! ? Gaseous atoms then om"ineto form "onds and energy isreleasedR its value $ill "e e'ualand o!!osite to that of "rea%ingthe "onds

∆Η r 5 Ste! = 1 Ste! ?

Beause2 in Ste! ? the route involvesgoing in the *PP*SITE +I(ECTI*Nto the defined hange of "ond enthal!y2

it#s value is su"trated.

SK) *,THE B*N+ENTHALPIES *,THE (EACTANTS

(EACTANTS

P(*+KCTS

AT*)S

∆Η

SK) *,THEB*N+ENTHALPIES *,THE P(*+KCTS



Calulate the enthal!y hange for the hydrogenation of ethene






∆Η? = 0 C5C "ond V == 5 == %Q

0 C1H "onds V = 5 =? %Q= 0 H1H "ond V 5 %Q

Total energy to "rea% "onds of reatants 5 ?@@ %Q





∆Η? = 0 C5C "ond V == 5 == %Q

0 C1H "onds V = 5 =? %Q= 0 H1H "ond V 5 %Q

Total energy to "rea% "onds of reatants 5 ?@@ %Q

∆Η = 0 C1C "ond V 5 %Q

0 C1H "onds V = 5 ? %Q

Total energy to "rea% "onds of !roduts 5 ?? %Q

A!!lying Hess#s La$ ∆Η= 5 ∆Η? U ∆Η 5 -?@@ U ??/ 5 U =? %Q



If you formed the !roduts from their elements you should need the same amountsof every su"stane as if you formed the reatants from their elements.

Enthal!y of formation tends to "e an e0othermi !roess

Enthal!y of reation from enthal!ies of formationEnthal!y of reation from enthal!ies of formation



Ste! = Energy is released as reatantsare formed from their elements.

Ste! ? Energy is released as !rodutsare formed from their elements.

∆Η r 5 1 Ste! = 9 Ste! ?

or Ste! ? 1 Ste! =

In Ste! = the route involves going in the*PP*SITE +I(ECTI*N to the definedenthal!y hange2 it#s value is su"trated.

SK) *,THEENTHALPIES *,,*()ATI*N *,

THE (EACTANTS

(EACTANTS

P(*+KCTS

ELE)ENTS

∆Η

SK) *,THEENTHALPIES*,,*()ATI*N*, THEP(*+KCTS


∆Η 5 Σ ∆Η f of !roduts U Σ ∆Η f of reatants∆Η 5 Σ ∆Η f of !roduts U Σ ∆Η f of reatants



Sam!le alulation

Calulate the standard enthal!y hange for the follo$ing reation2 given that thestandard enthal!ies of formation of $ater2 nitrogen dio0ide and nitri aid are 1?29 and 1= %Q mol1= res!etivelyR the value for o0ygen is WE(* as it is an element

?H?*-l/ 9 N*?-g/ 9 *?-g/ 6 HN*-l/

By a!!lying Hess#s La$ ... The Standard Enthal!y of (eation ∆ΗDr $ill "e...

P(*+KCTS (EACTANTS

0 ∆Η f of HN* minus -? 0 ∆Η f of H?*/ 9 - 0 ∆Η f of N*?/ 9 -= 0 ∆Η f of *?/

∆ΗDr 5 0 -1=/ 1 ? 0 -1?/ 9 0 -9/ 9 >

ANS:E( 5 1 ?? %Q


∆Η 5Σ

∆Η

f of !roduts UΣ

∆Η f of reatants∆Η 5

Σ

∆Η

f of !roduts UΣ

∆Η f of reatants



∆Η 5 Σ ∆Η of reatants U Σ ∆Η of !roduts∆Η 5 Σ ∆Η of reatants U Σ ∆Η of !roduts

Enthal!y of reation from enthal!ies of om"ustionEnthal!y of reation from enthal!ies of om"ustion

Ste! = Energy is released as reatants

undergo om"ustion.

Ste! ? Energy is released as !rodutsundergo om"ustion.

∆Η r 5 Ste! = 1 Ste! ?

Beause2 in Ste! ? the route involves goingin the *PP*SITE +I(ECTI*N to thedefined hange of Enthal!y of Com"ustion2it#s value is su"trated.

SK) *,THEENTHALPIES *,C*)BKSTI*N*, THE(EACTANTS

(EACTANTS

P(*+KCTS

*7I+ATI*NP(*+KCTS

∆Η

SK) *,THEENTHALPIES *,

C*)BKSTI*N *,THE P(*+KCTS



Enthal!y of reation from enthal!ies of om"ustionEnthal!y of reation from enthal!ies of om"ustion

Sam!le alulation

Calulate the standard enthal!y of formation of methaneR the standard enthal!ies ofom"ustion of ar"on2 hydrogen and methane are 1@2 1? and 1@> %Q mol1= .

C-gra!hite/ 9 ?H?-g/ 6 CH-g/

By a!!lying Hess#s La$ ... The Standard Enthal!y of (eation ∆ΗDr $ill "e...

(EACTANTS P(*+KCTS

-= 0∆Η

of C/ 9 -? 0∆Η

of H?/ minus = 0∆Η

of CH

∆ΗDr 5 = 0 -1@/ 9 ? 0 -1?/ 1 = 0 -1@>/

ANS:E( 5 1 %Q mol1=

∆Η 5 Σ ∆Η of reatants U Σ ∆Η of !roduts∆Η 5 Σ ∆Η of reatants U Σ ∆Η of !roduts



Calorimetry involves the !ratial determination of enthal!y hangesusually involves heating -or ooling/ %no$n amounts of $ater

$ater is heated u! reation is E7*THE()IC

$ater ools do$n reation is EN+*THE()IC

Calulation The energy re'uired to hange the tem!eratureof a su"stane an "e alulated using... ' 5 m 0 0 ∆Τ

$here ' 5 heat energy %Q

m 5 mass %g 5 S!eifi Heat Ca!aity %Q 1= %g 1= $ater is .=

∆Τ 5 hange in tem!erature

)EASK(ING ENTHALPY CHANGES)EASK(ING ENTHALPY CHANGES



Calorimetry involves the !ratial determination of enthal!y hangesusually involves heating -or ooling/ %no$n amounts of $ater

$ater is heated u! reation is E7*THE()IC

$ater ools do$n reation is EN+*THE()IC

Calulation The energy re'uired to hange the tem!eratureof a su"stane an "e alulated using... ' 5 m 0 0 ∆Τ

$here ' 5 heat energy %Q

m 5 mass %g 5 S!eifi Heat Ca!aity %Q 1= %g 1= $ater is .=

∆Τ 5 hange in tem!erature

E0am!le *n om!lete om"ustion2 >.=g of he0ane raised the tem!erature of =>>g $ater from ??DC to DC. Calulate its enthal!y of om"ustion.

Heat a"sor"ed "y the $ater -'/ 5 >.= 0 .= 0 ? 5 =>. %Q

)oles of he0ane "urned 5 mass X )r 5 >.= X

5 >.>>?>@

Enthal!y hange 5 heat energy X moles 5 =>. X >.>>?>@

ANS 5 >>> %Q mol 1=




E0am!le = 1 gra!hial

The tem!erature is ta%en every half minute "efore mi0ing the reatants.

(eatants are mi0ed after three minutes.

,urther readings are ta%en every half minute as the reation mi0ture ools.

E0tra!olate the lines as sho$n andalulate the value of ∆Τ.




E0am!le = 1 gra!hial

The tem!erature is ta%en every half minute "efore mi0ing the reatants.

(eatants are mi0ed after three minutes.

,urther readings are ta%en every half minute as the reation mi0ture ools.

E0tra!olate the lines as sho$n andalulate the value of ∆Τ.

E0am!le alulation

:hen >.=g of he0ane under$ent om!lete om"ustion2 it raised the tem!erature of =>>g ->.=%g/ $ater from ??DC to DC. Calulate its enthal!y of om"ustion.

Heat a"sor"ed "y the $ater -'/ 5 m C ∆Τ 5 >.= 0 .= 0 ? 5 =>. %Q

)oles of he0ane "urned 5 mass X )r 5 >.= X 5 >.>>?>@

Enthal!y hange 5 heat energy X moles 5 =>. X >.>>?>@ 5 >>> %Q mol 1=




E0am!le ?

?m of ?.>) HCl $as added to ?m of ?.>) Na*H in an insulated "ea%er. The initialtem!erature of "oth solutions $as ?>DC. The highest tem!erature reahed "y the

solution $as DC. Calulate the )olar Enthal!y of Neutralisation. The s!eifi heata!aity -/ of $ater is .= %Q 1= %g 1=

Na*H 9 HCl 6 NaCl 9 H?*




E0am!le ?




Tem!erature rise -∆Τ/ 5 > U ?@ 5 =

Jolume of resulting solution5 ? 9 ? 5 >m 5 >.> dm

E'uivalent mass of $ater 5 >g 5 >.> %g-density is =g !er m/

Heat a"sor"ed "y the $ater -'/ 5 m 0 0 ∆Τ = >.> 0 .= 0 =5 ?.= %Q




E0am!le ?




Tem!erature rise -∆Τ/ 5 > U ?@ 5 =

Jolume of resulting solution5 ? 9 ? 5 >m 5 >.> dm

E'uivalent mass of $ater 5 >g 5 >.> %g-density is =g !er m/

Heat a"sor"ed "y the $ater -'/ 5 m 0 0 ∆Τ = >.> 0 .= 0 =5 ?.= %Q

)oles of HCl reating 5 ? 0 ?X=>>> 5 >.> mol

)oles of Na*H reating 5 ? 0 ?X=>>> 5 >.> mol)oles of $ater !rodued 5 >.> mol

Enthal!y hange !er mol -∆Η/ 5 heat energy X moles of $ater 5 ?.= X >.>

5 . %Q mol 1=




(EJISI*N CHEC(EJISI*N CHEC

What should you be able to do?

E0!lain the difference between an endothermic and exothermic reaction

Knderstand the reasons for using standard enthalpy changes

(eall the definitions of enthalpy of formation and combustion

:rite euations representing enthalpy of combustion and formation

(eall and a!!ly !ess"s law

(eall the definition of bond dissociation enthalpy

Calulate standard enthalpy changes using bond enthalpy #alues

Calulate standard enthalpy changes using enthalpies of formation and combustion

no$ simple calorimetry methods for measuring enthalpy changes

Calulate enthalpy changes from calorimetry measurements

C$N %O& 'O $(( O) T!ESE?C$N %O& 'O $(( O) T!ESE? YES YES N*N*

ib enthalpy kh (1)

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