Chem Energetics1

7/26/2019 Chem Energetics1

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Chemical Energetics

Created by S. Colgan;

Modified by K. Slater

Resources:http://lincoln.pps.k!.or.us/lscheffler/Energetics.ppt"!#$%%Sta

ndard Enthalpy Changes


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IB Topic 5: Energetics5.1: Exothermic and EndothermicReactions

5.1.1 Defne the terms exothermic reaction,endothermic reaction and standard enthalpychange o reaction

'!o".

5.1.# $tate that com%&stion and ne&trali'ationare exothermic processes. 5.1.( )pply the relationship %et*een temperat&re

change, enthalpy change and the classifcation oa reaction as endothermic or exothermic.

5.1.+ Ded&ce, rom an enthalpy leel diagram,the relatie sta%ilities o reactants and prod&cts,and the sign o enthalpy change or the reaction.
http://upload.wikimedia.org/wikipedia/commons/d/d5/Archery_Target_80cm.svg


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!eat and Temperat&re!eat and Temperat&re

HeatHeatis energy that is transferred from oneis energy that is transferred from one

object to another due to a difference inobject to another due to a difference in

temperaturetemperature

TemperatureTemperatureis a measure of the averageis a measure of the averagekinetic energy of a bodykinetic energy of a body

Heat is always transferred from objects at aHeat is always transferred from objects at a

higher temperature to those at a lowerhigher temperature to those at a lowertemperaturetemperature

&


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5.1.1 Defne the terms exothermicreaction, endothermic reaction andstandard enthalpy change o reaction'!o".

Exothermic ReactionExothermic Reaction: ) process thatreleases heat to its s&rro&ndings.-rod&cts hae less energy than the

reactants

Endothermic ReactionEndothermic Reaction: ) process thata%sor%s heat rom the s&rro&ndings.-rod&cts hae more energy than thereactants.


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Standard Enthalpy Change of Reaction Standard Enthalpy Change of Reaction !!H"#H"#The heatenergy exchanged *ith the s&rro&ndings *hen areaction happens &nder standard conditions /T$T-0 see %elo*".

$ince the enthalpy change or any gien reaction*ill ary *ith the conditions, esp. concentrationo chemicals,'! are meas&red &nder standardconditions:

press&re 121.( 3-a temperat&re #54 #67 8 oncentrations o 1 mol dm9(The most thermodynamically sta%le allotrope *hich in the

case o car%on is graphite"

/nly'! can %e meas&red, not ! or the initial orfnal state o a system.


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(seudonyms )other names* for +

+eat of Reaction: +r,nheat produced in a chemical reaction

+eat of Combustion: +combheat produced by a combustion reaction

+eat of -eutraliation: heat produced in a neutraliation reaction

)hen an acid and base are mi,ed to get ater% p+ 0 1* +eat of solution: +solheat produced by hen something dissol2es

+eat of 3usion: +fusheat produced hen something melts

+eat of 4aporiation: +2apheat produced hen something

e2aporates +eat of Sublimation: +subheat produced hen something sublimes

+eat of formation: +fchange in enthalpy that accompanies the

formation of mole of compound from it5s elements )this has specialuses in chemistry6*



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5.1.# $tate that com%&stion andne&trali'ation are exothermicprocesses.

Combustion

E,othermic reaction

7eneral Combustion Reaction 3ormula:Compound )usually hydrocarbon* 8 9!

C9!8 +!9 8 energyC+: 8 !9!C9! 8 !+!9 8 $+ 0 ?$+ 0 ?1.&k=


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5.1.( )pply the relationship %et*eentemperat&re change, enthalpy change andthe classifcation o a reaction asendothermic or exothermic.

Exothermic

!eat o*s o&t o thesystem

$&rro&ndings heat &p

!eat change '!" ; 2negatie"

7!17< 1#=/# 7/# ! 9551# 3? mol91

!# < =/# !#/

>! 9#7@ 3? mol91

Endothermic

!eat o*s into thesystem

$&rro&ndings cool do*n

!eat change '!" A 2positie"

!#/s" !#/l"

>! !


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Exothermic processis any process that gi2es off heat D

transfers thermal energy from the system to the surroundings.

Endothermic processis any process in hich heat has to be

supplied to the system from the surroundings.

!+!)g*8 9!)g* !+!9 )l*8 energy

+!9 )g* +!9 )l*8 energy

energy 8 !+g9 )s* !+g )l*8 9!)g*

#.!

energy 8 +!9 )s* +!9 )l*


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5.1.+ Ded&ce, rom an enthalpy leel diagram,the relatie sta%ilities o reactants andprod&cts, and the sign o enthalpy change orthe reaction.

Exothermic Reactions

-rod&cts more sta%le than reactantslo*er energy".

>! !prod&cts !reactants

$ince the prod&cts hae less

energy than the reactants, the>! al&e is negatie.

Endothermic Reactions

-rod&cts less sta%le than reactantshigher energy"

>! !prod&cts !reactants

$ince the prod&cts hae moreenergy than the reactants, the>! al&e is positie.


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Enthalpy (H)is used to uantify the heat flo into or out of a

system in a process that occurs at constant pressure.

H0 H)products* D H)reactants*

H0 heat gi2en off or absorbed during a reaction at constant pressure

Hproducts F HreactantsHF + 2alue

Iirection of heat flo

Stability of reactantsStability of products

Aonding


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RE4HEB

Endothermic Exothermic

&efinition @ process that absorbsheat from thesurroundings

@ process that releasesheat into thesurroundings

Examples '" Combustion -eutraliation reactions

Change inTemperature

Iecreases Hncreases

!H value (ositi2e -egati2e

&irection of heat flow 3rom surroundings intosystem

3rom system intosurroundings

Stability of reactants More stable Less stable

Stability of products Less stable More stable

(onding Aond breaking Aond making


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IB Topic 5: Energetics5.#: alc&lation o Enthalpy hanges

5.#.1 alc&late the heat energy change *hen thetemperat&re o a p&re s&%stance is changed.

5.#.# Design s&ita%le experimental proced&res ormeas&ring the heat energy changes o reactions.

5.#.( alc&late the enthalpy change or a reaction&sing experimental data on temperat&re changes,C&antities o reactants and mass o *ater

5.#.+ Eal&ate the res&lts o experiments todetermine enthalpy changes.


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actors )ecting !eat F&antitiesactors )ecting !eat F&antities

The amount of heat contained by an objectThe amount of heat contained by an object

depends primarily on three factors#depends primarily on three factors#

The mass of materialThe mass of material

The temperatureThe temperatureThe kind of material and its ability toThe kind of material and its ability to

absorb or retain heat)absorb or retain heat)


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!eat F&antities!eat F&antities

Jhe heat reuired to raise the temperature of .


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5.#.1 alc&late the heat energy change *henthe temperat&re o a p&re s&%stance ischanged.

!eat Energy hange

q = m x c x T

C heat Go&les or calories"

m mass g"c specifc heat ? g91o91"

The amo&nt o heat reC&ired to raise thetemperat&re o 1 g o a s&%stance 1 o.

>T change in temperat&re


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!o* m&ch heat in Go&les *ill%e a%sor%ed *hen (#.2 go *ater is heated rom#5.2 o to 72.2 oH

q = m x c x T

C H

m (#.2 g

c +.17 ? g91o91

>T 72.29#5.2 55.2 o

C (#.2 x +.17 x 55.2 7,360 J

hen +(5 ? o heat is added to (.+

g o olie oil at #1 o, thetemperat&re increases to 75 o.hat is the specifc heat o olie

oilH

q = m x c x T

C +(5 ?

m (.+ g

c H>T 759#1 @+ o

+(5 (.+ x c x @+ 2.0 J g-1o-1


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!eat Transer -ro%lem 1!eat Transer -ro%lem 1

Calculate the heat that ould be reuired an aluminumCalculate the heat that ould be reuired an aluminum

cooking pan hose mass is + ,-- g" -).-' / g-).-' / g0101ooCC0101"'--"'--ooC 2 '-C 2 '-ooCC**

+ 3,4.,, /+ 3,4.,, /

!


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!o* m&ch heat in Go&les isreC&ired to raise thetemperat&re o #52 g omerc&ry 5# oH

) 1.55 g piece o stainless steela%sor%s 1+1 ? o heat *hen itstemperat&re increases %y 1J7o. hat is the specifc heat othe stainless steelH


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!o* m&ch heat in Go&les isreC&ired to raise the

temperat&re o #52 g omerc&ry 5# oH

q = m x c x T

C H

m #52 g

c 2.1+ ? g91o91 Ta%le 11.#"

>T 5# o

C #52 x 2.1+ x 5# 1!00J

) 1.55 g piece o stainless steela%sor%s 1+1 ? o heat *hen its

temperat&re increases %y 1J7 o.hat is the specifc heat o the

stainless steelH

q = m x c x T

C 1+1 ?

m 1.55 g

c H

>T 1J7 o

1+1 1.55 x c x 1J7 0."11 J g-1o-1


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5.#.# Design s&ita%le experimental proced&resor meas&ring the heat energy changes oreactions.

alorimeter: Reactions&sed to heat &p anexternal so&rce o*ater.

Temperat&re change o*ater, mass o materialand mass o *ater aremeas&red.

Kse q = m x c x T tosole or C then fnd theheat o reaction in3?Lmol o reactings&%stance.


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alorimetryalorimetry

Calorimetry involves the measurement ofCalorimetry involves the measurement ofheat changes that occur in chemicalheat changes that occur in chemicalprocesses or reactions)processes or reactions) Ietermines

the '+ by measuring temp 'Ps createdfrom the r,n The heat change that occurs when aThe heat change that occurs when a

substance absorbs or releases energy issubstance absorbs or releases energy isreally a function of three 5uantities#really a function of three 5uantities# Jhe massJhe mass Jhe temperature changeJhe temperature change Jhe heat capacity of the materialJhe heat capacity of the material

!


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!eat apacity and $pecifc !eat!eat apacity and $pecifc !eat

The ability of a substance to absorb or retain heatThe ability of a substance to absorb or retain heat

varies widely)varies widely)

The heat capacity depends on the nature of theThe heat capacity depends on the nature of the

material)material)

TheThe specific heatspecific heatof a material is the amount ofof a material is the amount of

heat re5uired to raise the temperature of 1 gram ofheat re5uired to raise the temperature of 1 gram of

a substance 1a substance 1 ooC or 6elvin"C or 6elvin"

!


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!eat Transer -ro%lem #!eat Transer -ro%lem #

Bhat is the final temperature hen < grams of ater atBhat is the final temperature hen < grams of ater at!

Solution:Solution: * Cold"+ * hot" mC T+ mC TLet J 0 final temperatureLet J 0 final temperature

)< g* ). = g)< g* ). = g??ooCC??*)J? !


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Ksing Thermochemical EC&ations

alci&m oxide com%ines *ith *ater to prod&ce calci&m hydroxideand heat exothermic reaction".

a/s" < !#/l" a/!"#s" < @5.# 3? #R a/s" < !#/l" a/!"#s" ! 9@5.# 3?

!o* many 3? o heat are prod&ced *hen J.#( g o a/ reactH

1" Balance eC&ation

#" Moles a/: J.#( [email protected] gLmol 2.1#6 mol(" Ratio &sing heat: x 3? @5.# 3?

!.$1 %J

2.1#6 a/ 1 a/

5.#.( alc&late the enthalpy change or a reaction&sing experimental data on temperat&re changes,C&antities o reactants and mass o *ater.


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Ksing Thermochemical EC&ations

$odi&m hydrogen car%onate a%sor%s 1#6 3? o energy anddecomposes to sodi&m car%onate, *ater, and car%on dioxideendothermic reaction".

#a!/(s" < 1#6 3? a#/(s" < !#/g" < /#g"#R

#a!/(s" a#/(s" < !#/g" < /#g" ! 1#6 3?

!o* many 3? o heat are needed to decompose #.#+ mola!/(s"H

1" Balanced eC&ation

#" Moles a!/(s" #.#+ mol

(" Ratio: x 3? 1#6 3? 1$$ %J

#.#+ a!/(s" # mol



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Ksing Experimental Data

In a ne&trali'ation reaction, #5.2 mN o *ater containing 2.2#5 mol!l is added to #5.2 mN o *ater containing 2.2#5 mol a/! ina oam c&p calorimeter. )t the start, the sol&tions and thecalorimeter are all at #5.2 o. D&ring the reaction, the highesttemperat&re o%sered is (#.2 o. alc&late the heat in 3?"released d&ring this reaction. )ss&me the densities o thesol&tions are 1.22 g mN91.

Kse q = m x c x T

m mass o sol&tion 52.2 mN x 1.22 g mN91 52.2 g

+.17 G g91o91

>T (#.2 #5.2 J.2 o

C 52.2 g x +.17 ? g91o91x J.2 o 1+@( ? 1.5 3?



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l l h h l h i


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Ksing Thermodynamic F&antities $tandard !eats o ormation"

+eat of reaction can be found by: sum the heats of formation of all

the products D sum of heats of formation of all the reactants

Hrxn+ Hf products2 Hf reactants

Hf0 standard enthalpy of formation. Energy reuired to form a

compound from its elements.

standardN is a term used a lot in chemistry. Ht usually means that

the 2alues are e,perimentally determined and compared to an

agreed upon reference 2alue

Since the +fis gi2en per mole% e must multiply by coefficients



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Ksing Thermodynamic F&antities $tandard !eatso ormation"

Ksing the ta%le o thermodynamic C&antities, calc&late the

heat o reaction or #$/#g" < /#g"

#$/(g"

!eat o reaction !prod&cts !o reactants

!eat o prod&cts: !$/(" 9(65.# 3?Lmol x # 9J62.+ 3?

!eat o reactants !$/#" < !/#"9#[email protected] 3?Lmol x #" < 2" 956(.7 8?

!eat o reaction 9J62.+ 3? 956(.7 3?" 916@.@ 3?



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-!98 & C9-!9 8 &C9!

+f products0 ) k= mol?* 8 &)?&$& k= mol?* 0 ?


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Ca)9+*!)s* 8 C9!)g*+!9)g* 8 CaC9&)s*

+f products0 )?!. k= mol?* 8 )?!


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IB Topic 5: Energetics5.( !essOs Na* P 5.+ Bond Enthalpies

5.(.1 Determine the enthalpy change o areaction that is the s&m o t*o or three reactions*ith 3no*n enthalpy changes.

5.+.1 Defne the term aerage %ond enthalpy

5.+.# Explain, in terms o aerage %ondenthalpies, *hy some reactions are exothermicand others are endothermic


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5.(.1 Determine the enthalpy change o areaction that is the s&m o t*o or threereactions *ith 3no*n enthalpy changes.

!essOs Na*: Reactions can %e added together in order to determineheats o reactions that canOt %e meas&red in the la%.

diamond" graphite"

This reaction is too slo* to %e meas&red in the la%. T*o reactions can%e &sed that can %e meas&red in the la%:

a" graph" < /#g" /#g" ! 9(6(.5 3?

%" diam" < /#g" /#g" ! 9(65.+ 3?

$ince graphite" is a prod&ct, *rite eC&ation a" in reerse to gie:

c" /#g" graph" < /#g" ! (6(.5 3?

o* add eC&ations %" and c" together:

diam" < /#g" < /#g" graph" < /#g" < /#g"

! 9(65.+ 3? < (6(.5 3? 91.6 3?

inal eC&ation: &diamond' &gra(hite' ) = - 1.* %J


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Qien the ollo*ing thermochemical eC&ations, calc&late the heat oreaction or:

#!+g" < !#/l" #!5/!l"

a" #!5/!l" < (/#g" #/#g" < (!#/l" ! 91(@J 3?

%" #!+g" < (/#g" #/#g" < #!#/l" ! 91+11 3?

( i h h l h


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Qien the ollo*ing thermochemical eC&ations, calc&late the heat oreaction or

#!+g" < !#/l" #!5/!l"

a" #!5/!l" < (/#g" #/#g" < (!#/l" ! 91(@J 3?

%" #!+g" < (/#g" #/#g" < #!#/l" ! 91+11 3?

$ince #!5/!l" is a prod&ct, *rite eC&ation a" in reerse order:

c" #/#g" < #!#/l" #!5/!l" < (/#g" ! 1(@J 3?

)dd eC&ations %" P c" together, cancelling o&t s&%stances on oppositesides o the arro*. )dd the heat al&es to o%tain the heat o

reaction.

#!+g" < !#/l" #!5/!l" ! 9++ 3?

5 ( 1 D t i th th l h


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s" < #!#g" !+g"

a" s" < /#g" /#g" ! 9(6( 3?

mol91

%" !#g" < = /#g" !#/l" ! 9#7@ 3? mol91

c" !+g" < #/#g" /#g" < #!#/l" ! 9762 3? mol91

5 ( 1 D t i th th l h


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s" < #!#g" !+g"

&s' + #2&g' #2&g' ) = -3*3 %J

#!#g" < = /#g" !#/l"" ! #9#7@ 3? mol91"

2)2&g' + #2&g' 2)2#&'' ) = -"72 %J

!+g" < #/#g" /#g" < #!#/l" ! 9762 3?mol91

#2&g' + 2)2#&' )$&g' + 2#2&g' ) = !*0

&s' + 2)2&g' )$&g' ) = -7" %J


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5.+.1 Defne the term aerage %ond enthalpy

Enthalpy changes o reactions are the res&lt o %onds %rea3ingand ne* %onds %eing ormed.

Brea3ing %onds reC&ires energy orming ne* %onds releases energy

ond entha(is the energy change reC&ired to %rea3 1 molo %onds or orm 1 mol o %onds %et*een t*o coalently%onded atoms in the gaseo&s state.

Bond enthalpies or &nli3e atoms *ill %e aected %y

s&rro&nding %onds and *ill %e slightly dierent indierent compo&nds so a/erage ond entha(iesare&sed.


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hemical Reactionshemical Reactions

8n a chemical reaction8n a chemical reaction

Chemical bonds are brokenChemical bonds are broken

9toms are rearranged9toms are rearranged $ew chemical bonds are formed$ew chemical bonds are formed

These processes always involveThese processes always involve

energy changesenergy changes


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".$.2 ond Entha(ies".$.2 ond Entha(ies

:ne approach to determining an enthalpy:ne approach to determining an enthalpy

change for a chemical reaction is to computechange for a chemical reaction is to computethe difference in bond enthalpies betweenthe difference in bond enthalpies betweenreactants and productsreactants and products

The energy to re5uired to break a covalentThe energy to re5uired to break a covalent

bond in the gaseous phase is called abond in the gaseous phase is called a bondbondenthalpy)enthalpy)

(ond enthalpy tables give the average energy(ond enthalpy tables give the average energyto break a chemical bond) 9ctually there areto break a chemical bond) 9ctually there are

slight variations depending on the environmentslight variations depending on the environmentin which the chemical bond is locatedin which the chemical bond is located


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Energ hangesEnerg hanges

Areaking chemical bonds reuiresAreaking chemical bonds reuires

energyenergy

3orming ne chemical bonds releases3orming ne chemical bonds releases

energyenergy


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ond Entha( Taeond Entha( Tae

The average bond enthalpies for several types ofThe average bond enthalpies for several types of

chemical bonds are shown in the table below#chemical bonds are shown in the table below#


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ond Entha(iesond Entha(ies

(ond enthalpies can be used to calculate the(ond enthalpies can be used to calculate theenthalpy change for a chemical reactionenthalpy change for a chemical reaction))

Energy is re5uired toEnergy is re5uired tobreak chemical bondsbreak chemical bonds))Therefore when a chemical bond is broken itsTherefore when a chemical bond is broken itsenthalpy changeenthalpy changecarries acarries a positive signpositive sign))

Energy is released when chemical bondsEnergy is released when chemical bondsformform) ;hen a chemical bond is formed its) ;hen a chemical bond is formed itsenthalpy changeenthalpy changeis expressed as ais expressed as a negativenegativevaluevalue

(y combining the enthalpy re5uired and the(y combining the enthalpy re5uired and theenthalpy released for the breaking andenthalpy released for the breaking andforming chemical bonds4 one can calculateforming chemical bonds4 one can calculatethe enthalpy change for a chemical reactionthe enthalpy change for a chemical reaction

h i d d h i


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Exothermic and EndothermicExothermic and Endothermic

-rocesses-rocesses

E,othermic processes release energyE,othermic processes release energy

CC&&++;;)g* 8 9)g* 8 9!!)g*)g* & C9& C9!!)g* 8 +)g* 8 +!!9 )g*9 )g*

< '-,= k/< '-,= k/

Endothermic processes absorb energyEndothermic processes absorb energy

C)s* 8 +C)s* 8 +!!9 )g*9 )g*


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Energ hanges inEnerg hanges in

endothermic andendothermic and

exothermic (rocessesexothermic (rocesses8n an8n anendothermicendothermicreaction there isreaction there ismore energymore energy

re5uired to breakre5uired to breakbonds than isbonds than isreleased whenreleased whenbonds arebonds areformed)formed)

The opposite isThe opposite istrue in antrue in anexothermicexothermicreaction)reaction)

5 + # Explain in terms o aerage %ond


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5.+.# Explain, in terms o aerage %ondenthalpies, *hy some reactions are exothermicand others are endothermic

I the amo&nt o energy reC&ired to %rea3 the %onds in thereactants is greater than the amo&nt o energy released *hen%onds are ormed in the prod&cts, the reaction isendothermic.

aerage %ond enthalpy reactants A aerage %ond enthalpyprod&cts

I the amo&nt o energy reC&ired to %rea3 the %onds in thereactants is less than the amo&nt o energy released *hen

%onds are ormed in the prod&cts, the reaction is exothermic.

aerage %ond enthalpy reactants ; aerage %ond enthalpyprod&cts

5 + # Explain in terms o aerage %ond


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Kse the ollo*ing aerage %ond enthalpies 3? mol91" todetermine the heat o reaction or (# < !( (!

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