lecture 2 - thermochemistry

7/24/2019 LECTURE 2 - Thermochemistry

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9.0 THERMOCHEMISTRY


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Concept of Enthalpy


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Important Terms

Heatis energy transferred between two bodies of

different temperatures

Systemis any specific part of the universe

Surroundingsis everything that lies outside the

system


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Open systemis a system that can exchange

mass and energy with its surroundings

Closed systemis a system that allows theexchange of energy with its surroundings

Isolated systemis a system that does not allowthe exchange of either mass or energy with itssurroundings


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Energyis the ability to do work

SI unit of energy is kg m2

s2

or !oule(J) Non SI unit of energy is "alorie(Cal)

Cal ! "#$" J


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Thermochemistry

A study of heat change in chemical reactions.

Two types of chemical reactions:

Exothermic Endothermic


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Exothermic reactions

Enthalpy of products


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Consider the following reaction:

A (g) ! (g) " C (g) H # ve

(reactants) (prod$ct)

reactantsenthalpy

reaction pathway

#-veH

products

Energy profile diagram for exothermic reaction


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%nthalpy of products #enthalpy of reactants& $His

positi%e

Energy is a&sor&edby the system from the surrounding

Endothermic %eactions


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Consider the following reaction

A (g) ! (g) " C(g) H # ve

(reactants) (prod$ct)

Energy profile of diagram endothermic reactions


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Enthalpy& H

The heat content of a system or total energy in thesystem

Enthalpy, H of a system cannot !e measured whenthere is a change in the system.

Example: system undergoes com!ustion or ionisation.


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Enthalpy of %eaction& 'H and

tandard Condition Ent'alpy o( rea"tion)

'he enthalpy change associated with a chemicalreaction#

Standard ent'alpy* +H, 'he enthalpy change for a particular reaction that

occurs at $*and atm(standard state)


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Thermochemical E$ation The thermochemical e"uation shows the enthalpy changes.

Example * H+,(s) " H+,(l) H # -./01

# mole of H+,(l)is formed from # mole of H+,(s)at $%C,

&H ' (.$# )*

However, when # mole of H+,(s)is formed from # mole ofH+,(l), the magnit$deof H remainsthe same with the

oppositesignof it.

H+,(l) " H+,(s) H # -./ 01


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Types of Enthalpies

There are many )ind of enthalpies such as:

Enthalpy of formation

Enthalpy of com!ustion

Enthalpy of atomisation

Enthalpy neutralisation

Enthalpy hydration

Enthalpy solution


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Enthalpy of 2ormation& 'Hf

The change of heat when / moleof a compound is

formed from its elements at their standard states.

H+(g) 3 ,+(g)" H+, (l) 'Hf # +4- 01 mol/

The standard enthalpy of formation of any element in

its most sta!le state form is 5E%,.

+H ./ ' $ +H Cl./ ' $


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Enthalpy of Com6$stion& 'Hc

The heat released when # mole of su!stance is

!urned completely in excess oxygen.

C(s) ,+(g) " C,+(g) +H"# 787 01 mol/


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Enthalpy of Atomisation& Ha The heat change when # mole of gaseo$s atomsis formed

from its element

Hais always positive !ecause it involves only !rea)ing of!onds

e.g:

0a,s/ 0a,g/ Ha' #$1 )* mol-#

2Cl.,g/ Cl,g/ Ha' #3 )* mol-#


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Enthalpy of 9e$trali:ation& 'Hn

The heat change when # mole of water, H. is formed from the

neutrali4ation of acid and !ase .

HCl,a"/ 0aH,a"/ 5 0aCl,a"/H.,a"/ &Hn' 67 )* mol#


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Enthalpy of Hydration& Hhyd

The heat change when # mole of gaseo$s ionsis

hydratedin ;ater.

e.g:0a,g/ 0a,a"/ Hhyd' 8$( )* mol-#

Cl-,g/ Cl-,a"/ Hhyd' 3(3 )* mol-#


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Enthalpy of ol$tion& Hsoln

The heat change when # mole of a s$6stanceis

dissolves in water.

e.g*9Cl,s/ 9,a"/ Cl,a"/ Hsoln' (1$ )* mol-#


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Enthalpy of $6limation& Hs$6l

'he heat change when one mole of a substance

sublimes (solid into gas)#

s/

5 g/

su!lH

Hsu&l

' #$( )* mol#


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Calorimetry

A method used in the la!oratory to measure the

heat change of a reaction.

Apparatus used is )nown as the calorimeter Examples of calorimeter

;imple calorimeter


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The outer ;tyrofoam cup

insulate the reaction

mixture from the

surroundings it is

assumed that no heat is

lost to the surroundings/ Heat release !y the

reaction is a!sor!ed !y

solution and the

calorimeter

imple calorimeter


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A 6om6 calorimeter


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Important Terms in Calorimeter

pecific heat capacity& c

pecific heat capacity& cof a su!stance is the amo$nt of

heatre"uired to raise the temperature of one gramof the

su!stance !y one degree Celsi$s(1g /C/). Heat capacity& C

Heat capacity&Cis the amo$nt of heatre"uired to raise

the temperature of agiven $antityof the su!stance !y

one degree Celsi$s(1

C/)


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+ ! mc,'

Heat released !y

su!stance'

Heat a!sor!ed

!y calorimeter

+ ! heat released by substance

m! mass of substance

C! specific heat capacity,' ! temperature change


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!asic rinciple in Calorimeter

Heat released

!y a reaction

'Heat a!sor!ed

!y surroundings

- Surroundings may re(er to

t'e)

i. Calorimeter itsel( or-

ii. T'e ater and "alorimeter- /rea"tion m"$T or C$T


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Example /

n an experiment, $.#$$ g of H.and excess of .were compressed

into a #.$$ = !om! and placed into a calorimeter with heat capacity

of 8.4 x /=1.C/. The initial temperature of the calorimeter was

6.$$$$C and finally it increased to 6.#66 $C. Calculate the

amount of heat released in the reaction to form H., expressed in )*

per mole.


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ol$tion

Heat released ' Heat a!sor!ed !y the

calorimeter

# C'T

' 1.$7 > #$8*$C-#/ > $.#66$C/

' #.8# > #$8*

' #8.# )*

H+(g) 3,+(g) " H+,(c)

mole of H. ' $.#$$ .$#(

' $.$81( mol


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moles of H+, # mole of H+

.=8- mol of H+, released /=./ 01 energy

/ mol H+, released # 01

# +4= 01

'Heat of reaction& 'H # > +4= 01 mol/

$81(.$

#.#8


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E1ample 2

# Calculate the amount of 'eat releasedin a reaction inan aluminum "alorimeter with a mass of 034.0gand

contains 5400.0 m6of water# 'he initial temperature of

the calorimeter is 27.08Cand it increased to 24.38C.


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?iven*

;pecific heat capacity of aluminum ' $.663*g-#%C-#

;pecific heat capacity of water ' 8.#7 *g-#%C-#

?ater density ' #.$ g m=-#

&T ' @.7 -6.$ /%C ' .7%C

ol$tion

# m;c;T mcccT

' #@$$.$ g/8.#7 *g-#%C-#/.7 %C/

3$7@.$ g/$.663 *g-# %C-#/.7%C/

' 8(@(.@# *

' 8.@ )*

.eat absorbed bywater

.eat absorbed byaluminium

calorimeter

.eat released ! /


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HE;;; =A?


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Hess =aw

Hesss =aw states that when reactants are converted to

products, the change in enthalpy is the same whether the

reaction takes place in one step or in the series of steps.

The enthalpy change depends only on the nature of thereactants and products and is independent of the ro$te

ta0en.

#HA B

C

3H

.H

+= 321 HHH


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i. @ist all the thermochemical e$ations involved

Alge!raic Bethod

#--#6($)*molH/

3/

/

@/(

.

#--7()*molH//

#/

.

#-313)*mol-H///

.

=++

=+

=+

gOH

gCO

gO

gHCiii

gOH

gO

gHii

gCO

gO

SCi

Step


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ii. rite the enthalpy of formation reaction for C+H-

/(

/3

/ gHC

fH

gH

sC

=

+


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iii. Add the given reactions so that the res$lt is the desiredreaction.

-78)*

3H

H

#H

fH

78)*-/(

/3

/

DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD

#6($)*3H/@/(/3g/Ciii/

-7()*3

H/

3/

3/

33/

)*-313#

H/

/

/

/

=

++=

=

+

+=++

=+

=+

gHC

fH

gH

sC

gOgHCgOHreverse

gOH

gO

gHii

gCO

gO

SCi


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Energy Cycle Bethod

0raw the energy "y"leand apply Hesss 6a to calculate the

unknown value#

+

C

(

s

)

7

H+

(

g

)

C+

H-

(

g

)

+

C

,+(g) 7H+,(g)

+

,+(g) 7B+,+(g)H

H

H

H

B

+

,+(g)

,

,

,

,

f

/

+

7

#

+

(

>

7

8

7

)

#

7

(

>

+

4

-

)

#

>

(

>

/

D

-

.

)


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#-)*mol-78

#6($767--@7(

%3

&H/%

H3/%#

H'%f

&H

=

+=


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Example /

The thermochemical e"uation of com!ustion of car!onmonoxide is shown as !elow.

C,s/ 2 .,g/ C,g/ '

given :

C,s/ .,g/ C.,g/ +H' -318 )* mol-#

C,s/ 2 .,g/ C.,g/ +H' -73 )* mol-#Calculate the enthalpy change of the com!ustion of car!on tocar!on monoxide.

H

H

H


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E1ample 2

Calculate the standard enthalpy of formation of methane if the

enthalpy of combustion of carbon& hydrogen and methane are

as follows1+H 2C(s)3 ! 455 kJ mol4

+H 2.(s)3 ! 45 kJ mol4

+H 2C."(s)3 ! 4675 kJ mol4

cH

cH

cH


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Example 7

tandard enthalpy of formation of ammonia, hydrogenchloride and ammonium chloride is -8(.# )* mol-#, -1.3 )*mol-#, 3#8.8 )* mol-#respectively. ?ritethe thermochemicale$ation for the formation of each s$6stance and calc$latethe enthalpy changefor the following reaction.

0H3g/ HCl g/ 0H8Cls/


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Exercise

/.Calculate the enthalpy of formation of !en4ene

if :

+H C.,g// ' -313.3 )* mol-#

+H H.,l// ' -76.6 )* mol-#

+H C(

H(,l/

/ ' -3(6.3 )* mol-# fH

fH

fH


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Born-Haber

Cycle


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=attice Energy, Hlattice

is the energy re+uired to completely separate one mole of asolid :ioni" "ompound;into gaseous ions

e#g1

NaCl(s) Na/(g) / Cl4(g) .lattice! /66 kJ mol4

(lattice dissociation)

Na/(g) / Cl4(g) NaCl(s) .lattice! 466 kJ mol4

(lattice formation) 'he magnitude of lattice energy increases as

the ionic charges increase the ionic radii decrease


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There is a strong attraction !etween small ions and

highly charged ions so the

H is more negative.

H for Bg is more negative than H for 0a.

!ecause Bg.is smaller in si4e and has !igger charge

than 0a, therefore

HlatticeBg/ F Hlattice 0a./


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0aand Cl-ions in the solid crystal are separated fromeach other and converted to the gaseous state Hlattice/

The electrostatic forces !etween gaseous ions and polarwater molecules cause the ions to !e surrounded !y watermolecules Hhydr/

.soln! .lattice/ .hdyr

Hydration Grocess of onic ;olid


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8attic

e%ne

rgy.eatof.ydra

tion

.eat of Solution


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Example *

>i%en-

i. Ent'alpy o( (ormation


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Example: A Born-Haber cycle for NaCl

Na(s) + Cl2(g)

Na(g) + Cl2(g)

NaCl(s)

energy

E=0

Na(g) + Cl(g)

Na+(g) + e + Cl(g)

Na+(g) + Cl-(g)

HaNa

HaCl

IonisationEnergy ofNa

Electron !nity ofCl

"attice energy

Hf NaCl

#ro$ Hess%s "a&'

HfNaCl= HaNa + HaCl +IENa+

ECl + "attice Energy-ve

+ve


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Calculation:

]

k!!!H

k"#$k122k%&&k1&'k$11H

EAHEHHH

HEAHEHH

la*ce

la*ce

)Cl(a+

&

fla*ce

la*ce)Cl(a+

&

f


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Exercise*

Constru"t a DornHa&er "y"le to e1plain 'y ioni" "ompound

lecture 2 - thermochemistry

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