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Enthalpy change

measurements

Practical Examples

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nit ( Definitions

Hf.- 'tandard enthalpy of formation The enthalpy change hen moleof a compound is formed from its

constituent elementsin their standard states, under standard conditions"

Hc.- 'tandard enthalpy of combustion The enthalpy change hen moleof a compound is completely burned

in oxygen, under standard conditions"

Hneut.- 'tandard enthalpy of neutralisation The enthalpy change hen mole of ateris formed hen an acid

neutralises an al/ali under standard conditions" H&0a!1& 2H-0a!1H(20l1

Remember STATE SYMBOLS are crucial in thermochemistry !

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5alculations

Heat transferred $ mass x 5p x temp" change H $ m x 5p x t Thus for burning ";g of ethanol if,

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Errors>the actual boo/ %alue is ?3@ +:7molAhy are the to %alues so differentB

" Cots of heat energy is

lost to the surrounding

(" lac/ soot is produced ?

incomplete combustion

3" 5opper bea/er gets hot

9" ccuracy of the balance

and thermometer

#nsulate the apparatus

and use a lid

se pure oxygen

#nclude the heat capacity

of copper in the

calculations

6ead to &7-

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more sophisticated 0Thiemann1

calorimeter

H t f 6 ti

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Heat of 6eaction0in%ol%ing a more reacti%e metal displacing a

less reacti%e metal1

The reaction beteen Finc

poder and copper sulphate

solution gi%es out heat energy

Fn & 5u(&Fn(&& 5u

Finc is added in excess atprecisely 3 mins and themixture stirred continuously

for a further = mins" graph is plotted and

extrapolated to establish themaximum temperature rise"

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Enthalpy of neutralisation

reactions 0titrations in%ol%ing acids7al/alis1 (;cm3of al/ali is pipetted

into the plastic cup"

cid is added ;cm3at a time

from a burette, the mixturestirred, and the temperature

recorded"

This is repeated until ;

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Temperature corrections

The graph is

extrapolated and the

maximum temperature

rise found" The %olume

of acid needed toneutralise the al/ali is

also noted"

H $ m x 5p x t

Gass of acid & al/ali Heat capacity of

ater

Gaximum

temp" rise

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5alculation cont>

#f mol7dm3of acid and al/ali is used and the

temperature rise is ="9o5 and (9";cm3of acid neutralises

the al/ali

:+ ) m 1 Cp 1 :t H $ 0(;&(9";1 x 9"( x ="9 $ 33< :

ut this is for (;cm3of al/ali

8or dm3the energy gi%en out ould be

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Errors

gain heat is lost to the surrounding"

Exact specific heat capacities are not used

? ater is assumed"

Experimental methods ha%e their on

errors

thermometers 7 burettes errorsError $ experimental %alue ? accepted %alue x