Chapter 4: Thermochemistry - University of...

Winter 2013 Chem 254: Introductory Thermodynamics

41

Chapter 4: Thermochemistry ....................................................................................................... 41

Enthalpy .................................................................................................................................... 41

Systematic approach to ΔrHTo (a formula) ................................................................................ 43

Relation Between ΔrUTo and ΔrHT

o ............................................................................................ 44

Ions in Solution ......................................................................................................................... 44

Chapter 4: Thermochemistry

Enthalpy

Chemical bonds represent internal energy

Chemical reaction means a change in internal energy

Exothermic Reaction Release Energy produced heat does work

Quantity of energy change: H constant pressure

U constant volume (for gases)

Focus on H

H : is a function of state, only depends on initial and final states

Initial: A B C D

,i iT P ,f fT P

Calculate in little steps: add them up

o

r TH , is defined for reaction as written

a) 2 ( ) 2 ( ) 3 ( )3 2g g gH N NH 298.15 ( )o

r H a

b) 2 ( ) 2 ( ) 3 ( )

3 1

2 2g g gH N NH 298.15

1( ) ( )

2

o

r H b H a


Chapter 4: Thermochemistry 42

Heat of formation/enthalpy of formation

298.15

o

f H (molecule)

: heat involved in forming 1 mole of molecule from elements in their

“elemental state

298.15 3

o

f H NH 2 ( ) 2 ( ) 3 ( )

3 1

2 2g g gH N NH

NOT 2 ( ) 2 ( ) 3 ( )3 2g g gH N NH

298.15 2 5

o

f H C H OH (s) 2 (g) 2 (g) 2 5 (l)

12 3

2C H O C H OH

Elemental states:

2 (g) 2 (g) 2 (g) 2 (g) (s) (s) (s), , , , , ,H N O F Li B P Tabulated in back of book 4.1, 4.2

From 298.15

o

f H , I can calculate 298.15

o

r H for any reaction

If I know

,P mC for any molecule, I can calculate o

r TH at any T

reactant

PC of 2 2 , 2 , 2

1 3 1 3( ) ( )

2 2 2 2P m P mN H C N C H

PC depends on T in general

f

i

T

i f PT

H T T C dT



If PC is assumed constant P f iH C T T

Systematic approach to ΔrHTo (a formula)

A B C Dn A n B n C n D

In : stoichiometric coefficient

0C D A Bn C n D n A n B

I In for products 0I

I In for reactants 0I , , ,I A B C D

Reaction : 0I

I

I

If I know I ,

,P mC and 298.15

o

f H I for each molecule I

, , 298.15

298.15( )

To o

r T i P m I I f

I I

H C dT H I

Steps I + III (in Joules) Step II + IV (in KJ)

(convert units before adding!)

Derivation of formula:

If PC depends on T (power series)

2

, 2

I I I I

P m

T TC a b c

K K



2

298.15:

TI I I

P I I I

I I I

C dT a b T c T dT

Relation Between ΔrUTo and ΔrHT

o

o

r TH : constant pressure processes

o

r TU : constant volume process

H U PV by definition

U H PV

( )U H PV

Apply to reaction energies at constant T

( )o o

r T r TU H PV Products Reactant( ) gas gasPV n n RT

Eg.

2( ) 2( ) 2 ( )

1

2g g lH O H O

2( ) 2( ) 2 ( )

1

2g g gH O H O

Reactants: 3

2PV RT Reactants:

3

2PV RT

Products: 0PV Products: 1

2PV RT

1

12

o o

r T r TU H RT

1

1 12

o o

r T r TU H RT

3

2

o o

r T r TU H RT 1

2

o o

r T r TU H RT

Ions in Solution

We would like to use heats of formation for reactions involving ions in solution

Elementary definition of reactions for f H ions:

1

( ) 2 ( ) ( ) ( )s l aq aqNa H O Na e

( ) 2( ) 2 ( ) ( )

1

2aq g l aqe Cl H O Cl

In reality such half reactions do not occur, e.g.

2

( ) 2 ( ) ( ) 2( ) ( )2s l aq g aqMg H O Mg H OH

NOT 2

( ) 2 ( ) ( ) ( )2s l aq aqMg H O Mg e



No such “half reactions” actually occur

Define artificial reference point:

2( ) ( ) ( )

1

2g aq aqH H e 0o

r TH for any T (??)

( ) ( ) ( )g aq aqHCl H Cl IH

2( ) 2( ) ( )

1 1

2 2g aq gH Cl HCl

IIH

2( ) ( ) ( )

1

2g aq aqH H e 0IIIH

2( ) ( )

1

2g aqCl e Cl Add reactions I II III IV

298.15 ( )

o

f aq r I r II r IIIH Cl H H H

Chapter 4: Thermochemistry - University of...

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