Lecture 23 - organic.cc.stonybrook.edu

Lec-25-Thermo2-Complete.ppt 10/17/2021

Thermochemistry

Chemical Reactions. Hess Law.

Calorimetry 1

1

Lecture 23Energy and Thermochemistry

Chapter 13

Office Hours:

Monday 2:30 – 4:30 PM Zoom Meeting accessed

from Blackboard.

Fernando Raineri

Chemistry Department Room 519

1


Thermochemistry


Calorimetry 2

2

Enthalpy Changes: Chemical Processes

( ) ( ) ( ) 1

2

-

2 2

1H 241.8kJ mog O g H O g

2lr H =+ → −

ΔrH at any temperature and 1 bar: standard enthalpies

of reaction Δ r H0

pure reactants at pure products at , ,

A CB D

T PT P

rc Ha b d+ + → + +

2


Thermochemistry


Calorimetry 3

Enthalpy Changes: Formation Reactions ΔfH

°

Standard molar enthalpies of formation:

enthalpy change ΔfH to produce a mol of substance

starting from the elements in their stable state at

standard conditions (1 bar). It is customary to

tabulate data at 25 ºC.

( ) ( ) ( ) 0 -112 2 22

H g O g H O 285.8kJ molf H+ → = −

( ) ( ) ( ) 0 -1

2 2 42C graphite 2H g C H g 52.26kJ molf H+ → =

( ) ( ) 0

2 2H g H g 0f H→ =

The enthalpy of formation of an element in its standard

state at 25°C is zero. 3

3


Thermochemistry


Calorimetry 4

( ) ( )( )2C s + 2H g

( )4CH ,gf H−

4

( ) ( ) ( ) ( )2 24 2CCH g O g2O g 2H O l+ → +

( ) ( )( )22O g O g+

( )22 O ,g 0f H− =

+

Standard Enthalpy of ReactionEnthalpy is a function of state.

For any given transformation: (initial) → (final)

ΔH is independent of the path.

4


Thermochemistry


Calorimetry 5

5

( ) ( ) ( ) ( )2 24 2CCH g O g2O g 2H O l+ → +

( ) ( )( )2C s + 2H g

( )4CH ,gf H−

( ) ( )( )22O g O g+

( )22 O ,g 0f H− =

+

( )22 H O,gf H ( )2CO ,gf H

Standard Enthalpy of Reaction

5


Thermochemistry


Calorimetry 6

6

( ) ( ) ( ) ( )2 24 2CCH g O g2O g 2H O l+ → +

( ) ( )( )2C s + 2H g

( )4CH ,gf H−

( ) ( )( )22O g O g+

( )22 O ,g 0f H− =

+

( )22 H O,gf H ( )2CO ,gf H

Standard Enthalpy of Reaction

( ) ( )( )

( ) ( )( )

2

4 2

2CO ,g 2 H O

CH ,g 2 O

,

g

l

,f f

fc f

H

HH H

H

+

= +

−

6


Thermochemistry


Calorimetry 7

7

( ) ( )0

Products a

0

R t

0

e c ants

r Rf fP HH RPH

=

−

pure reactants at p

0

, 1bar ure products at , 1bar

CA B D

T P T P

ra b Hc d

==

+ + → + +

( ) ( )( )

( ) ( )( )

0 0

sum over the p

00

sum over the reactan s

roducts

t

0 C D

A B

f

f f

r fc H d

a

H

H b

H

H

+

= + −

+

+

7


Thermochemistry


Calorimetry 8

Enthalpy Changes in Chemical Reactions:

Hess’s Law

Because enthalpy is a function of state, we can

combine any number of auxiliary reactions and their

enthalpies to calculate the enthalpy of a reaction of

interest.

8

8


Thermochemistry


Calorimetry 9

( ) ( ) ( ) 0

2 3 2 3 2 3CH CHCH g H g CH CH CH g 124kJ/molhH+ → = −

The standard reaction enthalpy for the conversion of

propene to propane at 25 °C is:

The standard reaction enthalpy of formation for liquid

water at 25 °C is:

( ) ( ) ( ) 0

2 2 2

1H g O g H O 286kJ/mol

2f H+ → = −

What is the standard reaction enthalpy of combustion

of propene at 25 °C?

( ) ( ) ( ) ( ) ( )0

2 3 2 2 2 2 3

9CH CHCH g O g 3CO g +3H O CH CHCH ?

2cH+ → =

9

( )1−

The standard reaction enthalpy for the combustion

propane at 25 °C is:

( ) ( ) ( ) ( ) 0

3 2 3 2 2 2CH CH CH g 5O g 3CO g 4H O 2220kJ/molcH+ → + = −

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Thermochemistry


Calorimetry 10

( ) ( ) ( ) ( ) ( )0

2 3 2 2 2 2 3


2cH+ → =

( ) ( ) ( ) 0

2 3 2 3 2 3CH CHCH g H g CH CH CH g 124kJ/molhH+ → = −

( ) ( ) ( ) 0

2 2 2

1H O H g O g 286kJ/mol

2f H→ + − =

( ) ( ) ( ) ( ) ( )0

2 3 2 2 2 2 3


2cH+ → =

( ) ( ) ( ) ( )

( )

( )

00

2 3

0

2 3

0

2

0 0

2 3 3 2 3 2

3

CH CHCH CH CH CH H O

kJ kJ kJ124 2220 286

mol m

CH CHCH

ol molCH CHCH

kJCH CHCH 2058

mol

h cc

c

c

fH H HH

H

H

− − −

−

= + −

= + −

=

10

9

2

( ) ( ) ( ) ( ) 0

3 2 3 2 2 2CH CH CH g 5O g 3CO g 4H O 2220kJ/molcH+ → + = −

10


Thermochemistry


Calorimetry 11

Measuring Enthalpy Changes: Calorimetry

Constant Pressure Calorimeter

0pq =

Due to the insulating

nature of the walls

there is no heat

exchanged between

the system (the

contents of the

calorimeter) and the

outside.

The process is at

constant pressure11

11


Thermochemistry


Calorimetry 12

Measuring Enthalpy Changes: Calorimetry

Constant Pressure Calorimeter

Ti

reactants

T

Extent of

reaction

Tf

products

ΔH

0pH q = =

ΔH1

ΔH2

21 0HH H= + =

( )2

final s,fi1 nal

1 H

m CH C T

H

−=

+=

−

12: heat capacity of the calorimeterC

12


Thermochemistry


Calorimetry 13

Measuring Enthalpy Changes: Calorimetry: Constant V Bomb Calorimeter

bomb calorimeterq q=−

Due to the insulating

nature of the walls

there is no heat

exchanged between

the inside of the

calorimeter and the

outside.

The process in the

bomb is at constant

volume

13

: heat capacity

of the calorimeter

C

13


Thermochemistry


Calorimetry 14

14

( ) ( )0 0 0

gasesr r r r rH U PV U n RT = + = +

( ) ( ) ( ) ( )7 8 2 2 2C H 9O g 7CO g 4 H O+ → +

( ) ( ) ( )2 2gasCO O 7 9 2n = − = − = −

0 0 2r rH U RT = −

As the process is at constant volume

calorimeter wat rbom er ,wateb ( )sq m C Cq TU = = =− − +

14


Thermochemistry


Calorimetry 15

15

Spontaneous and Non-Spontaneous Processes

Splash!

The falling down of the

object is a spontaneous

process.

The rising up of the object

is a non-spontaneous process.But not impossible.

15


Thermochemistry


Calorimetry 16

16

Spontaneous and Non-Spontaneous Processes

A spontaneous process is a process that occurs

naturally, without our help or intervention.

Some examples are:

❖ Melting of ice at 30 °C.

❖ Dissolution of small amounts of sucrose (table

sugar) in water (as in sweetening a coup of tea).

❖ Oxidation of iron in an environment with O2(g):

( ) ( )2 2 3

32 Fe O ( ) Fe O

2s g s+ →

16


Thermochemistry


Calorimetry 17

17

Correspondingly, a non-spontaneous process is a

process that does not occurs naturally, unless we help

or intervene.

The reverse of the processes cited above are examples:

❖ freezing of water at 30 °C.

❖ Precipitation of dissolved sucrose from water at 60

°C (if the amount originally dissolved was small).

❖ Reduction of iron oxide to neat iron:

( ) ( )2 3 2

3Fe O 2 Fe O ( )

2s s g→ +

Notice that if we intervene (by changing the conditions),

we may be able to make a non-spontaneous process

happen. Therefore, non-spontaneous doesn’t mean

impossible.

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Thermochemistry


Calorimetry 18

Directionality of Chemical Processes

18

It would be very useful if we could predict if a chemical

reaction

or a phase transition

C+ and consta+A D+ nB+ tT Pa b c d→

will proceed spontaneously (or naturally).

Phase and constantPhase T P→

The Second Law of Thermodynamics provides us with

the tools for developing a very useful spontaneity

criterion for processes that take place at constant

temperature and pressure.

18


Thermochemistry


Calorimetry 19

System and its surroundings

19

Surroundings

System

Energy can be exchanged

between system and

surroundings through the

walls that separate them.

19


Thermochemistry


Calorimetry 20

Second Principle of Thermodynamics

❖ The entropy S is an extensive function of state.

❖ In a reversible process the entropy of the universe is

constant.

❖ In a spontaneous or irreversible process the entropy

of the universe increases. 20

In the following “Universe” refers to the combination

( ) ( ) ( )System Surroundings Universe+ =

universe system surroundingsS S S= +

universe 0S =

universe 0S

20

Lecture 23 - organic.cc.stonybrook.edu

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