Chemical Energetics Notes Edited

Chemical
 Energe.cs
 Copied
from
NYJC
Chemistry
lecture
notes


The
standard
enthalpy
change
of
a
reac1on,
 ΔHr,
is
the
heat
energy
absorbed
or
evolved
 when
molar
quan..es
of
reactants
as
stated
 in
the
thermochemical
equa.on
react
 together
under
standard
condi.ons.

 The
standard
enthalpy
change
of
 combus1on,
∆Hc,
is
the
energy
change
when
 one
mole
of
a
substance
is
completely
burnt
 in
excess
oxygen
under
standard
condi.ons.

 ∆Hc
is
usually
exothermic.


Heat
evolved
by
fuel
=
Heat
absorbed
by
water
=
mcΔT

 
ΔHc
=
(+/‐)
Heat
evolved/
amount
of
fuel
used


The
standard
enthalpy
change
of
neutralisa1on,
 ΔHn,
is
the
energy
change
when
one
mole
of
 water
is
formed
during
the
neutralisa.on
of
an
 acid
and
an
alkali
under
standard
condi.ons.

 ΔHn
is
always
 exothermic.


Heat
evolved
=
Heat
absorbed
by
solu.on
=mcΔT
 ΔHn
=
‐
Heat
evolved
/
amount
of
water
formed




The
standard
enthalpy
change
of
forma1on,
 ΔHf
is
the
energy
change
when
one
mole
of
a
 compound
is
formed
from
its
cons%tuent
 elements
in
their
standard
states
under
 standard
condi%ons.

 ΔHr
=

∑nΔHf(products)
‐
∑mΔHf(reactants)




The
bond
dissocia1on
energy,
ΔHdiss
is
the
 energy
required
to
break
one
mole
of
covalent
 bond
between
two
atoms
in
the
gaseous
state.

 ΔHdiss
is
always
endothermic.


ΔHr
=
∑BE(bonds
broken)
–
∑BE(bonds
formed)

 The
standard
enthalpy
change
of
 atomisa1on,
ΔHat
is
the
energy
change
when
 one
mole
of
gaseous
atoms
is
formed
from
 its
element
under
standard
condi.ons.

 ΔHat
is
always
endothermic


The
first
ionisa1on
energy
of
an
element
is
the
 energy
needed
to
remove
one
mole
of
outermost
 electrons
from
one
mole
of
gaseous
atoms
to
 form
one
mole
of
singly‐charged
gaseous
ca.ons.
 Ionisa.on
 The
first
electron
affinity
is
the
energy
 energies
are
 change
when
one
mole
of
electrons
is
 always
 added
to
one
mole
of
gaseous
atoms
 endothermic.


to
form
one
mole
of
gaseous
anions.

 The
standard
laAce
enthalpy,
ΔHlaB
is
the
 energy
change
when
one
mole
of
an
ionic




Note:
 compound
is
formed
from
its
cons.tuent





ΔHlaB
is
 




always
 gaseous
ions
at
standard
condi.ons.

 exothermic


where

z+
=
charge
on
ca.on

z−
=
charge
on
anion
 r+
+
r−
=
interionic
distance
between
nuclei
of
the
two
ions

 +
z‐l
 lz 
LE

α
 r 
+
r + ‐




Charge









ASrac.on







LE
 

Size
of
ions



ASrac.on





LE
















Born
Haber
Cycle
‐
to
obtain
LE
 3
Steps
 1.
Atomisa.on
of
elements
 2.Forma.on
of
gaseous
ions
from
atoms
(I.E.,
E.A.)
 3.
Forma.on
of
ionic
laZce
from
gaseous
atoms
(LE)
 Na+(g)
+
Cl−(g)
 1st
I.E.


1ST
E.A.
 LE
/
ΔHlaS


Na(g)
+
Cl(g)
 ΔHat


Na(s)
+
½
Cl2(g)


ΔHf


NaCl(s)


The
standard
enthalpy
change
of
hydra1on,
 ΔHhyd
is
the
energy
change
when
one
mole
of
 gaseous
ions
is
surrounded
by
water
 molecules,
forming
a
solu.on
at
infinite
 dilu.on
under
standard
condi.ons.

 l
z
l
 
Always
exothermic,

















lΔHhydl

α





























 r


The
standard
enthalpy
change
of
solu1on,
 ΔHsol
is
the
enthalpy
change
when
one
mole
of
 compound
is
dissolved
by
solvent
(usually
 water)
such
that
further
dilu.on
produces
no
 more
heat
change
under
standard
condi.ons.



ΔH
of
dissolu1on
of
ionic
compounds,
ΔHsol

 Stage
1:
The
ions
are
pulled
far
apart
to
form
separate
ions
=
‐ΔHlaS
 Stage
2:
Water
molecules
are
bonded
to
the
gaseous
ions
forming
a
 solu.on
=
ΔHhyd




ΔHsol
=
ΔHhyd
−
ΔHlaB

 NaCl(s)
+
aq


Stage
1
 ‐ΔHlaS


ΔHsol


Na+(aq)
+
Cl‐(aq)


Na+(g)
+
Cl‐(g)
+
aq


Stage
2
 ΔHhyd


Entropy,
S,
is
the
measure
of
order
of
a
system.
 


Disorder












Entropy
 ΔS
=
Sfinal
–
Sini1al















Units
=
J
K‐1
mol‐1



Factors


Reason





temp



entropy
 The
higher
the
temperature,
the
more
energy
the
 system
possesses.
There
are
more
ways
to
 distribute
the
energy
“packets”
among
the
 par.cles
now.
As
a
result,
the
system
becomes
 more
disordered,
entropy
increases.

 Change
in
phase
 The
change
from
solid
to
liquid
to
gas
involves
 Solid


Liquid


Gas
 molecules
having
a
more
disordered
or
random
 




Entropy
 arrangement
within
the
chemical
system.
Hence
 entropy
increases
(ΔS
>
0).
Increase
in
entropy
 from
liquid
to
gas
is
greater
than
for
solid
to
liquid
 as
it
has
a
greater
increase
in
volume.

 

No.
of
par.cles
 

Entropy


The
greater
the
number
of
molecules,
the
more
 random
and
disordered
the
system
becomes.
 There
are
more
ways
of
arranging
the
molecules.
 Hence
entropy
increases
when
the
number
of
gas
 molecules
increases.



Factors


Reason


Mixing
of
 par.cles

 (Liquids,
 Gases)
 




Entropy



When
gases
mix,
each
gas
expands
to
occupy
the
whole
container,
 leading
to
higher
state
of
disorder.
There
are
now
more
ways
of
 arranging
the
gaseous
par.cles
within
the
system.
Hence
entropy
 increases.

 The
same
reason
(for
gases)
applies
to
the
mixing
of
liquids
with
 similar
polari.es.
Hence,
entropy
increases.


Dissolving
 When
an
ionic
solid
dissolves
in
water,
the
ions
experience
a
great
 of
Solids
 increase
in
disorder
as
they
break
away
from
the
solid
laZce,
 leading
to
increase
in
entropy.
However,
the
water
molecules
 Generally,
 become
more
ordered
around
the
ions
and
experience
a
decrease
 





Entropy

 in
the
disorder,
causing
a
decrease
in
entropy.
For
most
ionic
 solids
that
are
soluble
in
water,
the
disordering
processes
are
 usually
dominant.
Thus
the
overall
effect
is
an
increase
in
disorder
 and
entropy
of
the
system
upon
dissolving
in
water.

 




in
 volume
of
 gases
 




Entropy


A
gas
spontaneously
expands
to
occupy
the
whole
container.
 Expansion
of
volume
leads
to
increase
in
entropy.
This
is
because,
 in
a
larger
volume
of
space,
there
are
more
ways
for
the
 molecules
to
arrange
themselves.



ΔG
=
ΔH−
TΔS

 where
T
=
temperature
(in
Kelvin)

 ΔG
is
the
standard
free
energy
 change
needed
to
convert
reactants
 into
products
at
standard
condi.ons.

 If
ΔG

0,
reac.on
cannot
take
place
spontaneously.


 If
ΔG
=
0,
reac.on
reaches
equilibrium.

 The
temperature
at
which
the
reac.on
turns
 from
non‐spontaneous
to
spontaneous
can
be
 determined
by
seZng
ΔG
=
0
and
solving
for
T.



ΔH



ΔS


Examples


nega.ve
 posi.ve
 These
reac.ons
are
feasible
at
all
temperatures.
Reactants


are
metastable
under
all
condi.ons
and
only
exist
because
 the
ac.va.on
energy
of
the
reac.on
is
so
high.

 e.g.
organic
combus.on
reac.ons,
explosives,
some
 decomposi.on
reac.ons



posi.ve
 nega.ve
 These
reac.ons
are
not
feasible
and
have
to
be
driven.



e.g.
Photosynthesis


posi.ve
 posi.ve
 These
are
endothermic
reac.ons
which
may
not
be
feasible


at
room
temperature
but
will
become
feasible
if
the
 temperature
is
high
e.g.
mel.ng,
boiling,
decomposi.on
 reac.ons,
electrolysis,
dissolving
(in
some
cases).
 As
T
increases,
the
“–TΔS”
term
becomes
large
enough
to
 make
ΔG
nega.ve.



nega.ve
 nega.ve
 These
are
exothermic
reac.ons
which
are
feasible
at
low


temperatures
e.g.
condensa.on,
freezing,
combina.on
 reac.ons,
electrochemical
cells,
precipita.on.
At
low
 temperatures,
the
“–TΔS”
term
becomes
smaller
than
the
 ΔH
term
which
is
nega.ve,
in
order
to
make
ΔG
nega.ve.