# Single Sex School

Topics: Ethanol fuel, Energy development, Enthalpy Pages: 36 (1207 words) Published: March 14, 2013

6.1

The
Nature
of
Energy
6.2

Enthalpy
and
Calorimetry
6.3

Hess’s
Law
6.4

Standard
Enthalpies
of
FormaCon
6.5

Present
Sources
of
Energy
6.6

New
Energy
Sources

30

STANDARD ENTHALPIES OF FORMATION

Cgraphite(s)

Cdiamond(s)

ΔH
for
this
process
cannot
be
obtained

by
direct
measurement
in
a
calorimeter
but
can
be
calculated
from
heats
of
combusCon.

CH4(g)
+
2O2(g)

CO2(g)
+
2H2O(l)
ΔH°
for
this
process
can
be
calculated

from
the
standard
enthalpy
of
formaCon
(ΔHf°).

In
Appendix
4

31

STANDARD ENTHALPIES OF FORMATION

The
standard
enthalpy
of
forma1on
of
a
compound
is

the
change
in
enthalpy
that
accompanies
the
formaCon
of

one
mole
of
a
compound
from
its
elements

with
all
substances
in
their
standard
states.

1/2N2(g)
+
O2(g)

NO2(g)

ΔHf°
=
34
kJ/mol

ΔHf°
=
-­‐239
kJ/mol

C(s)
+
2H2(g)
+
1/2O2(g)

CH3OH(l)

In
Appendix
4

32

STANDARD ENTHALPIES OF FORMATION
Conven1onal
Deﬁni1ons
of
Standard
States

For
a
Compound,
  The
standard
state
of
a
gaseous
substance
is
a
pressure
of
exactly
1
atm.
  For
a
pure
substance
in
a
condensed
state
(liquid
or
solid),
the
standard
state
is
the
pure
liquid
or
solid.
  For
a
substance
present
in
a
solu>on,
the
standard
state
is
a
concentra>on
of
exactly
1
M.

For
an
Element,
  The
standard
state
of
an
element
is
the
form
in
which
the
element
exists
under
condi>ons
of
1
atm
and
25°C.

The
standard
state
of
oxygen
is
O2(g)
at
a
pressure
of
1
atm

The
standard
state
for
sodium
is
Na(s)

The
standard
state
for
mercury
is
Hg(l)
C(s)
+
2H2(g)

CH4(g)

1/2N2(g)
+
O2(g)

NO2(g)

H2(g)
+
1/2O2(g)

H2O(l)
C(s)
+
2H2(g)
+
1/2O2(g)

CH3OH(l)

33

STANDARD ENTHALPIES OF FORMATION
Examples
of
standard
enthalpies
of
formaCon
standard
state

1/2N2(g)
+
O2(g)

NO2(g)
( (

)
+