Chemistry Reference
In-Depth Information
Enthalpy of neutralisation (molar
heat of neutralisation)
This is the enthalpy change that takes place when
1 mol of hydrogen ions (H
+
(
aq
)) is neutralised.
H
+
(
aq
) + OH
−
(
aq
) → H
2
O(
l
) Δ
H
= −57 kJ mol
−1
This process occurs in the titration of an alkali by
an acid to produce a neutral solution (Chapter 8,
p. 124).
activation
energy (
E
A
)
CH
4
(g)
2O
2
(g)
H
CO
2
(g)
2H
2
O
(l)
progress of reaction
Figure 6.20
Energy level diagram for methane/oxygen.
Endothermic reactions are much less common than
exothermic ones. In this type of reaction energy is
absorbed from the surroundings so that the energy
of the products is greater than that of the reactants.
The reaction between nitrogen and oxygen gases
is endothermic (Figure 6.21), and the reaction is
favoured by high temperatures.
nitrogen + oxygen → nitrogen(
ii
) oxide
Questions
1
Using the bond energy data given in Table 6.3:
a
Calculate the enthalpy of combustion of ethanol, a fuel
added to petrol in some countries.
b
Draw an energy level diagram to represent this
combustion process.
c
How does this compare with the enthalpy of combustion
of heptane (C
7
H
14
), a major component of petrol, of
−4853 kJ mol
−1
?
d
How much energy is released per gram of ethanol and
heptane burned?
N
2
(
g
) + O
2
(
g
) → 2NO(
g
) +Δ
H
2
How much energy is released if:
a
0.5 mole of methane is burned?
b
5 moles of methane are burned?
c
4 g of methane are burned?
2NO
(g)
E
A
(
A
r
: C
=
12; H
=
1)
3
How much energy is released if:
a
2 moles of hydrogen ions are neutralised?
b
0.25 mole of hydrogen ions is neutralised?
c
1 mole of sulfuric acid is completely neutralised?
H
N
2
(g)
O
2
(g)
progress of reaction
Figure 6.21
Energy level diagram for nitrogen/oxygen.
●
Changes of state
In Chapter 1, p. 4, we discussed the melting and
boiling of a substance. The heating curve for water
is shown in Figure 1.11 on p. 5. For ice to melt
to produce liquid water, it must absorb energy
from its surroundings. This energy is used to break
down the weak forces between the water molecules
(intermolecular forces) in the ice. This energy is
called the
enthalpy of fusion
and is given the symbol
∆
H
fusion
. Similarly, when liquid water changes into
steam, the energy required for this process to occur
is called the
enthalpy of vaporisation
and is given
the symbol
∆
H
vap
. Figure 6.22 shows an energy
level diagram representing both the fusion and the
vaporisation processes.
Dissolving is often an endothermic process. For
example, when ammonium nitrate dissolves in
water the temperature of the water falls, indicating
that energy is being taken from the surroundings.
Photosynthesis and thermal decomposition are
other examples of endothermic processes.
In equations it is usual to express the
∆
H
value in
units of kJ mol
−
1
. For example:
CH
4
(
g
) + 2O
2
(
g
) → CO
2
(
g
) + 2H
2
O(
l
)
Δ
H
= −728 kJ mol
−1
This Δ
H
value tells us that when 1 mole of methane
is burned in oxygen, 728 kJ of energy are released.
This value is called the
enthalpy of combustion
(or
molar heat of combustion
) of methane.
