Chemistry Reference
In-Depth Information
C
H
HHH
BREAK BONDS
takes in energy
OOO
O
H
MAKE NEW BONDS
gives out energy
O
O
C
O
O
H
H
H
O
H
H
O
C
O
O
H
H
progress of reaction
Figure 6.19
Breaking and forming bonds during the combustion of methane.
Energy difference
=
Using the data in Table 6.3, which tells us how
much energy is needed to break a chemical bond
and how much is given out when it forms, we can
calculate how much energy is involved in each stage.
Table 6.3
Bond energy data.
energy required −
energy given out when
to break bonds
bonds are made
= 2734
− 3462
= −728 kJ
The negative sign shows that the chemicals are
losing energy to the surroundings, that is, it is an
exothermic reaction. A positive sign would indicate
that the chemicals are gaining energy from the
surroundings. This type of reaction is called an
endothermic
reaction.
The energy stored in the bonds is called the
enthalpy
and is given the symbol
H
. The change in
energy going from reactants to products is called the
change in enthalpy
and is shown as Δ
H
(pronounced
'delta H'). Δ
H
is called the
heat of reaction
.
For an exothermic reaction Δ
H
is negative and for
an endothermic reaction Δ
H
is positive.
When fuels, such as methane, are burned they
require energy to start the chemical reaction. This is
known as the
activation energy
,
E
A
(Figure 6.20).
In the case of methane reacting with oxygen, it
is some of the energy involved in the initial bond
breaking (Figure 6.20). The value of the activation
energy will vary from fuel to fuel.
Bond energy/kJ mol
−1
Bond
C
!
H
435
O
R
O
497
C
R
O
803
H
!
O
464
C
!
C
347
C
!
O
358
Bond breaking
Breaking 4 C
!
H bonds in methane requires
4 × 435 = 1740 kJ
Breaking 2 O
R
O bonds in oxygen requires
2 × 497 = 994 kJ
Total = 2734 kJ of energy
Making bonds
Making 2 C
R
O bonds in carbon dioxide gives out
2 × 803 = 1606 kJ
Making 4 O
!
H bonds in water gives out
4 × 464 = 1856 kJ
Total = 3462 kJ of energy
