chemical reactions
+
+
Every process that is connected to
the reorganization of chemical
bonds is called a chemical reaction.
Chemical reactions are often named
according to the predominant changes
that are induced, e.g. oxidation,
reduction, addition etc.
examples for chemical reactions
Common representation:
2H2 + O2
2Na + 2H2O
H2O + CO2
2H2O
2NaOH + H2
H2CO3
Most chemical reactions are reversible
and therefore lead to a chemical
equilibrium (symbolized by double arrows):
H2O + CO2
H2CO3
important types of chemical reactions
Acid-base-reaction: a reaction where a H+-ion is exchanged
between two partners
Oxidation:
one or several electrons are removed
from an element
Reduction:
one or several electrons are added
to an element
Redox-reaction:
oxidation of one element together with
a reduction of another
Acid-base-reaction:
a reaction where a H+-ion is
exchanged between two partners
acid
H+
+
base
+
H+
examples:
HCl + OH2 CH3-COOH + CO32-
HCl + NH3
Cl- + H2O
2 CH3-COO- + H2CO3
Cl- + NH4+
Acid-base-reaction:
The acidity of an aqueous solution of
an acid is measured by the pH-value:
HX + H2O
X- + H3O+
pH = - lg c(H3O+)
in words: the pH-value is the negative decadic logarithm of the
concentration of H3O+-ions in water.
Acid-base-reaction:
The pH-value usually varies on a scale
between 0 and 14, with pH = 7 being
neutral:
acidic
0
neutral
3.5
7
basic
11.5
14
Redox-reaction:
a reaction where an electron is
exchanged between two partners
e-
+
+
oxidized
ereduced
examples:
Zn + Cu2+
2Cu + O2
2H2 + O2
Zn2+ + Cu
2CuO
2H2O
X
(all values in V against H2)
Li
K
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
lithium
potassium
calcium
sodium
magnesium
aluminum
manganese
zinc
cromium
iron
- 2.96
- 2.92
- 2.76
- 2.71
- 2.34
- 1.33
- 1.10
- 0.76
- 0.51
- 0.44
H2
Xn+
low tendency
high tendency
The tendency of elements
to deliver an electron can
be measured by their
voltage against a
hydrogen electrode:
Cd
Co
Ni
Sn
Pb
H2
Cu
Ag
Hg
Au
Pt
H+
cadmium
cobalt
nickel
tin
lead
hydrogen
copper
silver
mercury
gold
platinum
- 0.40
- 0.28
- 0.23
- 0.16
- 0.12
0
0.34
0.79
0.85
1.36
1.60
chemical reactions for energy storage
Chemical reactions can be used to
store and to supply energy.
The amount of energy is determined
by the enthalpy change ∆H (in kJ/mol)
negative ∆H:
positive ∆H:
energy is supplied
energy is stored
energy storage: photosynthesis
terrestrial
J
surface
p.a.
8.1023
6 CO2 + 6 H2O
1021 J
hν
biomass
15.1010t
C6H12O6 + 6 O2
consumption of chemical energy
1021 J
biomass
15.1010t
C6H12O6 + 6 O2
primary
combustion
CO2
dissimilation
fossile
energy sources
mineral oil,
coal, etc.
6 CO2 + 6 H2O
combustion of fossile energy sources
biomass
15.1010t
primary
combustion
dissimilation
fossile
energy sources
CO2
mineral oil,
coal, etc.
C8H18 + 25/2 O2
C
+ O2
secondary
combustion
CO2
8 CO2 + 9 H2O
CO2
other sources of chemical energy
4 C + S + 20 KNO3
4 CO2 + SO2 + 20 NO + 10 K2CO3
black powder
H2NN(CH3)2 + 2 N2O4
3 N2 + 2 CO2 + 4 H2O
self-igniting rocket fuel
4 Al + 3 MnO2
2 Al2O3 + 3 Mn
“manganese thermite”
3 Mg + KClO3
3 MgO + KCl
old-fashioned photographic flash powder
235
92
U + 10 n
nuclear fission of uranium-235:
X + Y + 2 10n
∆H = -190.106 kJ/Mole
mass-reduction connected to chemical reactions
∆E = ∆m c²
∆m = ∆E / c²
H
example:
2 H.
H2
H
∆G = -406 kJ/Mol
∆H = -431 kJ/Mol
∆m = E / c² = -4,79.10-9 g
(that is: a recombination of 400 t of atomar hydrogen
would lead to a loss of 1 g of total mass).