Formation of Salts by REDOX-Reaction
(Introduction)
Definition
Reactions by which electrons are transferred
Origin of the term
Redox = reduction and oxidation
The two always take place together.
Oxidation: classical meaning: Reaction of an element with
oxygen
E.g. Oxidation of C to CO2, H2 to H2O, iron to iron-oxide
(rust)......
However, the concept oxidation is used more widely:
e.g., With the formation of NaCl from sodium and
chlorine
one says that sodium is oxidized,
meaning that electrons are taken away from sodium, or in
other words: sodium donates electrons.
Chlorine is reduced, meaning that electrons are given to
chlorine, or in other words: chlorine accepts electrons.
In some books one finds the following definitions:
- Oxidation means electron release/donation
- Reduction means electron acceptance
Non-metals generally oxidize metals because non-metals
have the higher electronegativity (EN)
Formation of Salts by REDOX-Reactions
(Electron transfer reactions)
At the example of the synthesis of cooking salt
Step by step derivation of the reaction equation:
Half reactions
Na -------------> Na+ + e-
x2
Oxidation: (Electron release)
Sodium-metal is oxidized to Na+ions.
Cl2 + 2 e-----> 2 Cl-
Reduction: (Electron acceptance)
Cl2 gas is reduced to Cl- ions.
------------------------------------------------------ --------------------------------------------2 Na + Cl2 (+ 2 e-) ----> 2 Na + + 2 Cl-(+ 2 e-)
Ion equation
2 Na + Cl2 -----> 2 NaCl
Material equation
Definitions
Reducing-agents reduce other materials.
I.e.(this means) they are electron donators and are oxidised themselves.
Oxidising-agents.......
Which is the reducing agent and which is the oxidizing agent with the reaction above?
Experiment: Combustion of magnesium
Observation:
ENERGY
It burns with a bright flame - brighter and hotter than the reaction
of sodium + chlorine
Conclusion: The released lattice energy of the reaction of
magnesium + oxygen is bigger than the one of Na + Cl2.
Explanation: Compare both ions.
Ion charge: Mg2 +> Na+
O2-> ClIon size:
Mg2 + <Na+
O2-< ClBoth criteria indicate that MgO should have the larger lattice
energy.
PRODUCT
a white powder, dissolves in water - 2 material classes are
possibly; conducts electricity in solution => salt-like material
Which ions are produced?
Mg ----> Mg2+ + 2 e-
x2
O2 + 4 e- ----> 2 O2-
---------------------------------2 Mg + O2 ----> 2 Mg2+ + 2 O2-
Half reactions
Oxidation: e- donation
Mg metal is oxidized to
Mg2 + ions.
Reduction: e- acceptance
O2 gas is reduced to O2 ions.
Ion equation
2 Mg + O2 ----> 2 MgO
Material equation
Mg acts as a reducing agent, O2 as an oxidizing agent.
Experiment: Aluminium and Bromine
Observations (focus on energy)
Product: a white smoke, therefore a white powder
The released lattice energy of this reaction is between the ones of
Na + Cl and Mg + O2,
because Br- is large but only weakly charged, whereas Al3+ is
small and highly/strongly charged.
Half reactions
Al ----> Al3+ + 3 eBr2 + 2 e- ----> 2 Br-
x2
x3
Oxidation: e- donation
Al-metal is oxidized to Al3 +
ions
Reduction: e- acceptance
Br2 is reduced to Br- ions.
-------------------------------2 Al + 3 Br2 ----> 2 Al3+ + 2 Br-
Ion equation
2 Al + 3 Br2 -----> 2 AlBr3
Material equation
Remark:
Non-metals form 2-atomic ions, except phosphorus forms P4- and
sulphur forms S8-molecules.
Different Reaction Rates of the three states of aggregation
Liquid bromine is more reactive than the gaseous chlorine or
oxygen because its smallest particles are much denser than with
gases.
At the same time liquids more reactive than solids because their
smallest particles are able to move freely in contrast to solids,
where the smallest particles are confined and only vibrate at their
lattice positions.