3c. Metal Displacement Reactions
(metal + salt solution)
-
A displacement reaction is
where a more reactive element displaces a less reactive element.
-
Displacement reactions involve a
simultaneous oxidation and reduction, which is why they are
called redox reactions.
-
In this experiment it is
important to understand the 'electron' definitions of oxidation and
reduction.
-
Oxidation is electron loss
- the atom/ion/molecule losing one or more electrons is said to be
oxidised.
-
Reduction is electron gain - the
atom/ion/molecule gaining one or more electrons is said to be
reduced.
-
You must learn these definitions!
-
There are lots more examples on
my
Introducing REDOX reactions
page.
-
All you need is a
selection of metals, salt solutions of the same metals (eg.
chloride, nitrate or sulphate) and rack of test tubes.
-
The physical state of
the metal in terms of granule size or area of sheet doesn't matter
in this experiment.
-
You just pop the bits of metals
into the solutions and carefully observe what happens on the
surface.
-
See also
metal extraction experiments in section 3(d)
3c. Results: Table of possible
observations
salt\metal |
aluminium film |
copper strip |
iron filings |
lead strip |
magnesium ribbon |
zinc granules |
aluminium
chloride |
no
reaction - same metal |
no reaction |
no reaction |
no reaction |
You may see a
slight change on the Mg surface |
no reaction |
copper sulphate |
faint pink coating
of copper |
no
reaction - same metal |
pink-orange-brown-dark? layer of copper on the iron filings,
blue colour fades |
You may see a
pink-orange-brown-dark? layer of copper on the lead strip, blue
colour fades |
pink-orange-brown-dark? layer of copper on the magnesium strip,
blue colour fades |
pink-orange-brown-dark? layer of copper on the zinc granules,
blue colour fades |
iron(II) sulphate |
theoretically
reacts - but doubt if you see anything |
no reaction |
no
reaction - same metal |
no reaction |
'dark' crystals
of iron on the magnesium ribbon |
'dark' crystals
of iron on the zinc granules |
lead(II) nitrate |
theoretically
reacts - but doubt if you see anything |
no reaction |
theoretically
reacts - but doubt if you see anything |
no
reaction - same metal |
'dark' crystals
of lead on the magnesium ribbon |
'dark' crystals
of lead on the zinc granules |
magnesium
sulphate |
no reaction |
no reaction |
no reaction |
no reaction |
no
reaction - same metal |
no reaction |
zinc sulphate |
theoretically
reacts - but doubt if you see anything |
no reaction |
no reaction |
no reaction |
'dark' crystals
of zinc on the magnesium ribbon |
no
reaction - same metal |
-
No reaction = no
observed change, in some cases where theoretically there should be a
reaction, you might not see any change (see Al note below).
-
Some of the best observations are
with the more reactive metals than copper.
-
The picture on the right shows
what happens if you put a strip of magnesium in copper sulfate
solution.
-
The solution starts of a deep
blue of the copper sulfate solution.
-
Then the blue colour fades as the
brown copper deposit forms on the magnesium.
-
Eventually most of the magnesium
dissolves and the solution becomes colourless as magnesium sulfate
is formed (which is colourless!).
-
Aluminium, again gives
problems with the observations because of the oxide layer inhibiting
the reaction with the salt solution of a less reactive metal.
-
The displaced copper can
display a variety of colours depending on how the
precipitate-coating forms, and how much of it - I'm afraid that's
the way it is!
-
In some cases the metal
crystals formed by the displacement reaction are very small and
scatter the light so that they can look quite dark - almost black,
rather than a silver-shiny precipitate.
3c. Conclusions and comments
-
Any colour change
indicates a displacement reaction has occurred,
-
AND the rule is that
-
a more reactive metal
will displace a less reactive metal from its salt solution
(irrespective of which salt)
-
The way that you deduce the order
is quite simple and logical.
-
e.g. zinc cannot displace
magnesium, but it can displace copper, and magnesium displaces both
zinc and copper,
-
therefore zinc is less reactive
than magnesium and zinc is more reactive than copper,
-
therefore you can
deduce the
reactivity order for these three metals is
Mg > Zn > Cu.
-
Of the series of metals
tested magnesium appears to be the most reactive because it
displaces all the other metals being investigated here.
-
Copper appears to be
the least reactive in this limited series because it does not
displace any of the other metals.
-
Therefore by considering
what will displace what from the results table, you can quite
legitimately deduce that in terms of reactivity order
-
magnesium >
aluminium > zinc > iron > lead > copper
-
BUT the observations
for aluminium and lead may not show up clearly.
-
What you can
definitely deduce from the observations in a more restricted experiment is the
reactivity series order
-
magnesium > zinc
> iron > copper
-
There can be problems in
observations of the reactivity of aluminium and lead due to a
protective oxide layer.
-
Examples of
displacement equations
-
The first metal on the
left is the most reactive, and the last metal on the right, is the
displaced less reactive metal.
- magnesium + copper
sulphate ==> magnesium sulphate + copper
- Mg(s) + CuSO4(aq)
==> MgSO4(aq) + Cu(s)
- The blue of the copper sulphate solution
fades as colourless magnesium sulfate is formed and the brown deposit of
copper increases on the surface of the magnesium.
- -
- zinc + copper
sulphate ==> zinc sulphate + copper
- Zn(s) + CuSO4(aq)
==> ZnSO4(aq) + Cu(s)
- -
- aluminium + copper(II) sulphate ==>
aluminium sulphate + copper
-
Again, these reactions can be
seen as redox reactions i.e. an oxidation-reduction reaction in
terms of electron loss and gain.
-
Oxidation is electron loss
and the neutral metal atom loses electrons to form a positive ion
(cation).
-
eg.
Mg
==> Mg2+ + 2e-,
Zn ==> Zn2+
+ 2e- and
Al ==> Al3+
+ 3e-
-
Reminder: The more easily the
electrons are lost, the more reactive the metal.
-
Reduction is electron gain
and the hydrogen ions gain electrons to form a neutral hydrogen
molecule.
-
You can then combine these half
reaction to give the full ionic equation that excludes
spectator ions like the chloride ion, which simply remain in
solution.
-
and for more on
redox
theory behind displacement reactions
3d. Simple Metal
Extraction Experiments (with reducing agents)
4. Action of heat on carbonates
Often, but not always, the more reactive a
metal, the more thermally stable is the metal carbonate i.e.
reactivity trend with water and acids:
sodium > calcium > copper,
thermal stability of carbonate: Na2CO3
> CaCO3 > CuCO3
BUT many carbonates do not fit into such a
sequence, so you can't regards this as a general rule
See notes on the
Limestone: Detailed
section on the thermal decompositions of carbonates page
and
advanced level chemistry
notes on the thermal decomposition of carbonates
OTHER ASSOCIATED PAGE
LINKS

SEE ALSO
2. RUSTING &
Introducing REDOX reactions
and 3.
Metal Reactivity Series
Experiments-Observations
and GCSE/IGCSE m/c QUIZZES on metal
reactivity
Foundation-tier Level
(easier) multiple choice quiz on the Reactivity Series of Metals
or
Higher-tier Level (harder) multiple choice quiz on
the Reactivity Series of Metals
and
GCSE/IGCSE
reactivity gap-fill worksheet or
Rusting
word-fill worksheet
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