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
TOP OF PAGE
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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