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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 sulfate) 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 sulfate |
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) sulfate |
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
sulfate |
no reaction |
no reaction |
no reaction |
no reaction |
no
reaction - same metal |
no reaction |
|
zinc sulfate |
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.
TOP of page
OR test yourself with some practice exam questions
Foundation-tier Level
(easier) multiple choice quiz on the Reactivity Series of Metals
Higher-tier Level (harder) multiple choice quiz on
the Reactivity Series of Metals
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 because oxide
layer inhibits the reaction with water, acids or with another
less reactive metal salt solution.
-
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
sulfate ==> magnesium sulfate + copper
-
Mg(s) + CuSO4(aq)
==> MgSO4(aq) + Cu(s)
- The blue of the copper
sulfate solution
fades as colourless magnesium sulfate is formed and the brown deposit of
copper increases on the surface of the magnesium.
- zinc + copper
sulfate ==> zinc sulfate + copper
-
Zn(s) + CuSO4(aq)
==> ZnSO4(aq) + Cu(s)
- aluminium + copper(II)
sulfate ==>
aluminium sulfate + copper
- 2Al(s) + 3CuSO4(aq) ==> Al2(SO4)3(aq)
+ 3Cu(s)
-
See also 'The
Reactivity Series of Metals'
-
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.
-
-
-
2Al(s) + 2Cu2+(aq)
==> 2Al3+(aq) + 3Cu(s) (a bit
tricky to balance!)
-
Note the electrons are not shown
because electrons lost = electrons gained, so cancel out.
-
and for more on
redox
theory behind displacement reactions
TOP of page
OR test yourself with some practice exam questions
Foundation-tier Level
(easier) multiple choice quiz on the Reactivity Series of Metals
Higher-tier Level (harder) multiple choice quiz on
the Reactivity Series of Metals
3d. Simple Metal
Extraction Experiments (with reducing agents)
-
Metals of lower reactivity (below aluminium) can be
displaced-extracted using carbon as the reducing agent.
- Heating oxides or carbonates with
powdered charcoal (mainly carbon)
- If you strongly heat copper(II)
carbonate with finely powdered charcoal (mainly carbon) you can
reddish-brown specks of copper in resulting mixture. The dark
green copper(II) carbonate turns black initially as copper(II)
oxide is formed, but this is then reduced to copper by the
charcoal.
- initially a thermal
decomposition: CuCO3 ==> CuO + CO2
- then the reduction
reaction (O loss):
2CuO + C ==> 2Cu + CO2
- You can of course start the
experiment with copper(II) oxide, but copper carbonate is closer
to the sort of naturally occurring copper ore that is mined.
- You can do a similar experiment
by heating lead(II) oxide with powdered carbon and you can get
silvery lead formed BUT in a fume cupboard please, since lead
fumes are very poisonous!
-
2PbO + C ==> 2Pb + CO2
- Lead is a sufficiently
unreactive metal for carbon to displace it.
- If you heat iron oxides with
powdered carbon nothing happens however strongly you heat the
mixture in a test tube.
- Although carbon is reactive enough to
displace iron, the temperature in the test tube isn't high
enough - you need a blast furnace!
- If you heat the white powder of
aluminium oxide with carbon powder, nothing happens because
carbon isn't 'reactive' enough to displace aluminium i.e.
aluminium is too reactive forming very stable compounds.
-
For more see notes on
Metal Extraction Fe, Cu, Al etc.
3e. Metal displacement reactions with
solid oxides plus reactive metal as the
reducing agent
- Here you can use a more reactive metal
to displace a less reactive one from its oxide.
- Your teacher can demonstrate two
metal extraction displacement reactions.
- Neither are used commercially,
but they do illustrate how chromium and titanium are extracted.
- Both these reactions are very
exothermic - lots of heat released
- The Thermit Reaction
- You ignite a mixture of brown
iron(III) oxide and silvery grey aluminium powder using a
magnesium fuse.
- The mixture goes off like a firework in a shower
of sparks - a VERY exothermic reaction!
- On examining the cold residue
you find a lump of iron and specks of white aluminium oxide.
-
2Al + Fe2O3
==> Al2O3 + 2Fe
- Aluminium is more reactive than
iron and so will displace iron from iron compounds.
- Heating a mixture of
magnesium powder and copper(II) oxide
- Silvery grey magnesium powder is
mixed with black copper oxide powder and heated strongly in a
test tube.
- The mixture glows red hot and on
examining the cold mixture you see specks of white magnesium
oxide and reddish-brown specks of copper.
-
Mg + CuO ==> MgO + Cu
- Magnesium is more reactive than
copper and so will displace copper from copper compounds.
TOP of page
OR test yourself with some practice exam questions
Foundation-tier Level
(easier) multiple choice quiz on the Reactivity Series of Metals
Higher-tier Level (harder) multiple choice quiz on
the Reactivity Series of Metals
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
What next?
Associated Pages
Index of notes on reaction of acids
GCSE
Level (~US grades 8-10) School Chemistry Notes
(students age ~14-16)
Find your GCSE
science course for more help links to revision notes
Advanced Level (~US grades 11-12) pre-university
Chemistry Notes
(for aged ~17-18)
Email doc
brown - comment? query?
OTHER ASSOCIATED PAGE
LINKS
to do with metal reactivity
2. RUSTING &
Introducing REDOX reactions
3. Metal Reactivity Series
Experiments-Observations
and GCSE level
m/c QUIZZES on metal
reactivity
Foundation-tier Level
(easier) multiple choice quiz on the Reactivity Series of Metals
Higher-tier Level (harder) multiple choice quiz on
the Reactivity Series of Metals
GCSE level reactivity gap-fill worksheet
and
Rusting
word-fill worksheet
Other QUIZ links to do with metals
(UK GCSE and ~US grade 9-10 level)
Higher level QUIZ on
Group 1 Alkali Metals and Transition Metals
Lower level QUIZ on
Group 1 Alkali Metals and Transition Metals
Higher level
QUIZ on The relative reactivity and extraction of metals
Lower level QUIZ
on The relative reactivity and extraction of metals
Higher level QUIZ on
Group 1 Alkali Metals, Transition Metals and relative reactivity &
extraction of metals
Lower level QUIZ on
Group 1 Alkali Metals, Transition Metals and relative reactivity &
extraction of metals
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