Zusammenfassung der Ressource
Electrolysis and Making salts
- Making salts
- Most chlorides, sulfates and nitrates
are soluble in water (with the
exception of lead chloride, lead
sulfate and silver chloride)
- Making soluble salts using a metal or an insoluble base
- Pick the right acid plus a metal or
an insoluble base (metal oxide or
metal hydroxide)
- E.g. if you want copper chloride mix hydrochloric acid and copper
oxide
- CuO (s) + 2HCl (aq) to CuCl2 (aq) + H2O (l)
- Add the metal, metal oxide or hydroxide
to the acid. The solid will dissolve in the
acid as it reacts. Excess solid will sink to
the bottom
- Then filter out the excess metal, metal
oxide or hydroxide to get the salt
solution. To get pure solid crystals of the
salt evaporate some of the water (to
make the solution more concentrated.
Then leave the rest to evaporate. This
is called crystallisation
- Making soluble salts using an Alkali
- You cant use the method above with alkalis (soluble bases) like
sodium, potassium or ammonium hydroxides because you cant tell
whether the reaction has finished
- You have to add exactly the right amount of alkali to just
neutralise the acid. You need to use an indicator to show when
the reaction is finished. Then repeat using exactly the same
volumes of alkali and acid so the salt isnt contaminated so the
salt isnt contaminated with indicator. Then just evaporate of
the water to crystallise it
- Making insoluble salts. Precipitation reactions
- If the salt you want to make is insoluble you can use a precipitation reaction
- You just need two solutions that contain the ions you need
- E.g. to make lead chloride you need a solution
which contains lead ions and one which contains
chloride ions. So you can mix lead nitrate
solution with sodium chloride solution ( all group
1 compounds are soluble)
- Pb(NO3)2 (aq) + 2NaCl (aq) to PbCl2 (s) + 2NaNO3 (aq)
- Once the salt has precipitated out (and is at the bottom of the
flask) all you have to do is filter it from the solution wash it then dry
it on filter paper. Precipitation reactions can be used to remove
poisonous ions from drinking water. Calcium and magnesium ions
can also be removed from water this way they make water hard.
Another use of precipitation is in treating effluent (sewage)
unwanted ions can be removed
- Electrolysis
- Electrolysis means splitting up with electricity
- If you pass an electric current
through an ionic substance thats
molten or in solution it breaks down
into the elements its made of.
- Electrolytes contain free ions there
usually the molten or dissolved ionic
substance
- Its the free electrons that carry electricity
- For an artificial circuit to be
complete theres got to be a flow of
electrons. Electrons are taken
away from ions at the positive
electrode and given to other ions at
the negative electrode. As ions
gain or lose electrons they become
atoms or molecules and are
released
- It requires liquid to conduct the electricity called the electrolyte
- Electrolysis reactions always
involve oxidation and
reduction
- Reduction is the gain
of electrons or the
loss of oxygen
- Oxidation is a
gain of oxygen or
a loss of
electrons
- Doesnt have to involve oxygen
- Electrolysis of molten lead bromide
- Electrolysis of sodium Chloride solution
- Reactivity affects the products formed by electrolysis
- Sometimes there are more than two free ions in the electrolyte
- E.g. if a salt is dissolved in water there will be some H+ and OH- ions
- At the negative electrode if metal ions
and H+ ions are present the metal ions
will stay in solution if the metal is more
reactive than hydrogen. This is because
the more reactive an element the
keener it is to stay as ions. So hydrogen
will be produced unless the metal is
less reactive in it
- At the positive electrode if OH-
and halide ions (Cl- , Br-, I-) are
present then molecules of
chlorine bromine or iodine will be
formed. If no halide is present
then oxygen will be formed
- When common salt (sodium
chloride) is dissolved in water
and electrolysed it produces
three products: hydrogen,
chlorine and sodium hydroxide
- At the negative two
hydrogen ions accept two
electrons to become one
hydrogen molecule
- At the positive electrode
two chloride ions lose
their electrons and
become one Chlorine
molecule
- The sodium ions stay in
solution because they are more
reactive than hydrogen.
Hydroxide ions from water are
also left behind. This means
sodium hydroxide is left in one
solution (NaOH)
- The Half equation
- Negative electrode: 2H+ + 2e- to H2
- Positive electrode: 2Cl- to Cl2 +2e-
or 2Cl- - 2e- to Cl2
- Useful Products
- Chlorine for the
production of
bleach and
plasitics
- Sodium hydroxide is a
very strong alkali and is
used widely in the
chemical industry e.g.
soap
- Extraction of Aluminium and Electroplating
- Electrolysis is used to remove Aluminium from
its ore
- Aluminium is a very
abundant metal but it
is always found
naturally in
compounds
- Its main ore is bauxite and after mining and purifying a white powder is left
- This is pure aluminium oxide Al2O3
- Cryolite is used to
lower the temperature
(and cost)
- Al2O3 has a very high melting
point at 2000 degrees. So
melting would be expensive
- Instead the aluminium
oxide is dissolved in
molten cryolite ( a less
common ore of
aluminium)
- This brings the temperature
down to 900 degrees which
makes it easier and cheaper
- The electrodes are made of
carbon (graphite) a good
conductor of electriciry
- Aluminium forms at the
negative electrode and oxygen
forms at the positive electrode
- Negative electrode: Al 3+ + 3e- to Al
- Positive electrode: 2O 2- to O2 + 4e-
- The oxygen then reacts with the carbon in the electrode to produce carbon dioxide.
This means that the positive electrodes gradually get eaten away and have to be
replaced every now and again
- Electroplating uses electrolysis
- Electroplating uses electrolysis
to coat the surface of one metal
with another metal e.g. you
might want to put electroplate
silver onto a brass cup to make
it look nice
- The negative electrode is the metal
object you want to plate and the positive
electrode is the pure metal you want it to
be plated with. You also need the
electrolyte to contain ions of the plating
metal. (The ions that plate the metal
object come from the solution while the
positive electrode keeps the solution
topped up)
- Uses for electroplating
- Decoration: Silver is attractive
but very expensive. Its much
cheaper to plate a boring brass
cup with silver than it is to make
the cup out of solid silver
- Conduction: Metals like
copper conduct electricity
because of this they are
often used to plate metals
for electronic circuits and
computers