Hey Guys! Here is your summary for
AQA Chemistry: Chemical Changes.
Now, if you want to get a checklist so we can go through
everything together, you can get the free one from my website (www.primrosekitten.com)
or you can get it over on Amazon.
We can list the metals by how reactive they are, with
the most reactive being at the top
and the least reactive
being at the bottom
Now, you need to remember these, if you have any good mnemonics
remembering these you can pop those in
the comments below, that will really, really help other people.
Things that are above carbon
need electrolysis
to be extracted. Whereas, things that are below carbon
can just be extracted by
reduction. However, things that are really, really unreactive
like silver, gold and copper are generally
found in the earth
as their pure ores,
unreacted with anything. Everything else
is generally going to be reacted with
oxygen, in the form
of metal oxides.
You can also use this to predict the products
from electrolysis. If the metal you are
using in the electrolysis
is more reactive than hydrogen, then you are going to get
hydrogen as a gas. If it is less
reactive, then you are going to get something else as a gas.
And we can use this to predict the products for
displacement reactions.
If we reacted magnesium chloride with calcium,
because calcium is more reactive
than the magnesium, the calcium is going to take the place
so we are going to get calcium
calcium chloride + magnesium
as our products.
However, if we reacted magnesium chloride
with aluminium, because magnesium is more reactive,
aluminium cannot take the place, it wil
not displace it, so no reaction is going to take place.
When you have a redox reaction, oxidation
is loss
of electrons;
Reduction
is gain
of electrons. (OILRIG)
A good way to remember what the electrodes are called is that the
positive
electrode is the anode
and negative
is
cathode. (PANiC)
At each electrode in electrolysis, we are going to
have oxidation and reduction taking place
and movement of electrons.
And the half-equations need to reflect this and they need to be
balanced.
The first thing we need to balance are the elements.
In the first one we have copper.
One on each side, that's
fine. But here we have 2+ charge, we need to make a
neutral charge.
The only thing we can add in is electrons,
which have a negative charge.
Because copper is 2+, we need to add in
two electrons. We are adding in electrons,
this is gain of electrons, so this is reduction.
And because copper is positive, it will go to
the negative electrode,
which is the cathode.
The second one is a bit more complicated because you can see
fluorine ion will go to a diatomic
fluorine molecule. First thing we need to do is to
balance the fluorines. Two go in there.
Now we need to balance the
atoms. We have two negative, that needs to go to a neutral.
So we need to lose something,
the only thing we can lose are electrons
and to balance out the charges, we need to lose two electrons.
This is loss of electrons,
so it is oxidation
Fluorine is negative so it will go to the positive
electrode,
and the positive electrode is the
anode.
You need to remember all of the equations,
remember the ions and be able to work out what is going
to come from a reaction.
So Acid + Metal --> Salt + Hydrogen
Acid + Metal Oxide --> Salt + Water
Acid + Metal hydroxide --> Salt + Water
Acid + Metal base --> Salt + Water
Acid + Metal Carbonate --> Salt + Water + Carbon Dioxide
To work out the formula of the salts, you need to know the formula of
all of your ions.
I've made flashcards
to help you with this,
you can watch the video, I'm afraid you're going to need to watch it over and over again
so that you learn it.
And then you're going to need to make sure you that
combine the ions in such as way that they are all neutral
overall.
For making a pure salt, we are going to be making copper sulfate.
This is mixed in sulphuric
acid and copper oxide
to make copper sulfate and water (Sulphuric acid + Copper oxide --> Copper sulphate + Water.
You're going to need to heat the sulphuric acid, stir in
the copper oxide, which is a black powder until it is in excess,
which basically means you can't dissolve it anymore.
Let it cool a bit and then you can
filter the solution to remove the excess copper oxide,
so that the black copper oxide powder will
stay in the filter paper. And then the
solution of copper sulphate will come out
down the bottom.
Once you have your solution of copper
sulphate, you can evaporate away the water,
to leave you with copper sulphate crystals.
Now this, the size of the crystals, will depend on how
quickly you do this. You're going to be left with
blue crystals.
The blue crystals here are the hydrated ones
and the white crystals around the edge are
the anhydrous ones.
On the pH scale, things that have a ph1
are acidic. pH7
is neutral and pH14
is an alkali.
The ions responsible for acidity are
hydrogen ions. The ions responsible for alkalinity are
are hydroxide ions.
The neutralisation equation
is incredibly important, it comes up alot,
and that tells us that hydrogen ions
hydrogen ions + hydroxide ions
can be neutralised to produce water.
To carry out a titration, pipette 25 cm3 (cm cubed)
of an alkali into a conical flask.
Add a phenolphthalein indicator or a methyl orange indicator.
Fill a burette with an acid of known concentration.
Take the initial reading on the burette and record it, while swirling
the flask, add drop by drop, using the tap,
acid into the alkali. When the first
permanent colour change, in this case, pink to clear happens, stop
adding acid. Record the final volume in the
burette and repeat this
until you get titres which are within the
0.05 cm3 (cm cubed).
There are two indicators you can use for titrations:
Phenolphthalein (which is the one you're seeing here), which
in an alkali, will be bright pink.
And in an acid will be clear or colourless.
Or Methyl Orange, which in an
alkali, you can see it is going this yellowy
colour.
And in an acid will be bright red.
Giving us a neutralisation point where it is an orange colour.
For titration calculations, we first need to
calculate the number of moles of acid used. We can use
this to find the number of hydrogen ions used in the reaction.
This is going to be equal to the number of hydroxide ions at the point
of neutralisation. We can use this
to calculate the number of moles of alkali used to concentrate
the calculation of acid.
We have 25 cm3 (cm cubed)
of alkali, which was
neutralised by 15 cm3 (cm cubed) of 0.2 moles of acid,
find the concentration of the alkali.
First thing I'm going to do is pull all the information out of
the question. Concentration of alkali is what we're trying
to find. Volume of the alkali = 25 cm3 (cm cubed).
Concentration of the
acid = 0.2 mol/dm3 (dm cubed).
Volume of the acid =
15 cm3 (cm cubed).
So the first thing we do is calculate the number of moles of acid
used. So if the number of moles
for acid used, we're going to use Concentration
of acid x Volume of the acid
0.2 x (15 ÷ 1000)
because we need it in dm3 (dm cubed)
0.2 x 0.015 = 0.003 mol.
If we look at our balanced equation, we can see that
acid and
alkali are in a 1:1 ratio in this equation.
So there is going to be an equal number of hydrogen
and hydroxide ions.
So we know that our moles of acid
are 0.003 moles, which means
are moles of alkali must also be 0.03 moles.
Now we know
the number of moles of alkali, we can use the concentration of our volume again,
rearranging that
because we know the moles and we know the volume
to find the concentration.
We can use Moles = Concentration x Volume again
and rearranging that because we know our moles
and we know our volume.
So Moles ÷ Volume = Concentration
So our moles we've just worked out
is 0.003.
Our concentration is 25 cm3 (cm cubed)
Divided that by 1000 to get
it in dm3 (dm cubed), so that it's to be
0.003 ÷ 0.025 = 0.12 mol/dm3 (dm cubed)
as our concentration of alkali.
There is a big difference between strength
and concentration. Strong acids are going to fully
dissociate into hydrogen ions and other ions.
The strong acids are: Hydrochloric acid,
Nitric acid, Sulphuric acid, Hydrobromic acid,
Hydroiodic acid and Chloric (VII) acid.
I would expect you to know that hydrochloric acid
is HCl, Nitric acid is
HNO3
and Sulphuric acid
is H2SO4. The other ones we don't have to worry about too much.
Everything else is all weak acid, which
means only partially dissociates.
Here we have strong and weak acids at
high and low concentrations. So for our strong
acid we can see our hydroxide ions and our hydrogen
ions are fully dissociated.
They're not touching each other - they are separated.
Here we have them at a high concentration, which means there are lots
of hydroxide and hydrogen ions,
compared to very few water molecules.
Here we have our strong acid, again fully
dissociated but at a low concentration,
meaning there aren't very many
hydrogen or hydroxide ions in a lot of water.
For our weak acids, they are only
partially dissociated so some of the hydrogen
hydroxide ions are separated and some of them
haven't,
meaning that we are going to get some which are still together
and some that are separated. At a high concentration,
there are going to be lots of acid particles
for very few particles of water, whereas
at a low concentration there aren't going to be
very many acid molecules
per molecule of water.
Here we have sodium chloride. Now, ionic compounds
have to be molten or dissolved
to be able to conduct electricity, because it's when it's
in its solid state, you can see that this sodium
and these chlorine's are not going anywhere - they are very, very
fixed.
However, in a liquid or a
molten or a dissolved state, when these ions are
free to move around, that is when
they are going to be conducting electricity, and that is when you can do electrolysis.
Aluminum electrolysis
is a slightly different form of electrolysis, we have one electrode
up here, this is our positive anode and
another down here, this is our negative cathode.
The molten aluminium and the cryolite
Cryolite - a compound that is added to reduce the melting point of molten aluminium oxide.
It is added into this reaction vessel
and we get one reaction taking place down here and another reaction
taking place at the top.
At the bottom, at the negative cathode,
we are going to be attracting the positive
aluminium ions, they are going to be picking
up electrons and turning them into aluminium
atoms. This is 3+,
so it needs to pick up three electrons.
And then at the top, at the carbon electrode,
we are going to attract the negative
oxygens, they are going to be
losing electrons and turning into oxygen
gas.
Because we have on this side, two oxygens on that side, we need
two on that side,
which means we now have four negative charges, so we need to lose
four electrons as well.
This is a carbon electrode
up here and we are starting a
reaction which causes oxygen gas to be evolved.
Eventually, the oxygen gas will react with
the carbon electrode and we are going to lose the electrode
as carbon dioxide. So the carbon
dioxide will wear
away the electrode eventually, so this will need to be replaced.
The molten aluminium collects at the bottom
and can be taken off like that and
that is how we purify aluminium.
The common setups for electrolysis that you need to know are
sodium chloride, sodium sulphate, copper chloride and
copper sulphate. For sodium chloride, the
products you are going to get are hydrogen gas,
chlorine gas
and sodium hydroxide.
For sodium sulfate, the products you are going to get
are going to be hydrogen and
oxygen gas. For copper chloride,
you are going to get copper
and chlorine gas.
And for copper sulphate, you are going to get
copper and oxygen gas
When we set up electrolysis, you
need positive and negative electrodes, lightbulb there just to check
that electricity is flowing. You can see bubbles
collecting around the positive and negative electrodes,
sometimes this might be a metal collecting,
as in the case of
copper collecting here and here,
in copper sulphate and copper chloride.
You can test for all of the different gases
coming off, for example: hydrogen, chlorine
and oxygen.
The test for hydrogen gas is a squeaky pop.
The test for oxygen gas is relighting a glowing splint.
And the test for chlorine gas is that it bleaches damp, litmus paper.