Electrolysis

Electrolysis is a way of splitting a compound (2 or more elements that are chemically bonded) using electricity. I'm a huge fan of looking at the etymology of scientific words to try and work out what it means, so let's look at this word:

ELECTRO-lysis 

The electro part suggests that we are going to be using electricity (seems sensible enough!)

Electro-LYSIS

The suffix is a little harder to determine a meaning for so let's look at some other words that end in -lysis:

Para-LYSIS, Ana-LYSIS are two reasonably common words. If something is paralysed then it has stopped working, or is broken. When you analyse something you are breaking/splitting it down. So -lysis means "splitting". Putting these together give us a definition: using electricity to split a compound

An example

It's probably easier to start with an example of electrolysis in action before we look at the finer points of this process. We will look at what happens during the electrolysis of water (H2O). Let's think of water as being made from 2 different parts, a H+ part and an OH- part. The diagram below shows how the H+ part joins to the OH- part.

Hopefully you're ok with this so far (take my word for it if you're having any niggling doubts.)

If we could somehow attract the OH- away from the H+ and the H+ away from the OH- then we could split water apart. Put your chemistry brain into gear now and try to work out what could attract OH-, and H+.......

How are you doing? Worked it out yet? What could attract an OH-?? Look at it carefully, it's got a minus after it, because it has a negative charge....yes, that's right; you could use a positive charge to attract it. Same goes for the H+, but the other way round. A negative charge could attract the H+. So we set up an electrolysis cell like this:

Ignore the names of the red bit and blue bit for now, we will come to that later. Notice how the OH- is attracted to the positive side (red) and the H+ is attracted to the negative side (blue). 

As a basic introduction to electrolysis that's about it; it's a way of splitting a compound using electricity. 

Definitions you need to know:

Electrolyte:        the substance that you are electrolysing.

Ion:                     An atom that has charge (either + or -). There are 2 types of ion, a cation and an anion.

Anion:                A Negative ion (see how the A and the N are in the name of the ion?)

Cation:              A positive ion ( say it like this to remember it; the cation is "pussy-tive*")

Electrode:         the positive or negative "thing" that you put into the electrolyte. There are 2 types; anode and                                  cathode but more on that in a min.)

Anode:              This is the electrode that the anion is attracted to, if you can remember that the anion is A                                        Negative ion then you can work out that the anode must be the positive electrode.

Cathode:          This is the electrode that the cation is attracted to. The cation is "pussy-tive" so it must be                                          attracted to the negative electrode, i.e the cathode is negative. 

It is complicated and no one expects you to just understand all of this really quickly. Try to learn the first 5 definitions and then you can work out whether the anode or cathode is positive.

*Apologies to Mr Twist, my Chemistry teacher when I was at school. He taught me that about 20 years ago and it just seems to have stuck for me. I can't take any credit for this cunning Jedi mind trick.

Why would we bother?

The crust of our planet is jammed full of useful things, like metals. some of them can just be found in the ground, exactly how you'd want them, without any other things combined with them. The most unreactive elements tend to be found like this, like gold and silver. We say that they are found native. When I think about this I like to imagine what it would've been like to be human on this planet soon after the planet become hospitable. A little stroll would see you encounter all sorts of different metals just sticking out the ground where they were left after the Earth formed. By the time you'd got back home your pockets would be filled with gold, silver, platinum....and other unreactive metals. 

But not many metals are so unreactive that they are just found lying around. Most metals are found combined with other elements, such as oxygen. For example iron is found as iron oxide and never just as iron on its own. If a rock has enough metal in it to make enough money from extracting it then it is called a metal ore. Just check you're happy with the terms native  and ore.

Ok, so some metals are found native but most are fund as ores. To extract a metal from its ore you need to displace the other bit. For medium reactive metals this is easy enough, you just react it with something like carbon. Take a look at the Reactivity Series to see which metals can be extracted with carbon.

These metals can't be displaced with carbon because they are MORE reactive.

Metals that are less reactive than carbon can be displaced using carbon.

These metals are found native

Which metals do you think we extract using electrolysis?

Any metal that is more reactive than carbon has to be extracted using electrolysis. Let's take a look at Aluminium.

Aluminium

This metal can only be extracted using electrolysis. It is found in the ground in an ore called bauxite.

To extract the aluminium from this rock we first have to heat it until it melts. This requires temperatures of over 2000oC, which is both expensive and difficult to achieve. To reduce costs we add something called cryolite that melts at about 900oC and helps extract the aluminium, at lower cost.

Once the bauxite is melted we can start to electrolyse it (an electrolyte has to be either molten (melted) or dissolved so it has free ions to conduct electricity).

Passing electricity through it means that the Aluminium ions (they're positive) will be attracted to the negative electrode, and the oxygen ions (they're negative) are attracted to the positive electrode. (I've deliberately not used the terms anode/cathode or anion/cation yet, but feel free to work out which is which!)

Ok, so you either haven't got a clue what's going on, or you want to check if you're right. Well the positive ion is the aluminium, so that's the CATion (pussy-tive, remember!). The cation is attracted to the cathode, so the cathode is negative. The aNion is the Negative ion, which is oxygen, is attracted to to the anode, so the anode is positive. 

Downsides of Electrolysis

It is really expensive to run the whole process, you need huge amounts of electricity to split a metal ion from its ore. The electrodes themselves wear down over time and they can be quite expensive, add in the cost of heating the ore until it's molten (or dissolving it in a suitable solvent) and the whole thing becomes incredibly expensive. It's also worth considering the environmental impact, with digging out the ore, transporting it and then using all that electricity. It's no wonder companies prefer to recycle aluminium of possible.

Just in case it wasn't complicated enough....

I won't lie, this bit takes some thinking about to get it sorted in your head. During electrolysis there are 2 reactions happening at the same time (simultaneously, indeed!). One ion loses electrons and the other ion gains electrons. I'm going to give you some definitions at this stage. It should help.

Oxidation iLoss (of electrons)

Reduction iGain (of electrons)

We use the first letter of each of these to come up with a mnemonic to help remember this:

O.i.L.R.i.G

Ok, so that's easy enough(!). One ion is oxidised and one is reduced (i.e one loses electrons and one gains electrons. Another new word for you here; Redox, which is simultaneous reduction and oxidation.

Let's look at this in practice for aluminium oxide.

Aluminium is in group 3 (it is, trust me) so it has a charge of 3+ (all metals have a positive charge).

For aluminium to become an atom it has to gain 3 electrons (electrons are negatively charged).

Oxygen is in group 6 so it has a charge of 2-(again, trust me). For an oxygen ion to become an atom it needs to lose 2 electrons.

Here's that in an equation:

The reaction at the cathode is just like I said, Aluminium ion needs to gain 3 electrons to become an atom. But what on earth is going on with oxygen? Why are there 2 of them, why does it lose 4 electrons? Fear not, I'll explain.

 

Oxygen always forms molecules of O2 (unless it's really made to form molecules of ozone, O3, but let's not worry about that just now). We say that it forms diatomic molecules (di meaning 2), quite a lot of gases do this; nitrogen, hydrogen, fluorine, chlorine....). So we have to make O2 as our product, so we have to have 2 oxygen ions at the start of the equation. Each ion has 2 spare electrons, so in total they lose 4 electrons. I told you this was tough.

Still one last thing to think about. If aluminium is gaining 3 electrons and oxygen is losing 4 electrons then we have a problem We can't just have a spare electron floating around without somewhere to go, so we need to balance out the number of electrons. To do this we have to multiply the number of each ion until the number of electrons is equal. The smallest number that 4 and 3 have in common is 12, so if we have 4 aluminium ions they will gain 12 electrons in total. If we multiply oxygen ions by 3 then 6 oxygen ions will make 3 oxygen molecules and lose 12 electrons. Probably easier to show this as an equation:

Did I say there's one more thing to think about? I meant two more things....

The 2 equations we've just produced are call HALF EQUATIONS but they now need to be joined together. When we do this the electrons cancel out leaving us with this: 

Right, now we're done. For aluminium oxide anyway.

You also need to know about the electrolysis of brine to produce some really important chemicals. For this tale we have to travel to Cheshire first. Cheshire is a curious place because under the ground there are vast salt deposits (salt is sodium chloride (NaCl)). The good people of Chehsire were very fond of digging this salt up to use in a variety of ways. In Roman times salt was used like money (it's where the term SALary comes from), but we have moved on a bit since then. Fed up of digging out salt and then watching the salt mines collapse and take their buildings into these vast sink holes some clever people had the idea of solution mining. First, let's look at a house built over a salt mine in Cheshire.

As you can see, the subsidence caused by salt mining was catastrophic. Then someone had an idea to flood the mines, which would dissolve the salt and then pump the salty water (called brine) out, before re-flooding the mines to keep them from collapsing. This led to the development of a major industry, now known as the chlor-alkali industry, that still has its base in Cheshire.

 

Brine is just another name for salt water (i.e. NaCl dissolved in water). You may have seen the word brine on tinned tuna, because it's what tuna is often stored in. When brine is electrolysed we get some really interesting products formed. Let's have a look at the overall electrolysis cell.

This seems far more complicated that the electrolysis of bauxite to make aluminium, there are suddenly new ions floating around in the mix. To understand this we perhaps need to start at the beginning; what ions are there at the start of the reaction?

 

NaCl will be split into Na+ and Cl- ions (remember that metals are always positive)

Water (H2O) will be split into H+ and OH- ions 

With 4 ions floating around in the solution then we have some decisions to make. There are 2 positive ions (H+ and Na+) and 2 negative ions (Cl- and OH-), which will be attracted to the electrode? The rule is that the LEAST reactive ion is attracted to the electrode. H+ is less reactive than Na+ so it is attracted to the negative electrode (cathode), whilst Cl- is less reactive than OH- and so is attracted to the positive electrode (anode).

 

The reactions at each electrode are called half equations.
The half equations are written so that the same number of electrons occur in each equation.

2H+  +  2e-       H2 (hydrogen gas at the (-)cathode).
2Cl-  -  2e-       Cl2  (chlorine gas at the (+)anode).

Hydrogen ions gain electrons (reduction) to form hydrogen atoms.
The hydrogen atoms combine to form molecules of hydrogen gas.

Chloride ions lose electrons (oxidation) to form chlorine atoms.

 

The chlorine atoms combine to form molecules of chlorine gas.

The overall reaction is
2NaCl(aq)  +  2H2O(l)    2Na+(aq) +  2OH-(aq) +  Cl2(g) +  H2(g)

Other things that you need to know:

Testing the gases made in electrolysis of brine-

Test for Hydrogen- it burns with a squeaky pop,

Test for chlorine- it bleaches litmus paper.

Uses of the products of electrolysis of brine.

Meir , Stoke-on-Trent ST3 7DF, UK

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