Half-equations [Chemistry]

[chantellehugs]

I have my Chemistry exam for C4, C5 and C6 and I've basically spent the whole of today attempting to understand electrolysis and half-equations. Can anyone explain them in depth to me, it would be extremely appreciated because I'm worried that I'll go into the exam not knowing about it, and then questions might come up on it..

[Chippiewill]

I'm not too great at chemistry but I do understand this bit kinda well.

In an Ionically bonded molecule you have two ions of opposing charge, for example Aluminium Oxide has Al(+)O(-) (Pretend the symbols in brackets are superscript). And in electrolysis you have two electrodes of opposing charge, and Anode(+) and a Cathode(-). The two Ions in the molecule are attracted to the different electrodes. The Aluminium(+) is attracted to the negatively charged Cathode and the Oxygen(-) is attracted to the positively charged Anode.

From this you have the two half equation for each of the ions at the two poles:

2O(-) -> O2 + 2e(-)
Al(+) + e(-) -> Al

(e for electron. from or to the electrode).

Half equations are used here to demonstrate that some electrons are moving around, otherwise you'd never see the electrons:

2AlO -> O2 + 2Al

--

The science there probably wasn't too great, but from an understanding point of view I think it makes sense.

[GirlNextDoor15]

Electrolysis

Use a table for this. I always use a table to solve questions involving electrolysis and it's quite easy for me.

If you get questions about electrolysis of aqueous solutions, you have to know the ions in the solution. That's first.

Then, determine which one is cation(positively charged ion) and which one is anion(negatively charged ion). Basically, positively charged ions/cations will go to the cathode electrode and negatively charged ions/anions will go to the anode electrode. How to determine which electrode is cathode or anode? Cathode is usually connected to the negative side of dry cells/power supply. Anode is usually connected to the positive side of dry cells/power supply. -ve side of dry cells is the shorter line and +ve side of dry cells is the longer line.

After determining that, then look at the electrochemical series. Whichever (anion/cation) ion placed lower in the electrochemical series will be discharged.

Then, make half equations, reason it out and you're done.

Tips: Memorise the electrochemical series.
Remember the factors that affect the electrolysis of an aqueous solution.

What I explained just now is just the basic part of electrolysis of an aqueous solution. Different factors will affect the selective discharge of ions during the electrolysis of an aqueous solution.

[chantellehugs]

THANK YOU, THANK YOU, THANK YOU! I just read what you both explained and then I read my textbook and slowly I understood it! I've tried some examples from my textbook with half equations and I got them correct, all I need to do is memorise the charges on the elements that aren't in the groups on the periodic table! Thank you both, I'm so relieved and happy that I finally conquered electrolysis, I can get on with the rest of my revision in peace

I'm not too great at chemistry but I do understand this bit kinda well.

In an Ionically bonded molecule you have two ions of opposing charge, for example Aluminium Oxide has Al(+)O(-) (Pretend the symbols in brackets are superscript). And in electrolysis you have two electrodes of opposing charge, and Anode(+) and a Cathode(-). The two Ions in the molecule are attracted to the different electrodes. The Aluminium(+) is attracted to the negatively charged Cathode and the Oxygen(-) is attracted to the positively charged Anode.

From this you have the two half equation for each of the ions at the two poles:

2O(-) -> O2 + 2e(-)
Al(+) + e(-) -> Al

(e for electron. from or to the electrode).

Half equations are used here to demonstrate that some electrons are moving around, otherwise you'd never see the electrons:

2AlO -> O2 + 2Al

--

The science there probably wasn't too great, but from an understanding point of view I think it makes sense.

Electrolysis

Use a table for this. I always use a table to solve questions involving electrolysis and it's quite easy for me.

If you get questions about electrolysis of aqueous solutions, you have to know the ions in the solution. That's first.

Then, determine which one is cation(positively charged ion) and which one is anion(negatively charged ion). Basically, positively charged ions/cations will go to the cathode electrode and negatively charged ions/anions will go to the anode electrode. How to determine which electrode is cathode or anode? Cathode is usually connected to the negative side of dry cells/power supply. Anode is usually connected to the positive side of dry cells/power supply. -ve side of dry cells is the shorter line and +ve side of dry cells is the longer line.

After determining that, then look at the electrochemical series. Whichever (anion/cation) ion placed lower in the electrochemical series will be discharged.

Then, make half equations, reason it out and you're done.

Tips: Memorise the electrochemical series.
Remember the factors that affect the electrolysis of an aqueous solution.

What I explained just now is just the basic part of electrolysis of an aqueous solution. Different factors will affect the selective discharge of ions during the electrolysis of an aqueous solution.