electrolysis: Help!!!!!! (1 Viewer)

[Yaz88]

ok im confused..... i dnt get this like i get basics such as :

·The cathode (reduction electrode) is negatively charged
·The anode (oxidation electrode) is positively charged.
·Electrolysis only occurs when the voltage applied to the cell is greater than the calculated cell potential (ie emf sign is negative as it needs electricity input to make chemical energy)
·The more reactive metal becomes the cathode while the less reactive metal from the standard potentials becomes the anode (in galvanic cells it’s the other way around eg Ca+2 would be the anode while Cl- would be the cathode)
·However anode is still the site of oxidation and cathode is still the site of reduction, it is just that the metals are switched.

but i dont get things like the generalisations like... - potassium ions, sodium ions, calcium ions and aluminium ions are never reduced.

or the actual reactions ..like why does water get oxidised rather than the Cl- ions because of its concentration....etc

so can sumone kindly explain that to me....
as my teacher is a complete idiot and failed to teach us anything..

 

[Rax]

Ok...well I hope you went ok in your trial...
To the point.
The Cathode is the site of reduction
The Anode is the site of oxidation
In a electrolytic cell the cathode is negative and the anode is positive. This is the opposite to a Galvanic cell.
Ok you can put whatever you want on the Cathode and the anode but it wont always work. As you described about the generalisations
ie "but i dont get things like the generalisations like... - potassium ions, sodium ions, calcium ions and aluminium ions are never reduced"
I actually have never heard of this before but if I think about it....and It may depend on the actual reaction ie if its in a (aq) solution with inert electrodes, if its a molten solution, if it is a Potassium electrode.
Potassium is a metal so you would expect it to be able to be reduced but it may take a high voltage.
Ok now with the Cl- ion stuff I know. Depending on the concentration of the solution there are certain things which may happen. In a not too concentrated solution (I am not too sure what the exact concentration is) say >1 Molar (dissolved in water) the regular reactions should occur.
IE If you look at the Standard Reduction potential table Cl ions get oxidised at -1.36 V
water gets oxidised at -1.23 V. This means that as it takes less voltage the water will be oxidised first. If more voltage is applied then the Cl ions will get oxidised.
In a concentrated solution, due to the amount of Cl ions and the lack of water the Cl ions will become oxidised to produce Chlorine gas.
If I have said anything wrong (I hope I havent) then anyone can add it and I can answer more questions if you want and I hope I know the answer. lol
Ok Good luck man

 

[shobadaynature]

" i dont get things like the generalisations like... - potassium ions, sodium ions, calcium ions and aluminium ions are never reduced."

The potassium, sodium, calcium and aluminium ions are high on the standard potentials table - or high in reactivity. On the table, potassium needs - 2.xx V is needed to reduce it, which is a LOT of voltage. Water, on the other hand, is only -1.23 V to reduce.
Since the ion with the <b> least voltage </b> needed to reduce occurs, in other words, the most positive number on the chart is reduced, then water will be reduced in preference to potassium. Water needs much less voltage input into the electrolytic cell to reduce, whereas the potassium ion needs much more voltage.

hope this helps

 

[Rax]

I believe I stated that already but eh I know I aint that clear

 

[shobadaynature]

it's cool, just adding stuff...hehe

 

[Yaz88]

isnt that only when its dilute?

 

[bobsvill]

i dont know much about stuff.... but i do know that Cl- gets reduced instead of water if it is over 2 mol/L concentration.

 

[Roobs]

well K, Ca, Na, Al and the like will spontaneously react with water, so thats another reason they will never be reduced in aqueous solution.

The Redonx tavle gives u potentials for "standard conditions" --1atm, 1mol/L and 25C, if you change these conditions, you change the potentials (concentration has a big impact), so depending on cencentration + voltage different reactions occur-- Just justifying what people have allready said.

 

[angmor]

so what if they tell us to work out some potentials from an redox reaction, but the concentrations were very high or something....do we still use the same potentials in the chart?

 

[Riviet]

so what if they tell us to work out some potentials from an redox reaction, but the concentrations were very high or something....do we still use the same potentials in the chart?

Basically for concentrations that aren't under standard conditions, the standard reduction potentials table becomes redundant so you really have to work it out yourself and make an educated prediction based on the info you're given in the question.

 

[mitochondria]

Basically for concentrations that aren't under standard conditions, the standard reduction potentials table becomes redundant so you really have to work it out yourself and make an educated prediction based on the info you're given in the question.

Mm... There's no reason why you can't work out the cell potential when the concentrations are given to you - and for that you need the standard reduction potentials.

It's just that the Nernst equation is not part of the HSC syllabus... (Or is it?)