# Homework Help: Electrolysis of water

1. Nov 17, 2011

### PeetPb

greetings

I'd like you to check my calculations. The task is to calculate the time needed to produce 10 moles of hydrogen from water by electrolysis with a current of 1Amp.
I first wrote the equation of the reaction that occurs on the cathode
2H2O+2e- = H2 +2OH-
Then I derived the equation for time from Faraday's law
t=(F*n*z)/I
Since there are two electrons exchanging , z=2, I evaluated the equation and get
t= 1929.7 seconds which is .536 hours.

Is this correct ?

thanx

2. Nov 17, 2011

### Staff: Mentor

Approach is OK, but somehow you are off by orders of magnitude.

Please note this should land in the homework section.

3. Nov 17, 2011

### PeetPb

oh, right thanks , I somehow used the wrong constant ... so again when I plug the numbers along with the units I get
t=(96485.3*2*10 C mol)/(1 mol A) the moles cancel and I'm left with C/A which should be seconds. The actual number is about 536 hours ..... is this right ? it seems to be a little lot to me ... I got the Faraday's constant from http://www.wolframalpha.com/input/?i=F&a=*C.F-_*Unit.dflt-&a=UnitClash_*F.*FaradayConstantValue--"

shall I rewrite the thread or would you move it there ?

thanx

Last edited by a moderator: Apr 26, 2017
4. Nov 17, 2011

### Staff: Mentor

Hundreds of hours sounds much better. And the thread is already in HW subforum.

5. Nov 18, 2011

### PeetPb

I was just wondering ... shouldn't the time depend on the amount of water that we are electrolysing or on the surface area of the electrodes ?

6. Nov 18, 2011

### Staff: Mentor

Doesn't matter how much water is present - what does matter is how much water decomposed.

As long as the current is constant, electrode size doesn't matter. If you were to compare two experiments, where we supply the same potential, but electrodes are different, then yes - larger electrode will produce more gas. But it will produce more gas because the current will be higher.

7. Nov 18, 2011

### PeetPb

thank you very much ... I got it finally :)