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Topic: Year 12 Chemistry Electrolysis of Pure Water vs Salt Solution  (Read 1260 times)

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Offline jayathv

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Year 12 Chemistry Electrolysis of Pure Water vs Salt Solution
« on: October 31, 2018, 10:17:44 PM »
So I've been trying to write an assignment and I keep getting stuck on one point. Our experiment was to increase the salt concentration (na2co3) and observe how it affected the rate of producing hydrogen gas from the splitting of water. We got told it was increased because the higher concentrations led to a higher conductivity but we got no more explanation than that.

From what I understand:
There is a battery connected to two copper plates to the negative and positive sides. the electrodes are then put in water. What I don't understand is how the electrodes become charged. I understand that electrons in the battery want to move from the negative side to the positive side, so they would move into the copper plate from the negative side making it have a strong negative charge. The electrons in the other copper plate would be pulled out and sent to the positive side of the battery. This would cause the electrodes to have strong electrical charges from what I'm guessing.

(Side question: If you connected to copper plates and didn't put them in water, literally just connected them to each side of the battery and didn't complete the circuit would the plates still become positive and negatively charge?)

Then H20 would split into H+, OH- and O(2-) because of being next to the charged electrodes, the water would just split. Then the H+ would gain an electron at the negative electrode and become H2 gas and leave the system. The O(2-) would drop its electrons off at the positive electrode and  and become just O and then O2 molecule.
What I don't understand is why this doesn't happen in pure water, I don't understand the relationship between current, voltage, how the anode and cathode become charged etc. If I add a salt such as nacl I get that the na+ becomes na and the cl- becomes cl2 but I don't know how this helps make the electrodes more positive and negative and how it makes water split up more. From what I understand there's some kind of relationship between the nacl or whichever salt you choose and helping to make the terminals stronger at being negative and positive. But I don't understand why exactly.

Thank you

Offline Traumatic Acid

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Re: Year 12 Chemistry Electrolysis of Pure Water vs Salt Solution
« Reply #1 on: November 01, 2018, 12:20:09 AM »
Hiya

I can't answer if the electrodes would or would not become charged without completing a circuit.

Voltage however is the difference in electric potential. In the case of a single cell battery the voltage is determined by the difference in the reduction potential between the cathode and the anode. Whichever one has a higher reduction potential (the cathode) will draw electrons from the anode.
Current on the other hand is a measure of "how much" electricity is actually flowing through the circuit (i.e the rate of flow of electrons).

Voltage and current are connected. Think of current as the actual particles and the voltage as pressure behind them. Much like a water system, if you open a tap, water will flow. But if you add more pressure the water tank then the water will flow faster and go further.
If you have a huge amount of electricity but no voltage to push it it will not be able to flow through a circuit, hence you cannot use batteries of smaller voltage to run a larger circuit board (for example) than it's designed for, the low voltage won't be able to push the current through it (i.e the resistance of the circuit is too high).

Now pure water is not actually a very good conductor of electricity, it offers a high level of resistance to electrical currents. A small battery will not be able to drive electricity far through water. It might be able to form a circuit through a very very small distance. Electrolytes such as the salts you were using increase the conductivity (decrease the resistance) of the water by introducing ions into the solution making it easier for electricity to flow through it with a lower voltage. Acids and bases can also be used as electrolytes to increase the conductivity of water for as long as they form ions in solution.
I'm not 100% sure how this works but if I recall correctly the anode donates electrons to the ions whereas the cathode takes electrons from the ions. This is what causes the negative and positive clouds around the electrodes. The electrolyte pretty much carries electrons from the anode to the cathode.

My apologies if I at any point confused cathode and anode. Remember that electricity is the flow of negatively charged particles along a charge gradient.

Hope this helps! :)

Offline Borek

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Re: Year 12 Chemistry Electrolysis of Pure Water vs Salt Solution
« Reply #2 on: November 01, 2018, 04:34:53 AM »
If there is a potential difference between electrodes, they are definitely charged, that comes from the very basic definition of potential difference, which requires non-zero electric field to exist. No charge separation, no electric field, no potential difference.

What I don't understand is why this doesn't happen in pure water

Sure it does.

Trick is, once the charge from the electrode was used to split water it is water near the electrode that becomes charged. That stops further transfer of charge into water molecules in contact with the electrode (remember: same charges repel). New charge won't get into solution till the old charge flows away in the direction of the other electrode. Flowing charge is a current, the lower the resistance, the easier it is for the current to flow. Does it click now?
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Offline Borek

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Re: Year 12 Chemistry Electrolysis of Pure Water vs Salt Solution
« Reply #3 on: November 01, 2018, 04:38:11 AM »
If you have a huge amount of electricity but no voltage to push it

Huh? What is a "huge amount of electricity"?

There is a grain of truth here and there in what you wrote, but in general it is more confusing than helpful.
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Offline Traumatic Acid

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Re: Year 12 Chemistry Electrolysis of Pure Water vs Salt Solution
« Reply #4 on: November 02, 2018, 09:53:20 PM »

Huh? What is a "huge amount of electricity"?

There is a grain of truth here and there in what you wrote, but in general it is more confusing than helpful.

hmm, yeah I see what you mean. I mean to say that electrons wont really go anywhere unless there is a great enough difference in electrical potential between the electrodes. Why would they?

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