What was happening on the lab?

[danne89]

Hi! We'd a lab in school today, but no matter of how much I think about it, I can't get what happened.

We'd to construct our experiment on our own, supported by our teacher. We chose to connect electricity to a water and NaCl solution. I can't the English word for that, and my dictionary don't include it, so ...

Anyway, we used coppar elements, you know those who have contact with the solution itself, in the water. (My English vocabulary is quite finit, and also my dictionary...)

We was expecting some cind of chlorine gas, and hydrogen and oxygen gases too. Maybe we got them, but we couldn't observed them. What we got, and it was way more interesting, was one green substance. We believe it may be some salt with copper, due to its green color. But I can't set up the forumlas for that. My work looks like this current:

$$Na^+ + e^- \rightarrow Na$$
$$2Cl^- - 2e^- \rightarrow Cl_2$$
$$2H^+ + 2e^- \rightarrow H_2$$
$$O_2^- - e^- \rightarrow O_2$$
$$Na + O \rightarrow NaO$$ I'm not sure about this, but it would one occur there water touches air.
Maybe this:
$$Na + H_2 O \rightarrow ~NaOH + H$$ ?? I dunno. Help me please!

 

[gravenewworld]

You oxidized the copper I'm guessing. Just think of the Statue of Liberty in America. The Statue of Liberty is actually made of copper , but it is all green because it has been oxidized by the oxygen in the air.

Cu +O2---->Cu2O


If I remember correctly, If you run an electric current through water you will get hydrogen and oxygen gas as a product, which is where the oxygen would come from to oxidize the copper.

 

[DB]

Was this green substance a gas or solid? Because $$Cl_2$$ is green.

 

[danne89]

The solution got a green color. Can it be CuCl2? Like this: $$CuO + 2H^+ + 2 Cl^- \rightarrow CuCl_2 + H_20$$

 

[archie]

electrolysis of NaCl

What's happening when u electrolysis NaCl (it's called Sodium Chloride in english, table salt) soln is really different from electrolysing it in molten state.

When u electrolyse molten NaCl ( approx. 800 centrigrade) sodium/natrium, Na metal will be obtained at the cathode while Chlorine gas will be evolved at the anode.

2 NaCl (l) ---electrolysis--> 2 Na(l) cathode + Cl2 (g) anode

However if u electrolyse NaCl soln, chlorine gas will still be formed at cathode but sodium won't be obtained at the cathode. Instead, hydrogen gas, H2, and sodium hydroxide NaOH will be formed at the cathode.

NaCl -> Na+ + Cl-
at anode : 2Cl- ---> Cl2(g) + 2e
at cathode : 2Na+ + 2H2O + 2e --> 2 NaOH(aq) + H2(g)

About the green stuff you observed during the experiment, here's that might be going on.
Since you're using copper (Cu) electrode and chlorine gas is produced during this electrolysis reaction, Cl2 attacked Copper electrode forming green Copper (II) Chloride, CuCl2

Cu + Cl2 ---> CuCl2(green)

If u use inert electrode, such as graphite (pencil lead) then you'll observe swimming pool odour of Cl2 and there won't be any green coloration in your electrolysis cell

 

[Gokul43201]

You MUST NOT use copper electrodes for this experiment. You must use inert electrodes (made of platinum, for example, which is the standard electrode material), or electrodes of any metal with a reduction potential greater than the applied potential. In this case Aluminum electrodes would work, if you are careful to make sure the voltage does not exceed 1.66V.

In this experiment, you want to make O2 and H2. For this, you must go all the way up to 1.23V. To get chlorine gas you have to apply 1.36V. However, the reduction (actually negative reduction potential, or oxidation potential) potential for Cu is just 0.34V. So, you start forming $Cu^{2+}$ ions which are responsible for the green solution.

Here's a table of standard reduction potentials

This process/experiment is known as ELECTROLYSIS. That is the English word you are looking for. Never perform an electrolysis experiment without first knowing how much voltage (or potential difference) you have to apply across the electrodes.

In this lab, how did you apply the voltage ? What did you use as a voltage source ? Were you using an adjustable DC power supply or some fixed voltage supply (a battery ? ) ? You really should be using a low range (about 0 - 5V or so) adjustable DC supply.

 

[archie]

usage Platinum electrode

The usage of Pt electrode for electrolysing NaCl soln is not really recommended. Though Pt is very inert metal, it's prone to the attack of Chlorine that will be formed at the anode. Your expensive anode may be corroded.
If u are electrolysing water to obtain hydrogen and oxygen then you may use Pt electrodes, but use other compounds/salt (i.e. Sodium Sulphate or dilute sulphuric acid ) rather than NaCl.

 

[Gokul43201]

Good point !

In this particular experiment though, I suspect that only a tiny amount of salt was used, primarily to aid in the dissociation of water.

Yes graphite electrodes are cheap and safe.

PS : Welcome to the forums, archie. I see you're new here !

 

[danne89]

Actualy, the purpuse of this lab was to observ chlorine gas. Which, by the way, didn't succesed.

About the voltage. We just connected about 30 V DC a minute or so.

When I shake the green solution, it almost became brown. I wonder, can it still be CuCl2? I don't have any AgNO3 to test with. :(