What caused the decrease in volume of Cu Nitrate in a heated Daniell Cell?

[Da Apprentice]

I ran an experiment in which a Daniell Cell (using Cu and Mg electrodes with Cu Nitrate and Mg Nitrate as the electrolytes (both 0.1mol)) was heated on both sides by using two beakers filled with water, the water that heated the cell was heated to about 92C and then left to cool. The experiment was run for about 2 days after which the Cu Nitrate (0.1mol) electrolyte decreased in volume by 30% (from 50ml-35ml). Some of the reasons I believe to have caused this are that the Cu Nitrate evaporated - but I'm not sure if this is a viable factor becasue aparenetly Cu Nitrate doesn't boil until at about 170C. *can the Cu nitrate be considered separate to the water it's mixed with? - Another factor i thought might influence the volume is the loss of the Cu nitrates ions to the Cu, but this wouldn't have such an effect as to decrease the volume by 30%, would it? I was wondering if anybody had any other possible explanations for this decrease in volume?

Thanks,

Edit: Also why is it that the Cu Layer on both the top and bottom of the bearker formed? Shouldn't it have completely bonded to the Cu metal? (image attached) Is this result common in Daniell Cell testing?

Edit 2: Sorry i found what the reason for the loss in mass was, the salt brige has absorbed some the Cu Nitrate, As for the Cu layer being formed I'm still unsure as to why this occured

 

[Borek]

Some of the reasons I believe to have caused this are that the Cu Nitrate evaporated - but I'm not sure if this is a viable factor becasue aparenetly Cu Nitrate doesn't boil until at about 170C.

Where did you got the 170 deg C from?

can the Cu nitrate be considered separate to the water it's mixed with?

It has to. I have a feeling you are treating the solution as if it was copper nitrate alone - that's not the situation.

 

[Da Apprentice]

Where did you got the 170 deg C from?

From Wikipedia: http://en.wikipedia.org/wiki/Copper(II)_nitrate
- column to the right, properties, boiling point : 170 °C (trihydrate, decomposes)

It has to. I have a feeling you are treating the solution as if it was copper nitrate alone - that's not the situation.

I kinda was... how should it be treated? seprately as a copper nitrate and water?

I discovered what actualy decreased the Cu nitrate, however I'm not sure why the cu sediment appeared, shouldn't any cu atoms formed bond with the existing cu atoms?

 

[Borek]

From Wikipedia: http://en.wikipedia.org/wiki/Copper(II)_nitrate
- column to the right, properties, boiling point : 170 °C (trihydrate, decomposes)

Copper nitrate is a solid. When heated, it melts and decomposes. What starts to boil is this hydration water in which salt starts to dissolve. Or at least that's my guess, that's what other salts do. Calling it a boiling point is not the best idea.

In general, when you dissolve a salt in water, and the solution starts to boil, it is water that boils. At 1 atm pure water boils at 100 deg C, water in which some non-volatile substance is dissolved boils at higher temperature (this is called boiling point elevation, important property that you will probably learn about later). This can get more complicated in the case of highly concentrated solutions of not too stable salts, but for now you don't to worry about it.

I discovered what actualy decreased the Cu nitrate, however I'm not sure why the cu sediment appeared, shouldn't any cu atoms formed bond with the existing cu atoms?

"Decreased Cu nitrate" is incorrect. What you have seen is that volume of the solution decreased. You are right that it was sucked by the paper, but it is quite possible that part of the water just evaporated. It doesn't mean that copper nitrate disappeared - its amount in the solution can be exactly the same as before.

It is hard to tell anything about observed reddish color. It doesn't have to be copper, looks more like a rust to me. Have you used distilled (DI, RO) water, or just a tap water?

 

[Da Apprentice]

"Decreased Cu nitrate" is incorrect. What you have seen is that volume of the solution decreased.

Sorry bad wording, a volume decrease is what i meant.

You are right that it was sucked by the paper, but it is quite possible that part of the water just evaporated.

I read this (if you do read this look at the second answer - i think this is actually the better answer):
http://in.answers.yahoo.com/question/index?qid=20100527233427AA1nuhz
and I can see how the conecpt works but aproximately what change in volume would occur in only 50ml of water heated at about 90 deg C? Is there a formula that relates to this? Can the heat capacity formula Q=mc\wedgeT be rearanged for mass to find this?

It doesn't mean that copper nitrate disappeared - its amount in the solution can be exactly the same as before.

Yes it can be exactly the same as before, but it wouldn't be would it? The experiment was run over 2 Days throughout which the Cu nitrate was giving up ions to the copper wouldn't the concetration (moles) of the solute have to have dropped?

It is hard to tell anything about observed reddish color. It doesn't have to be copper, looks more like a rust to me.

Our teacher thought it was Copper, what tests are there to check if it is Cu? - Also how could the rust form? this couldn't be from the Cu could it?

Edit: Sorry, I actually think it may be rust: it appears the aligator clips holding the metals have been sumberged in the ionic solutions, if you see the above picture of the salt bridge next to it are the electrodes, you can see where the clip was. Further there is the pictrue of the aligator clip (sorry it's a bit blurry): most of the orange substance is rust, but do you think it's possible that some of it is also Cu metal?

Have you used distilled (DI, RO) water, or just a tap water?

Yes we did use distilled water to create the Cu Nitrate, I've seen the setup they use to distil the water and I'm pritty sure it does both Ro and DI but I didn't take a good look at it. From what I saw the system heats tap water and then recondenses it, it might have other sections that I didn't see.

 

[Borek]

I can see how the conecpt works but aproximately what change in volume would occur in only 50ml of water heated at about 90 deg C? Is there a formula that relates to this? Can the heat capacity formula Q=mc\wedgeT be rearanged for mass to find this?

In theory you can calculate what mass of water can evaporate assuming all heat was used for evaporation, that's not difficult - but it doesn't make sense. First, you can be sure evaporation was not the only way of giving off heat, second, the beaker was left fro two days - put another beaker in teh same place (and in teh same conditions) and check what happens. Wate dries out, doesn't it?

Yes it can be exactly the same as before, but it wouldn't be would it? The experiment was run over 2 Days throughout which the Cu nitrate was giving up ions to the copper wouldn't the concentration (moles) of the solute have to have dropped?

You are right - amount of the copper present in the solution should drop down, I was mainly pointing out to errors in your earlier analysis.

Our teacher thought it was Copper, what tests are there to check if it is Cu? - Also how could the rust form? this couldn't be from the Cu could it?

Edit: Sorry, I actually think it may be rust: it appears the aligator clips holding the metals have been sumberged in the ionic solutions, if you see the above picture of the salt bridge next to it are the electrodes, you can see where the clip was. Further there is the pictrue of the aligator clip (sorry it's a bit blurry): most of the orange substance is rust, but do you think it's possible that some of it is also Cu metal?

It is possible. Check what happens if you put it in solution of HCl (1M will probably do), rust should easily dissolve, copper should not.

Yes we did use distilled water to create the Cu Nitrate, I've seen the setup they use to distil the water and I'm pritty sure it does both Ro and DI but I didn't take a good look at it. From what I saw the system heats tap water and then recondenses it, it might have other sections that I didn't see.

OK, I asked because such experiments are sometimes done using just a tap water, and it can contain substantial amount of iron.

 

[Da Apprentice]

second, the beaker was left fro two days - put another beaker in teh same place (and in teh same conditions) and check what happens. Wate dries out, doesn't it?

I think i'll run this experiment - measuring the volume decrease.
Thanks for all the help.