[supaumar]

Hi, we're doing a project in chemistry with Zinc and a solution of Copper (II) sulfate. In groups of 2, we were supposed to test an aspect of the reaction. My partner and I decided to test and see if a difference in pH would change the final temperature of the reaction. We used HCl for acidity and I believe NaOH for basicity(?).

We found that the pH didn't really make a difference in the final reaction temperature. Just trying to find out if that was supposed to happen or if we did something wrong.

So my question is...

Should a difference in pH affect the final reaction temperature between Zinc and Copper (II) Sulfate?

Thanks a lot for your help.

[Arkcon]

Well, you've been given the assignment, and you conducted the experiment.  It's not one that I'd have thought of doing, so the expert on the topic is you.  So whatever you observed, has to be correct, withing your own experimental error.  For the fun of it, can you tell us what pH values you selected and performed?  Did you observe, anything else, during the procedure?

[supaumar]

Yeah, sure.

I didn't actually calculate a pH, but measured acidity and basicity in drops.

I used 10,25, and 35 drops of both HCl and NaOH.

We did however observe a difference in the appearance of the reaction. The acidic reaction fizzled while the basic reactions became somewhat opaque. Also, in the basic reactions, the color of the copper (II) sulfate reaction lightened (from the blue to a lighter hue).

In both reactions the zinc turned a reddish brown.

Something else, the temperature of the reaction did increase 1-2 degrees from the temperatue of the copper II sulfate.

Hope that was 'fun'.

Anyways, if anyone does know how the results should end, please enlighten me. I just don't want to be totally off.

[Arkcon]

Perhaps, to help you draw conclusions useful to your write up, you can tabulate your data -- pH in units of drops in one column, temperature result in another.  Did you do the same pH points multiple times, so you can see if there is natural deviations in your temperature readings?  Do you have a value for control, that is no adjustment?  Did you do it multiple times?  Is the difference between your pH adjustment readings no wider than the difference between multiple readings at one point?  Being able to answer these questions will prevent you from being "totally off".

[supaumar]

Well, I suppose I'm not "totally off". I did have a control (no pH manipulation). I also tested each pH level 3 times. My results varied from 0-3 degrees, completely sporadically. I'm guessing that this is what's supposed to happen.

Is the difference between your pH adjustment readings no wider than the difference between multiple readings at one point?...would you please explain that?

Thanks again for your help and time.

[Arkcon]

So, if when you do it 3 times for each point, the variation is 0-3 deg between trials, and the variation from pH point to pH point is no more than 2 deg, then what conclusions are you going to draw?

[supaumar]

I guess this leads me to conclude that changing the pH had no difference upon the reaction between zinc and copper (II) sulfate.

Thanks.

[AWK]

I guess this leads me to conclude that changing the pH had no difference upon the reaction between zinc and copper (II) sulfate.

Thanks.

The reaction you are talking about is:
Zn(s) + CuSO₄ = ZnSO₄ + Cu(s)
After adding HCl Zn can react according to equation
Zn + 2HCl = ZnCl₂ + H₂(g)
and these two reaction are competetive
When you add NaOH
The main reaction will be:
CuSO₄ + 2NaOH = Cu(OH)₂(s) + Na₂SO₄
If NaOH is in an excess then
Zn(s) + 2NaOH + 2H₂O = Na₂[Zn(OH)]₄ + H₂(g)
If Zn is in excess the slowest eventual reaction would be:
Zn(s) + Cu(OH)2 = Cu + Zn(OH)2(s)

Heats od some reactions are rather small so in a such primitive conditions you cannot decide that pH changes the temperature of reaction (heat is evolving)

[AWK]

If the reaction proceeds spontaneously it should be an exothermic one.

http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation