HCl+Mg reaction question?

[jasonm08]

Hi everyone, I have a question about the reaction between HCl and Mg. Earlier this school year (11th grade), I did a lab that was purposely designed wrong by my teacher Involving these chemicals. Basically we massed some HCl and some Mg ribbon seperately, and then we Reacted them and, by the law of conservation of mass (there were no nuclear reactions happening obviously), we were supposed to be able to calculate the ratio of the mass of hydrogen gas to the mass of Mg. There was an Excess of HCl for the lab purposfully (could this be the problem?). I really not sure what the problem was but when we reacted the Mg and HCl, H gas was most definately not the only thing coming out of there, because there was a gas that I could most definately see, and most definately smell ( It smelled like rotten eggs). Everywhere I have looked, I have just found things like HCl ----> H2+ MgCl (yeah duhh!). So, that being said, I have a few questons.

1. What was the other gas coming out?
2. What was the design flaw?
3. How do I fix the lab?
4. Is there anything else you think that I should know?

Thanks in advance for any help,

Jason

[jasonm08]

I have been sitting here thinking about this for a while, and I think I may have come up with the answer... I may be wrong but... I think that it IS because of the excess of HCl. I think because of the violent exothermic reaction, some of the excess HCl evaporates out causing the gas that you can see and that has a bad odor. So, you have to put just the right amount of HCl with the magnesium for it to work properly.

[jake-the-peg]

I think you're absolutely right.

Note though that classically hydrogen sulphide gives the smell of rotting eggs. HCl is a similarly nasty - and dangerous gas. Choking, eyewatering.

When I was about your age my chemistry teacher demonstrated the formation of HCl by cutting a hole in a washing up bottle and covering it with clear tape- and filling the bottle with hydrogen and chlorine. he then demonstrated the reaction could be initiated by light by bringing a burning piece of magnesium ribbon in front of the window.

I don't think I'll ever forget the resulting explosion and clouds of white choking smoke - I think he got in trouble for that then, these days he'd probably have been sued.

But I'll never forget the smell of HCl

[brachiopod]

Are you absolutely sure you were using hydrochloric acid and not, (in error, or due to contamination), sulphuric acid?

[Gef]

The excess HCl ensures that the mass of hydrogen produced depends only on the mass of magenesium used (2g of hydrogen for every 24g of magnesium).

A tiny amount (not enough to affect the results) of hydrogen sulphide (rotten eggs smell) is produced by impurites in the magnesium.

If the neck of the reaction vessel is not loosely plugged with cotton wool then a fine aerosol of HCl/water can escape although not enough to affect the results. I think this is what you saw.

[jasonm08]

Yes, it was HCl, and what happened was DEFINATELY enough to affect the results. The class a a whole had a huge error percentage.

[Bert]

Jason,
I'm sorry, but the answer to your fourth question is that you should know how to spell 'separately' and 'definitely'. If you can remember how to spell 'infinity', then words like 'infinitely', and 'indefinite' become easy to get right.

[jasonm08]

Words that have to do with the number of something are spelled the similarly. I see what you're saying. Ha! I would have spelled infinity right... Well now I know.

[jake-the-peg]

Save it for Art and Literature Bert - all the spelling pedants hang out there!

[Gef]

What do you mean by the results being affected?

What was the aim of the experiment?

Can you give us any results?

[jasonm08]

The purpose of the lab was: "determine the mass of hydrogen gas to the mass of hydrogen gas to the mass of magnesium metal when magnesium reacts with an excess of hydrochloric acid."

abs. deviation for 4 groups of two was calculated. Average % error was 2490%. Now, we have done other percision labs befor and after this, and the results were definitly one whole hell of a lot better than this, so it's not really the quality of the people doing it. After the lab was over, the teacher stated:
"there was something wrong with the lab design on purpose. Figure out what it was."

[jasonm08]

oops, one too many "the mass of hydrogen gass" in that last post.