-1
$\begingroup$

I'm doing a reaction of zinc metal and sulfuric acid to get zinc sulfate. I've been trying to learn the chemistry lately so I tried to measure out correct molar amounts. My understanding from the equation is that it's equimolar:

Zn + H2SO4 = ZnSO4 + H2

Sulfuric acid has a molar mass of 98.079 g/mol and zinc metal is 65.38 g/mol. The density of sulfuric acid is 1.84 g/ml (yeah cubic centimeter but it's a ml), 98.079 / 1.84 = 53.3 ml/mol. I believe my acid to be about 96%-ish, so to give myself a fudge factor I measure out 60ml of acid, diluted with 120ml of water. I put in about 65 grams of zinc metal (over time). When the bubbling had mostly stopped I had a pretty much solid mangle of zinc metal at the bottom of the jar, and even some air bubbles trapped underneath where the liquid couldn't get to anymore. There was also a white precipitate.

I'm confused why my (mostly) equimolar amounts left SO much zinc after, far more than even 10 or so grams.

Edit: I filtered the liquid still left in the jar through coffee filters. Cheap ones, but I layered them and stuffed one into the stem. It took a long time to filter, and is black...I also added more distilled water and some 35% acid to dissolve the rest of the zinc in the jar. It's bubbling away happily now.

$\endgroup$
2
$\begingroup$

If you use about equimolar amounts of zinc and the acid, the reaction would about asymptotically converge to depletion of the substance with the lower molar amount. The white precipitate is a mark the solution is saturated with zinc sulphate heptahydrate. The solution becoming thickened by the suphate may further decrease the rate of zinc dissolution. You have said it "mostly stopped", that is what I am talking about. It got too slow in the end, as there is just little acid left.

Remember the activity coefficient of $\ce{H+}$ raises very fast with acid concentration, so e.g. 10 times diluted acid reacts much slower than 10 times.

I suggest to use bigger acid excess, and then let it partially evaporate, separate crystals and wash them to get rid of the acid. ( like icy water or methanol or ethanol, then evaporate the liquid ).

| improve this answer | |
$\endgroup$
  • $\begingroup$ Excellent, thank you! Yeah there was still some very slow fizzing when I finally poured it off. I guess there was really no reason to use molar amounts, I could easily just have added zinc slowly until the fizzing stopped. Lessons for the future! Thank you for explaining it so well! $\endgroup$ – HaLo2FrEeEk Mar 24 at 9:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.