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Topic: Acid question  (Read 15312 times)

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Offline noiseordinance

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Acid question
« on: April 12, 2009, 02:03:31 PM »
I'm batting a thousand. Heh.

I'm struggling with a question about acids. Here's how it's presented:

Quote
Zinc hydroxide is insoluble in water but dissolves when a nitric acid solution is added. Why? Write balanced total ionic and net ionic equations, showing nitric acid as it actually exists in water and the reaction as a proton-transfer process.

I'm not sure where to begin on this. I read the acid chapter in my book but I don't see any example questions like this. I do know that most hydroxides are insoluble in water, so that's no surprise. I also know that HNO3 is a strong acid, but I don't know if I need to write a 2-part reaction (nitric acid reacts with water first) or a 3-part reaction (nitric acid + zinc hydroxide + water)... Can anyone lend a pointer?

As always, thanks a million...
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Offline leve

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Re: Acid question
« Reply #1 on: April 12, 2009, 03:09:00 PM »
Nitric acid, HNO3 is a strong acid so you know that it dissociates completely into H+ and NO3-

Zinc Hydroxide partially dissociates so what would be the reaction?
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Offline noiseordinance

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Re: Acid question
« Reply #2 on: April 12, 2009, 11:05:47 PM »
Well, I guess part of my confusion is this... do I write it like this?

Zn(OH)2 + HNO3 + H2O ----> . etc...?

The hard part for me is that if zinc hydroxide does not dissolve in water, yet it dissolves in nitric acid, what role does water have in this reaction? Would I just end up with an extra heavy water molecule, like this?

1 Zn(OH)2 + 1 HNO3 + 1 H2O ----> 1 Zn(NO3)2 + 1 H3O + 1 H2O

Wait, that's not even balanced, lol... argh! I'm not sure how to distribute ions in something like this...
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Offline Squirmy

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Re: Acid question
« Reply #3 on: April 13, 2009, 12:16:39 AM »
Try showing each of the processes leve described in separate equations.

Then we can think about trying to combine the two to get the whole picture.
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Offline noiseordinance

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Re: Acid question
« Reply #4 on: April 13, 2009, 12:36:23 AM »
Ok, so two processes... got it. So if the zinc hydroxide does not dissolve in water, but does dissolve in nitric acid, then I'm assuming my first reaction is going to be zinc hydroxide + nitric acid... Just a stab at this:

Zn(OH)2 + HNO3 ---> Zn(NO3)2 + H2O + H3O

Is this right?
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Offline Squirmy

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Re: Acid question
« Reply #5 on: April 13, 2009, 12:39:34 AM »
Quote from: leve on April 12, 2009, 03:09:00 PM
1. Nitric acid, HNO3 is a strong acid so you know that it dissociates completely into H+ and NO3-

2. Zinc Hydroxide partially dissociates so what would be the reaction?

numbers added by me...can you write equations for 1 and 2
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Offline noiseordinance

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Re: Acid question
« Reply #6 on: April 13, 2009, 12:58:58 AM »
Interesting, so the acid disassociated in water first, prior to having an effect on the Zinc Hydroxide? Ok, so here's another stab...

1. HNO3 + H2O ---> NO3- + H3O+

and then, assuming that the above is correct for the first step, would I make the next reaction involving Zn(OH)2 + NO3- + H3O+, or would it just be Zn(OH)2 + H3O+?
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Offline Squirmy

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Re: Acid question
« Reply #7 on: April 13, 2009, 01:15:13 AM »
The question asks for total ionic equation as well as net ionic equation.

It's always best to start with total ionic equation and then take out spectator ions to get the net ionic equation. So, at least to start, you should show NO3-. You may or may not need to take it out for the net ionic equation.
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Offline noiseordinance

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Re: Acid question
« Reply #8 on: April 13, 2009, 01:33:24 AM »
Ok, so I take my Zn(OH)2 + NO3- + H3O+... How do I know which product reacts with which? Do I get something like this?

Zn(OH)2 + NO3- + H3O+ ---> Zn(NO3)2 + H2O + OH-

??? Argh, I'm horrible with this acid stuff. :***(   LOL

PS - I know that's not balanced yet, but I'm taking a shot in the dark here..
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Offline Squirmy

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Re: Acid question
« Reply #9 on: April 13, 2009, 01:48:34 AM »
For ionic equations, soluble salts are shown in ionic form. Considering the zinc hydroxide dissolves in HNO3, how does that change your representation of Zn(NO3)2?

Also, reactions between hydroxides and acid form a salt and water...not more hydroxide.
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Offline noiseordinance

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Re: Acid question
« Reply #10 on: April 13, 2009, 02:05:50 AM »
I am so freaking lost... I just don't get this at all... so, from what I am picking up, I leave the NO3 alone, it just floats around all ionic-like and the Zn(OH)2 directly reacts with the H3O+... however, since H3O+ has an extra hydrogen, maybe that gets donated to the Zn(OH)2 to create that molecule of water you're talking about, but that would leave me with an extra hydrogen, so I'm guessing that stays with one of the water molecules to reform H3O... I feel like I'm going in circles here, but here's another attempt...

Zn(OH)2 + NO3- + H3O+ ---> Zn2+ + NO3- + H2O + H3O

Crap, that doesn't even balance. Ugh. So I just can't figure out what to do with all these hydrogens and oxygens. I'm going to have a heck of a time explaining this one to my girlfriend when I finally get this solved. :P
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Offline Squirmy

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Re: Acid question
« Reply #11 on: April 13, 2009, 02:20:36 AM »
Quote from: noiseordinance on April 13, 2009, 02:05:50 AM
so, from what I am picking up, I leave the NO3 alone, it just floats around all ionic-like

Yes...things that "float around all ionic-like" in both the reactant and product are your spectator ions. They get taken out in the net ionic equation.

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and the Zn(OH)2 directly reacts with the H3O+

Yep...remember, though, you have two OH's on zinc.

Quote
however, since H3O+ has an extra hydrogen, maybe that gets donated to the Zn(OH)2 to create that molecule of water you're talking about, but that would leave me with an extra hydrogen, so I'm guessing that stays with one of the water molecules to reform H3O...

Not so much. Keep it simple, and worry about balancing later: hydroxide + acid  :rarrow: salt +  water
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Offline Squirmy

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Re: Acid question
« Reply #12 on: April 13, 2009, 02:29:02 AM »
Or maybe it would help if I wrote that:

MOH  +  HX  :rarrow: MX  + H2O
 
Where M is a metal and X is an anion
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Offline noiseordinance

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Re: Acid question
« Reply #13 on: April 13, 2009, 02:33:05 AM »
First off, thanks again for continued help on this. There are a few of you guys on here that are lifesavers, especially since the tutor center isn't open when I'm not at work / school. Anyways, so the nitrate stays isolated, and it sounds like the zinc does too, so really I just  need to figure out what / how to combine / react the (OH)22- and the H3O+. So if the H3O+ donates a hydrogen, it becomes H2O, and one of the hydroxides also becomes H2O, leaving me with a single hydroxide, but I have a feeling my logic is wrong...

Ok, so I'm looking at your next post you just made... So the metal does end up associating with an anion, in this case O... so that would give me ZnO + 2 H2O... Holy crap, is that correct? LOL, if so, I don't know why that was so hard for me...
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Offline Squirmy

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Re: Acid question
« Reply #14 on: April 13, 2009, 02:45:05 AM »
Maybe I should've stopped before that last post  :)

How about an example of this type of problem, but different?

NaOH  +   HCl  :rarrow:  NaCl  +  H2O

In total ionic equation, this would be

Na+(aq) + -OH (aq)  +  H3O+(aq) + Cl-(aq):rarrow:  Na+(aq)   + Cl-(aq)  +   H2O(l)

The net ionic equation would be:

-OH (aq) + H3O+(aq) :rarrow: H2O(l)

That's a little different from your reaction because NaOH completely dissociates but Zn(OH)2 does not.
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