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Topic: How can I work out what the products are going to be?  (Read 9203 times)

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Offline Bel-p

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How can I work out what the products are going to be?
« on: May 16, 2006, 06:28:43 PM »
I always seems to have problems working out the right hand side of an equation - what products are produced by the reactants.  I have an extremely bad memory so I try to rely on logic where possible, rather than memorising things.

So, an example was today when we had to write a balanced equation for the reaction of ammonia with nitric acid:

NH3 + HNO3 --> NH4NO3

OK, so I knew what the products were and I knew the formulas for them so I knew the left side, but I couldn't think what they might produce, so I couldn't get the right hand side of the equation. 

With ionic compounds you can split them into ions and then work out that the cations go with the annions, and I think you can use the activity series to work out which goes with with etc.  But with this example above I don;t think I can do that. 

Can anyone help please???

Offline Borek

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Re: How can I work out what the products are going to be?
« Reply #1 on: May 16, 2006, 06:32:49 PM »
With ionic compounds you can split them into ions and then work out that the cations go with the annions

Try the same approach - nitric acid dissociates and the salt produced is dissociated too.
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Offline rctrackstar2007

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Re: How can I work out what the products are going to be?
« Reply #2 on: May 16, 2006, 06:38:04 PM »
Remember that strong acids dissociate all the way and weak acids don't so there may be some ion left on the right side
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The world is like an atom. The not-quite-as-intelligent people are the nucleus all packed together sharing a common...everything. We, we are the electrons. Granted we're not as smart as these engineers and what-not so we're most likely in the first orbital, but we're the electrons of this giant atom. We all have differing intelligences and ideas and we are separated from the nucleus which makes us better because no one really cares about how a nucleus acts. It's the electrons that make chemistry, except for nuclear chem, of course, which I am a big fan of.

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Offline Bel-p

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Re: How can I work out what the products are going to be?
« Reply #3 on: May 16, 2006, 07:28:12 PM »
Ammonia + Nitric acid --> ammonium nitrate
NH3(g) + HNO3(aq) --> NH4NO3(aq)

Do I start like this:
N3+ + 3H+ + H+ + NO3-  -->  ??

Or perhaps the NH3 doesn't split up (dissociate), in which case, um....

I'm afraid I still don't understand this.  I've tried looking for information on this sort of thing on the internet but can't find it.   I know this is probably above my level (I'm doing GCSE chemistry in the UK), but I really want to understand it.  I'd be really grateful if someone could explain this to me, then perhaps I can start it to try and logically work out how other reactions work.

Offline rctrackstar2007

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Re: How can I work out what the products are going to be?
« Reply #4 on: May 16, 2006, 07:31:16 PM »
Ammonia + Nitric acid --> ammonium nitrate
NH3(g) + HNO3(aq) --> NH4NO3(aq)

Do I start like this:
N3+ + 3H+ + H+ + NO3-  -->  ??

Or perhaps the NH3 doesn't split up (dissociate), in which case, um....

I'm afraid I still don't understand this.  I've tried looking for information on this sort of thing on the internet but can't find it.   I know this is probably above my level (I'm doing GCSE chemistry in the UK), but I really want to understand it.  I'd be really grateful if someone could explain this to me, then perhaps I can start it to try and logically work out how other reactions work.

that's not dissociation, that is a polyatomic ion and that will stay together. dissociation is something like H2CO3 becoming H+ and CO32- in solution
AP Chemistry Squad Member [002]

The world is like an atom. The not-quite-as-intelligent people are the nucleus all packed together sharing a common...everything. We, we are the electrons. Granted we're not as smart as these engineers and what-not so we're most likely in the first orbital, but we're the electrons of this giant atom. We all have differing intelligences and ideas and we are separated from the nucleus which makes us better because no one really cares about how a nucleus acts. It's the electrons that make chemistry, except for nuclear chem, of course, which I am a big fan of.

-Your's truly, 2006;
  written to describe the HS chem student apart from the average being

Offline Bel-p

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Re: How can I work out what the products are going to be?
« Reply #5 on: May 16, 2006, 07:56:33 PM »
Ammonia + Nitric acid --> ammonium nitrate
NH3(g) + HNO3(aq) --> NH4NO3(aq)

OK, so:
NH3 + H+ + NO3-  -->  ??

but I don't understand what goes with what and why.  In fact, why does anything react here at all?

I'm on the verge of giving up.

Offline rctrackstar2007

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Re: How can I work out what the products are going to be?
« Reply #6 on: May 16, 2006, 08:00:09 PM »
Ammonia + Nitric acid --> ammonium nitrate
NH3(g) + HNO3(aq) --> NH4NO3(aq)

OK, so:
NH3 + H+ + NO3-  -->  ??

but I don't understand what goes with what and why.  In fact, why does anything react here at all?

I'm on the verge of giving up.

you have the correct equation finished, it results in ammonium nitrate. it does this because it is a combination reaction. you need an activity series to know why it does react but it's because it's high enough up on it that it wants to react. you won't be given a rxn on the AP chem test that doesn't react
AP Chemistry Squad Member [002]

The world is like an atom. The not-quite-as-intelligent people are the nucleus all packed together sharing a common...everything. We, we are the electrons. Granted we're not as smart as these engineers and what-not so we're most likely in the first orbital, but we're the electrons of this giant atom. We all have differing intelligences and ideas and we are separated from the nucleus which makes us better because no one really cares about how a nucleus acts. It's the electrons that make chemistry, except for nuclear chem, of course, which I am a big fan of.

-Your's truly, 2006;
  written to describe the HS chem student apart from the average being

Offline mrdeadman

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Re: How can I work out what the products are going to be?
« Reply #7 on: May 16, 2006, 08:18:17 PM »
im sorry to say it, but you pretty much have to memorize what things will do, just like the solubility rules, you have to memorize them.
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Offline rctrackstar2007

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Re: How can I work out what the products are going to be?
« Reply #8 on: May 16, 2006, 08:23:49 PM »
that is the basic truth behind it. lots and lots of practice will make it come to you soo easily tho
AP Chemistry Squad Member [002]

The world is like an atom. The not-quite-as-intelligent people are the nucleus all packed together sharing a common...everything. We, we are the electrons. Granted we're not as smart as these engineers and what-not so we're most likely in the first orbital, but we're the electrons of this giant atom. We all have differing intelligences and ideas and we are separated from the nucleus which makes us better because no one really cares about how a nucleus acts. It's the electrons that make chemistry, except for nuclear chem, of course, which I am a big fan of.

-Your's truly, 2006;
  written to describe the HS chem student apart from the average being

Offline Bel-p

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Re: How can I work out what the products are going to be?
« Reply #9 on: May 16, 2006, 08:39:13 PM »
I know I had finished the equation, but I was told the answer, I didn't get there myself!

I do have the reacivity series (and have memorised that with the help of a great mnemonic), and am trying to use that as aid to what reacts with what, but not getting very far. 

rctrackstar2007 when you say
Quote
but it's because it's high enough up on it that it wants to react.
what do you mean by "it"?  I've only got elements on my activity series, do you mean hydrogen?

MrDeadman, perhaps you're right I just need to memorise these things - but that's exactly what I'm trying to avoid -
a) because I have an appalling memory  and b) because I want to understand these things. 
I am the sort of person that needs to understand something fully in order to take it in.  Also I'm doing this more for my own interest than anything else, so I don't just want to memorise loads of stuff in order to pass an exam.

The problem is not just with this reaction, but many others.  It's a problem that has been bugging me since I started (September).   Perhaps I'll try and find another easier one and see if people can explain that to me... 

Offline mike

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Re: How can I work out what the products are going to be?
« Reply #10 on: May 16, 2006, 08:45:44 PM »
Bel-p: Borek was setting you on the right track with his post.

You can still use your method here.

NH3 + HNO3 ---->

and earlier you realised that HNO3 will dissociate to: H+ and NO3-

and you wrote:

NH3 + H+ + NO3- -----> ??

So it is acid and base reacting: NH3 = base, H+ = acid, NO3- = spectator ion


NH3 + H+ + NO3- -----> NH4+ + NO3-

right?
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Offline Bel-p

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Re: How can I work out what the products are going to be?
« Reply #11 on: May 16, 2006, 08:58:09 PM »
So it is acid and base reacting: NH3 = base, H+ = acid, NO3- = spectator ion

NH3 + H+ + NO3- -----> NH4+ + NO3-
OK, thanks Mike... but why does the hydrogen ion get together with ammonia? 
NH3 + H+ --> NH4+
Maybe I'm being a bit dim or slow...  very sorry.
Actually it's 2am here and I must go to bed now, but I'll look again tomorrow.

Offline mike

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Re: How can I work out what the products are going to be?
« Reply #12 on: May 16, 2006, 09:13:15 PM »
Ammonia, NH3 is an example of a lewis base, ie it is an electron pair donor.

Ammonia has two electrons which it can donate to the proton  :NH3

This forms a new covalent bond between the N and the H:

H-NH3+

Octet rules, lewis structures, molecular geometry etc are some of things that can help you explain why NH4+ is a "real" product (ie it can possibly form).

Another thing to remember is that just because you write a reaction equation (assuming you have followed all of the aforementioned rules) doesn't mean that the products you have written are favourable in that situation. You can make some educated assumptions usually, but often it would require an experiment.

Feel free to continue this discussion tomorrow :)

« Last Edit: May 17, 2006, 03:19:52 AM by mike »
There is no science without fancy, and no art without facts.

Offline Borek

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Re: How can I work out what the products are going to be?
« Reply #13 on: May 17, 2006, 03:11:11 AM »
OK, thanks Mike... but why does the hydrogen ion get together with ammonia? 
NH3 + H+ --> NH4+

As mrdeadman already stated - that's one of these things you have to memorize.
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Offline Bel-p

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Re: How can I work out what the products are going to be?
« Reply #14 on: May 17, 2006, 05:52:46 AM »
Hm, I've been searching on the internet and reading lots of stuff and thanks to that and the information from Mike etc I'm beginning to understand this a little I think.   

A very useful site I found was:
http://www.chemguide.co.uk/physical/acidbaseeqia/theories.html

It's given me a little understanding about the different theories of acids & bases anyway.  Also it's got diagrams of the
H-NH3+ bonding that Mike talked about, which is really helpful.  At least it'll help me remember that NH3 and H+ like to bond.

Thanks to everyone for trying to *delete me*

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