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Topic: How to write ionic and net ionic equations?  (Read 31494 times)

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Offline customx

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How to write ionic and net ionic equations?
« on: June 16, 2010, 11:44:31 AM »
My teacher used the following example in class:

H2SO4 + NH4OH ---> (NH4)SO4 + H2O

                      ---> (NH4)2SO4 + H20 (why do you add 2 to the ammonium? because sulfate has a -2 charge and NH4 +1?)

H2SO4 + 2NH4OH ---> (NH4)2SO4 + 2H2O  (why are there 2 molecules of H2O? I'm confused on how this is balanced)


Ionic equation= 2H + So4 +2NH4OH ---> 2NH4 + SO4 + 2H2O (is the ammonium hydroxide not broken up because NH4OH is a weak base? Are only strong bases broken? Are all salts broken up because they are strong electrolytes?)


Thanks! ;D

Offline opti384

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Re: How to write ionic and net ionic equations?
« Reply #1 on: June 16, 2010, 12:04:18 PM »
My teacher used the following example in class:

H2SO4 + NH4OH ---> (NH4)SO4 + H2O

                      ---> (NH4)2SO4 + H20 (why do you add 2 to the ammonium? because sulfate has a -2 charge and NH4 +1?)

H2SO4 + 2NH4OH ---> (NH4)2SO4 + 2H2O  (why are there 2 molecules of H2O? I'm confused on how this is balanced)


Ionic equation= 2H + So4 +2NH4OH ---> 2NH4 + SO4 + 2H2O (is the ammonium hydroxide not broken up because NH4OH is a weak base? Are only strong bases broken? Are all salts broken up because they are strong electrolytes?)


Thanks! ;D

First of all, the equation will be

H2SO4 + NH4OH --->  (NH4)2SO4 + H20

because as you said, sulfate ion has the charge of 2- and ammonium ion has a +1. 

Then, you should balance the number of atoms on each side. That is why you write the overall equation as

H2SO4 + 2NH4OH --->  (NH4)2SO4 + 2H20

Also, being a weak base does not mean that ammonium hydroxide does not break up. The difference between a weak base and a strong base is that the strong base readily gives off OH- (in the Arrhenius sense). In other words a strong base has a larger pKb than other bases. Moreover, all salts do not break up as you think. For example, in this equation,

Ca(OH)2 + CO2  :rarrow: CaCO3 + H2O

the salt CaCO3 is a precipitate.

Offline customx

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Re: How to write ionic and net ionic equations?
« Reply #2 on: June 16, 2010, 12:32:59 PM »

First of all, the equation will be

H2SO4 + NH4OH --->  (NH4)2SO4 + H20

because as you said, sulfate ion has the charge of 2- and ammonium ion has a +1. 

Then, you should balance the number of atoms on each side. That is why you write the overall equation as

H2SO4 + 2NH4OH --->  (NH4)2SO4 + 2H20

Also, being a weak base does not mean that ammonium hydroxide does not break up. The difference between a weak base and a strong base is that the strong base readily gives off OH- (in the Arrhenius sense). In other words a strong base has a larger pKb than other bases. Moreover, all salts do not break up as you think. For example, in this equation,

Ca(OH)2 + CO2  :rarrow: CaCO3 + H2O

the salt CaCO3 is a precipitate.


I just figured out how to write subscripts, so thanks for changing that.

I still don't understand why the 2NH4OH isn't broken up in the net ionic equation. My class is just learning how to write these equations, and my teacher only gave this example and one more. The other example is as follows:

HCl(aq) + NaOH(aq) ---> NaCl + H2O

My teacher classified the NaOH as a strong base, and then wrote the following ionic equation:

H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) ---> Na+(aq) + Cl-(aq) + H2O

He then said that all salts that come from any acid based reaction are strong electrolytes, which led me to believe that they are broken up. If the base was weak, then it shouldn't be broken up. Is he wrong?

Offline tamim83

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Re: How to write ionic and net ionic equations?
« Reply #3 on: June 16, 2010, 01:01:13 PM »
NH4OH is a problem.  It should be water soluble (it usually comes as a liquid)

NH4OH  --> NH4+ + OH-

But then:

NH4+ + OH- --> NH3 + H2O

NH4OH is pretty much concentrated ammonia (NH3), which is a weak base.  So the reaction should be

H2SO4 + 2NH3 --> (NH4)2SO4

The net ionic equation should then be

H+ + NH3 --> NH4+

So although NH4OH is a weak base (or at least is the reagent form of the weak base), I hesitate to call it a "weak electrolyte";  meaning that I would split it into ions when writing a total ionic equation.  I think this example is a little confusing.  

For your other question, if you want to know if a salt (which can refer to any ionic compound) is going to be a strong electrolyte, you may want to check out the solubility rules.  These will tell you whether or not the salt will be water soluble (a strong electrolyte).  Salts formed in acid-base neutralizations are often strong electrolytes, but I hesitate in saying that they always are.  

Hope this helps some.  


Offline customx

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Re: How to write ionic and net ionic equations?
« Reply #4 on: June 16, 2010, 01:29:49 PM »
OK I've got one more. I just tried to solve this problem but got one part wrong.

I balanced the equation and got:

Ca(OH)2(aq) + 2CH3CO2H (aq) ---> 2H2O + Ca(CH3CO2)2

Then I wrote the ionic equation:

Ca+(aq) + 2OH-(aq) + 2H+(aq) + 2CH3CO2-(aq) ---> 2H2O(L) + Ca2+(aq) + 2CH3CO-2

When I checked the answer though, it says that 2CH3CO2H(aq) is not broken up. Is this because it's a weak acid? I'm going crazy >:(




Offline tamim83

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Re: How to write ionic and net ionic equations?
« Reply #5 on: June 16, 2010, 02:02:59 PM »
HCH3CO2 is acetic acid, which is a weak acid.  Weak acids are weak electrolytes since they don't dissociate 100% in water.  Therefore, you can't break it up into ions when writing an ionic equation. 

It helps to know which acids and bases are "strong".  For your course level, there are not very many.  There are at least six strong acids: HCl, HBr, Hi, H2SO4, HNO3, and HClO4.  All group IA and some group IIA metal hydroxides are strong bases (the group IIA metal hydroxide case depends on solubility). By default, the other acids and bases you will encounter are considered "weak" in aqueous solution. 

Offline customx

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Re: How to write ionic and net ionic equations?
« Reply #6 on: June 16, 2010, 02:12:36 PM »
Thanks a lot.

Is it safe to assume then that any time I encounter H3PO4, HF, or CH3CO2H that they can't be broken up in a net ionic equation?

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Re: How to write ionic and net ionic equations?
« Reply #7 on: June 16, 2010, 02:24:55 PM »
some group IIA metal hydroxides are strong bases (the group IIA metal hydroxide case depends on solubility)

Actually they are all quite strong - that is, they are almost completely dissociated, just their solubility is low.
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