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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Lt.Pliskin on October 07, 2005, 12:14:56 AM

Title: Hess's Law with net ionic equations
Post by: Lt.Pliskin on October 07, 2005, 12:14:56 AM
Alright, I have three equations I carried out during a lab I did.

Combining HCl and NaOH

HCl + NaOH --> NaCl + H2O
H+(aq) + Cl -(aq) + Na+(aq) + OH-(aq) -->  Na+(aq) + Cl-(aq) + H2O(liq)
H+(aq) + OH-(aq) --> H2O(liq)


Combining Ammonium Chloride and Sodium Hydroxide

NH4Cl + NaOH --> NH3 + NaCl + H2O
NH3(aq) + H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) --> NH3(aq) + Na+(aq) + Cl-(aq) + H20 (liq)
H+(aq) + OH-(aq) --> H2O(liq)



Combining a solution of ammonia (NH3OH) and HCl

NH4OH +HCl --> NH4Cl + H20
NH3(aq) + OH-(aq) + 2H+(aq) + Cl-(aq) --> NH3(aq) + H+(aq) + Cl-(aq) + H2O(liq)
H+(aq) + OH-(aq) --> H20(liq)


Alright, using the heats of reactions obtained from the lab, I came up with:

H+(aq) + OH-(aq) --> H20(liq)            -158.23 kJ/mol

H+(aq) + OH-(aq) --> H20(liq)            -80.65 kJ/mol      
------------------------------------
H+(aq) + OH-(aq) --> H20(liq)            -267.54 kJ/mol


My question is, how would I use hess's law to verify that the third one can be found using the heats of reactions of the first two. Usually, a Hess's law problem involves rearranging the reactants and products to get the final quation. How would that work with these?

Any help is greatly appreciated!  
Title: Re:Hess's Law with net ionic equations
Post by: mike on October 07, 2005, 12:29:21 AM
I think maybe all you should have to do is reverse the second equation and add it to the first equation.
Title: Re:Hess's Law with net ionic equations
Post by: mike on October 07, 2005, 12:30:11 AM
NH4OH is just NH3 and H2O
Title: Re:Hess's Law with net ionic equations
Post by: Lt.Pliskin on October 07, 2005, 12:35:00 AM
So, it would be

Combining a solution of ammonia (NH3OH) and HCl

NH4OH +HCl --> NH4Cl + H20
NH3(aq) + H2O(liq) + H+(aq) + Cl-(aq) --> NH3(aq) + H+(aq) + Cl-(aq) + H2O(liq)
Nothing ---> Nothing?


H+(aq) + OH-(aq) --> H20(liq)            -158.23 kJ/mol

H20(liq) -->   H+(aq) + OH-(aq)           80.65 kJ/mol      
------------------------------------
Nothing --> nothing                           -267.54 kJ/mol

The two equations cancel each other out, so Im assuming that when NH4OH is added to HCL, no reaction occurs?
Title: Re:Hess's Law with net ionic equations
Post by: mike on October 07, 2005, 12:36:59 AM
HCl + NaOH ---> NaCl + H2O

NH4OH + NaCl ---> NH4Cl + NaOH

----------------------------------------

NH4OH + HCl ---> NH4Cl + H2O
Title: Re:Hess's Law with net ionic equations
Post by: mike on October 07, 2005, 12:38:46 AM
This is like:

NH3 + HCl ---> NH4+ + Cl-
Title: Re:Hess's Law with net ionic equations
Post by: Lt.Pliskin on October 07, 2005, 12:49:37 AM
OH, ok, I see, thanks!

Now, I have to use net ionic equations witht he calculations, how would those look?

I got:

 H+ + Cl- + Na+ + OH- -->  H20 + Na+ + Cl-

 NH3 + H20 + Na+ + Cl- --> NH3+ H20 + Na+ + Cl-
-----------------------------------------------------------
NH3 + H20 + H+ + Cl- --> NH3 + H+ + Cl-  + H2O      which basically means nothing happens :-\


Does the NH4OH dissasociate into those ions? How about the NH4Cl?

Thankyou for your help thus far, I think Ive almost got it! :)
Title: Re:Hess's Law with net ionic equations
Post by: Lt.Pliskin on October 07, 2005, 12:52:47 AM
sorry, we seem to keep missing each other

H+ OH- -->  H20

H20 --> H20
-----------------------------------------------------------
H+ OH- --> H2O  

Is this right

**modified to remove spectator ions**
Title: Re:Hess's Law with net ionic equations
Post by: mike on October 07, 2005, 01:09:04 AM
H+ + NH3 ---> NH4+

You are reacting H+ with NH3 not OH-
Title: Re:Hess's Law with net ionic equations
Post by: Lt.Pliskin on October 07, 2005, 01:20:53 AM
I am totally, lost...

Thanks for your patience with me thus far.

What is the net ionic equation for the NH4CL + NaOH -->  NH4OH  + NaCl?
Title: Re:Hess's Law with net ionic equations
Post by: mike on October 07, 2005, 01:29:52 AM
NH4+ + Cl- + Na+ + OH- ----> NH3 + Na+ + Cl- + H2O
Title: Re:Hess's Law with net ionic equations
Post by: Lt.Pliskin on October 07, 2005, 01:39:12 AM
:D Thank you so much! I hope I havent been too much of a bother. I can finally get some sleep now!