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Topic: How do you find the molar enthalpy for reactions?  (Read 21889 times)

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Offline jawille

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How do you find the molar enthalpy for reactions?
« on: November 04, 2007, 08:29:01 PM »
This is a pre-lab exercise for my Gen Chem Lab. 

I have already found the net ionic equations to all of these reactions, but the change in enthalpy for each reaction has got me stuck.  I'm needing a great deal of help here.  Anything you can give me is greatly appreciated.  All I need is to understand how to find the molar enthalpy for each reaction.  Again even the enthalpy for the first reaction would be great.  Any help is appreciated. 

Reaction 1: An aqueous solution of sodium hydroxide reacts with an aqueous solution of hydrochloric acid, yielding water and sodium chloride. 

NaOH (aq) + HCl (aq) -> NaCl (aq) + H20 (l)

Reaction 2: An aqueous solution of sodium hydroxide reacts with an aqueous solution of hydrochloric acid, yielding water and sodium chloride. 

NaOH (aq) + NH4Cl (aq) -> NH3 (aq) + H20 (l) + NaCl (aq)

Reaction 3: An aqueous solution of hydrochloric acid reacts with aqueous ammonia, NH3, yielding aqueous ammonium chloride. 

NH3 (aq) + HCl (aq) -> NH4Cl (aq)
« Last Edit: November 04, 2007, 10:32:18 PM by jawille »

Offline Yggdrasil

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Re: How do you find the molar enthalpy for reactions?
« Reply #1 on: November 10, 2007, 01:38:21 PM »
Look up Hess's law in your chemistry text book or on wikipedia (http://en.wikipedia.org/wiki/Hess%27s_law).  To apply Hess's law to these reactions, you would need to look up the standard enthalpies of formation for each of the compounds (usually found in the appendix at the back of your chemistry textbook).

If you do this correctly, you should find out that the enthalpy of reaction 3 is the enthalpy of reaction 1 minus the enthalpy of reaction 2 (remember to correctly account for positive and negative values in this calculation).  Do you know why this is?

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