Hess's Law HELP PLEASE!

[ManzaAP]

Hello!

I'm writing up the post-lab for a thermochem experiment, and I have to combine two net ionic equations to reach the third equation then add up the enthalpy changes. But I need a bit of help working out the equations:

(a) NaOH + HCl > NaCl + H2O

(b) NH4Cl +NaOH > NH3 + NaCl + H2O

© NH3 + HCl > NH4Cl

 

So I have to algebraically combine the first two equations to get to the third equation, but I'm not really sure how

 

Thanks!

[Runninfarmer]

I think you just reverse the bottom equation (which changes the sign on the enthalpy) and then add it to the first equation. Then you cancel out reactants. I'm not fully sure if thats true but someonelse can follow me up on that,

[Psycho]

I would think you reverse equation B

 

(a) NaOH + HCl > NaCl + H2O

(b) NH3 + NaCl + H2O > NH4Cl + NaOH

© NH3 + HCl > NH4Cl

 

To that then you can cancel to get the third equation, meaning you would change the signs on equation B.

 

I'm quite impressed with myself for doing that.

[Runninfarmer]

I meant the bottom equation as being equation B

[thedarkshade]

You have to know whether the reactants are acidic or basic. When you have an "acid + base" reaction the outcome will be the salt plus water, though in some cases water will be needed only to equalize the reaction.