Enthelpy of a Neutralization Reaction

[elasticities]

Homework Statement

Say there was an experiment between NaOH and HCl. The reaction is a neutralization reaction resulting in NaCl and H2O. The question is: What if the two reactants weren't aqueous, but if NaOH was in solid pellets instead?

Homework Equations

Neutralization reactions are usually exothermic.

The Attempt at a Solution

I initially thought there would be no enthalpy change because it's just changing the states...and according to Hess' law..it doesn't matter.

But the answer, I think, is that the enthalpy change is supposed to be greater (more negative)...meaning a lower final enthalpy.

But I need help explaining why? Something to do with dissolving, breaking bonds between Na+ and Cl-...and water...

 

[Borek]

You are on the right track. What are reaction products?

 

[elasticities]

You are on the right track. What are reaction products?

The reaction is a neutralization reaction resulting in NaCl and H2O...right?

 

[elasticities]

Another question on the tangent of thermochemistry -

Why are fractions allowed in thermochemical equations but not otherwise?

 

[Borek]

The reaction is a neutralization reaction resulting in NaCl and H2O...right?

Yes. What happens when you have water and salt?

As to the fractions - enthalpy is often reported per mole of reaction or mole of reactant - these don't have to be identical. Sometimes using fractions for other substances you can "normalize" reaction so that it contains one mole of the substance you are interested in.

Note that technically whether coefficients are smallest integers or not doesn't matter, it is stoichiometric ratio of reagents that is important. Using smallest integer is just a convention that makes comparison easier and put some order.

 

[elasticities]

Yes. What happens when you have water and salt?

The salt dissolves...? I'm sorry...I still don't get it. :S

Thanks for explaining my other question, by the way. :) Much appreciated.

 

[Borek]

Salt dissolves, that's right. Dissolution is (especially in the case of anhydrous salts) almost always exothermic.