[gleeman]

The following reaction is under consideration
NaOH(s) + H2O(l) => Na+ + OH- + H20 + HEAT

The question:
What is the reason that the heat is formed in this reaction?

I can only guess that:
1. the entropy increases i.e. disorder increases
2. the bonds are being broken and the reaction is exothermic
but these are not the reasons.

Why the bonds are being broken?
Is it because water is polar substance and NaOH is base?
And is NaOH then non-polar?

I appreciate your help.

[vladi307]

This is not a reaction but a dissolution of the sodium hydroxide in water. You must find the enthalphy of the solvation ΔΗsol. This is negative because the process is exothermic.

[gleeman]

Correct.
But why does the dissolution happen?

I have done a lab about this dissolution but I cannot comprehend why it happens. I have calculated that the your mentioned enthalpy of solution for NaOH is negative with the results of my experiments - but without these results is it possible to claim that this dissolution happens and why?

[vladi307]

You must know that the interaction between Na+ and OH- is ionic.Since you solute NaOH the Na+ cations interact strongly with the oxygen of the water and the OH- with the hydrogens of the water. This is the dissolution of a substance. The solvent separate the ions outside the  pattern.

[gleeman]

Thanks for your reply!

Could you say is the following prediction of the latter reaction then correct?
I predict that when NaOH(s) is added into H2O(l) heat will be formed as NaOH dissociates into sodium cations and hydroxide anions. I assume this as NaOH is completely ionic, containing sodium ions and hydroxide ions and water is polar substance. If this is true, the enthalpy of solution for NaOH(s) must be negative i.e. the dissolution is exothermic.

So is the reason for why the dissolutin happens only that NaOH is ionic and water polar, which leads to the fact that the dissolution happens easily?

[enahs]

http://en.wikipedia.org/wiki/Solvation
http://en.wikipedia.org/wiki/Enthalpy_change_of_solution

[vladi307]

If you know, in chemistry we have an empirical rule: polar substances are soluble in polar solvents and non-polar substances in non-polar solvents. In my lab we have tested two salts (KCl and NaCl) and calculated their solvation enthalpies in the water. May I ask you what's your prediction about these processes.

[gleeman]

If you know, in chemistry we have an empirical rule: polar substances are soluble in polar solvents and non-polar substances in non-polar solvents. In my lab we have tested two salts (KCl and NaCl) and calculated their solvation enthalpies in the water. May I ask you what's your prediction about these processes.

I predict that both substances will dissolve in water because both substances are partially ionic and polar and as water is very good solute as it is polar. But I think two latter substances are also more polar than ionic as the electronegativity is > 2.0 in both substances. Therefore, it is very likely the reaction will happen.

Furthermore, I predict that the temperature will increase in the water when NaCl is added into water and temperature will decrease when KCl is added into water.
  Here, I do not know why the temperature will increase in the former dissolution and decrease in the latter dissolution.
I think this is because of Gibbs free energy.

Thanks for your reply!

[vladi307]

A small experiment, take 1 gram of sodium chloride( from your kitchen) and put it in a glass. With a thermometer measure the temperature before and after the dissolution of the salt. Tell me the results please. It have funny. See that after the experiment http://www.uvm.edu/~mcase/courses/chem36/Lecture19_20_21.pdf

[gleeman]

A small experiment, take 1 gram of sodium chloride( from your kitchen) and put it in a glass. With a thermometer measure the temperature before and after the dissolution of the salt. Tell me the results please. It have funny. See that after the experiment http://www.uvm.edu/~mcase/courses/chem36/Lecture19_20_21.pdf

I am not sure but I think that the Temperature increased when the salt was inserted into water, am I right? The guide says that the dissolution is spontaneous for salt hence this explains why the reaction happens but is the reason why the temperature increased that the reaction is exothermic and therefore the bonds are formed which gives energy and heat.

Could you tell me, please. Is it possible to say why the temperature of water then increased after NaCl was inserted?

Thanks for the guide - it is superb!

[vladi307]

The temperature decrease because the solvation is  small endothermic. The dissolution is spontaneous because the entropic character of this process overlap the enthalpic. The guide that I sent you explain very well that.

[gleeman]

Ou, the reaction is really endothermic just like you say.

Thanks for the reply!
I really appreciate your guidance!