What Must Happen to the Sodium Acetate for It to Dissolve in the Water Again

Now as for the question;

since i study chemistry i'd besides like to know why sodium acetate releases heat and ammonium nitrate absorbs it.

I think i will take a stab at it

So, as far equally sodium acetate goes, we must first consider what is happening. WEll, the sodium acetate is forming a solid and is heating up the sorounding solution, but hte question is, "why?"

WEll, that is a supurb question and is excellent for demonstrating the power of thermodymanics (equally information technology can easily be answered with just a basic grasp of enthalpy and entropy and gibbs free energy).

So nosotros know that gibbs complimentary energy of a reaction (delta G --> /_\G) is equal to the sum of the enthalpy of that reaction and the negative entropy of a reaction multiplied by the temperature at wich it happens, right? That is, we have;

/_\M = /_\H - /_\Due south (T)

ok, we also know that for a process to proceed spontaneously, information technology must accept a negative delta G. Right?

Ok, so durring the formation of the solid, the sodium acatate is falling out of solution and forming an ordered lattice (crystals). IN the course of this, the soduim acetate is having information technology degrees of liberty (spacially) severely diminished (going from free in solution, to trapped in a latice). THis means that in the form of the reaction the system is loosing entropy (becoming more ordered).

Ok, so we know that the organization is becoming more ordered, correct? That must mean that /_\S is negative. But since nosotros subtract out the entropy in the equaion, we come across that the overal contribution of the entropy volition be a positive number.

Well, we know that the reaction does proceed spontaneously (once it is started) becuase when you click the metal disc the pack gets hot. So, /_\ Thousand must be negative. In order for /_\G to be negative, we see that /_\H must be nagative every bit well (in order to counter-act the positive contribution of the entropy). So we know that /_\H must be negative.

Well, what do we call a reaction with a negative /_\H? Exothermic ANd a reaction that is exothermic releases heat.

So we have just shown, thermodynamically that the reaction of sodium acetate (or any other substance, for that matter) forming crystals out of solution must be exothermic. It just and then happens that for sodium acetate, it is quite exothermic.

Now every bit far as Ammonium Nitrate (A.N.) goes, there is non quite the same elegant explination that volition evidence why it must be endothermic. But lets look at it anyways.

OK, so for A.N. what exercise we accept? WE accept a solid inside of a container that when it is broken it gets dissoved? (I think that is right). And so, using the same reasoning as above, nosotros see that when we are disolving a crystal, we are increaing the entropy of the arrangement. That is becuase the molecules are freed from a constricted latice and put into solution.

Now that must mean that entropy is positive (inacreasing). That means that the contribution of entropy to the gibbs free energy volition be negative. Right? WEll, if entropy is negative, enthalpy could be positive and the overall gibbs free energy would still be negative.

So we see that the reactoin could be negative, though information technology is not forced to be. Thus, in the case of A.N. it just so happens that the enthalpy of disolving actually is negative.

Of course it turns out that all disolving of substances is endothermic, just like all crystalizing is exothermic. Only I am not quite sure that you are in a identify where i could expalin information technology (an i am not sure that i take time to write all that correct now anyways, sorry)

The take abode message existence that, practice to a subtract in entropy durring freezing (crystalization), thermodynamics shows us that crystalization is nessesarily an exothermic reaction.

Promise that answers your question -- enquire more than if y'all got 'em

tanbobsely75.blogspot.com

Source: https://www.scienceforums.net/topic/2231-sodium-acetate-and-water-heat/

Share this post