[forpageletter]

So I just had a Chemistry exam, and I got tripped up by some stuff.

#1) There are a question about a precipitation reaction. I think it was NaCO3 (aq) + CaNO3 (aq) ----> CaCO3 (s)

It said that "the precip. rxn was endothermic." Then asked, "What was the change of Entropy (delta S) for the system? (negative, positive, or no change)"

"What was the change in kinetic energy for the system? (negative, positive, or no change)"

"Did potential energy of system increase or decrease after precipitation? (negative, positive, or no change)"

Then the final question was: "The system is the water and the ions. Are the attractive forces within the system greater before or after the precipitation?"

In my notes, I have the system defined as the reactants and products, but if the system is defined as the water and the ions, how can you include the products of the reaction (solid CaCO3 ) in the system, if they have precipitated out of ion form? Also, is water really included in the system? I'm almost positive that my lab instructor said that the water is considered the surroundings (when a salt is dissolved).


#2) "After a solution of HF (aq) is titrated with NaOH, based on the remaining ions, will the final solution be basic, acidic, or neutral?"

I was unsure if this was neutral, (because obviously the point of titration was to neutralize the HF acid with a base), or if it was basic, because the HF and NaOH form H2O, but NaF is soluble and therefore stays as Na+ and F- ions, and since F- ions are slightly basic (whereas Na+ are not acidic?) the solution will be more basic.

#3) Also, "Categorize CN- + H2O ---> HCN + OH- as arrhenius, bronsted lowry, lewis acid base, or some combination of these"

[Astrokel]

In my notes, I have the system defined as the reactants and products, but if the system is defined as the water and the ions, how can you include the products of the reaction (solid CaCO3 ) in the system, if they have precipitated out of ion form? Also, is water really included in the system? I'm almost positive that my lab instructor said that the water is considered the surroundings (when a salt is dissolved).

I don't get what you are trying to say.

#2) "After a solution of HF (aq) is titrated with NaOH, based on the remaining ions, will the final solution be basic, acidic, or neutral?"

I was unsure if this was neutral, (because obviously the point of titration was to neutralize the HF acid with a base), or if it was basic, because the HF and NaOH form H2O, but NaF is soluble and therefore stays as Na+ and F- ions, and since F- ions are slightly basic (whereas Na+ are not acidic?) the solution will be more basic.

Correct, final solution will be basic due to hydrolysis of F^-. Na⁺ is a neutral ion.

#3) Also, "Categorize CN- + H2O ---> HCN + OH- as arrhenius, bronsted lowry, lewis acid base, or some combination of these"

Use the definitions

[eunChae]

In my notes, I have the system defined as the reactants and products, but if the system is defined as the water and the ions, how can you include the products of the reaction (solid CaCO3 ) in the system, if they have precipitated out of ion form? Also, is water really included in the system? I'm almost positive that my lab instructor said that the water is considered the surroundings (when a salt is dissolved).

(ion1) + (ion2) --> water + solid
water is generally considered as surroundings but when you examine the potential energies you have two type(particularly in for your question): 1_intermolecular forces between water and ions in the reactants side 2_ intramolecular forces within water molecule and solid in the products side. And the difference btw (1) and (2) will give you the enthalpy change. Therefore you have to use both water and precipitates to find out the potentials and consider them as a system, I think.

[zxt]

So I just had a Chemistry exam, and I got tripped up by some stuff.

#1) There are a question about a precipitation reaction. I think it was NaCO3 (aq) + CaNO3 (aq) ----> CaCO3 (s)

It said that "the precip. rxn was endothermic." Then asked, "What was the change of Entropy (delta S) for the system? (negative, positive, or no change)"

"What was the change in kinetic energy for the system? (negative, positive, or no change)"

"Did potential energy of system increase or decrease after precipitation? (negative, positive, or no change)"

Then the final question was: "The system is the water and the ions. Are the attractive forces within the system greater before or after the precipitation?"

In my notes, I have the system defined as the reactants and products, but if the system is defined as the water and the ions, how can you include the products of the reaction (solid CaCO3 ) in the system, if they have precipitated out of ion form? Also, is water really included in the system? I'm almost positive that my lab instructor said that the water is considered the surroundings (when a salt is dissolved).

Please write formula correctly.

First I doubt this rxn is endothermic,since less ions are moving in the system less chaotic the system will be, and S can be regarded a symbol to describe the chaos of one system, and generally a cooler system has a less S value, so this rxn should be exothermic I think. Even though this rxn is endothermic, according to  :delta:S= Q/ :delta:T,
Q>0, :delta:T>0, so the  :delta:S>0.

As I said if  :delta:S>0 and Q>0, then generally ions will move more chaotically and with heat absorbed, the average Kinetic energy of  the ions and molecules will rise. So you know……

Because  :delta:U=Q+W, Q>0, W= :delta:(PV), as to liquid system, we regard P and V constant, so W=0. In addition, potential energy increased.

You know that solid like CaCO₃ doesn't stay totally solid in aq system. It partially ionizes as the equilibrium:
CaCO₃(s) ::equil::Ca²⁺(aq)+CO₃^2-(aq). And electrostatic force will be weakened after the extent of thermal movement increased.

[zxt]

Here one thing should be modified,   :delta:S=∫δQ/T.  :delta:S=Q/ :delta:T is not correct.