MHB Lattice energy, hydration energy, solution energy

  • Thread starter Thread starter WMDhamnekar
  • Start date Start date
  • Tags Tags
    Energy Lattice
AI Thread Summary
The discussion focuses on the enthalpy of solution for potassium chloride (KCl), highlighting that its dissolution is endothermic with a calculated value of +30 kJ/mol, contrasting with the standard value of -17.22 kJ/mol. Participants debate whether the temperature increases or decreases when KCl dissolves, concluding that it decreases due to the energy required for dissolution. The conversation also touches on interactions during dissolution, confirming ion-dipole interactions for calcium chloride in ethanol and dispersion forces for toluene in benzene. Additionally, the enthalpy of hydration is compared between sodium and calcium ions, with calcium having a larger absolute value, and similar reasoning is applied to potassium nitrate (KNO3), which also results in a temperature decrease upon dissolution.
WMDhamnekar
MHB
Messages
376
Reaction score
28
Hi,

The enthalpy of crystallization of $KCl$ (Potassium Chloride) is + 715 kJ/mol. The enthalpies of hydration for Potassium and Chloride are -322 and -363 kJ/mol respectively. So, enthalpy of solution of $KCl \Delta H_{sol}=\Delta H_{lattice} - \Delta H_{hyd}$

$\Delta H_{sol}=715 kJ/mol -685 kJ/mol= 30 kJ/mol.$

But if you see table of enthalpy of solution, it is -17.22 kJ/mol for $KCl(s).$ Obviously, our experiment value 30 kJ/mol indicates dissolving $KCl$ into water is endothermic reaction. Should the temperature increase or decrease when $KCl$ dissolves?

My answer is temperature increase when $KCl$ dissolves.

What types of interaction are present when $CaCl_2$ dissolves in ethanol($CH_3CH_2OH)$?
My answer: Ion-dipole

What is the major type of interaction when toluene$C_7H_8$ dissolves in benzene$C_6H_6?$
Answer is unknown.

Which ion has a larger absolute value of enthalpy of hydration, $Na^+$ or $Ca^{2+}$?
My answer: $Ca^{2+}$

Which ion is larger, $Na^+$ or $Cl^-?$
My answer is $Cl^-$

When $KNO_3$ is dissolving in water, will the temperature decrease or increase?
My answer:Temperature will increase.

Are my answers correct?
 
Mathematics news on Phys.org
Dhamnekar Winod said:
Hi,

The enthalpy of crystallization of $KCl$ (Potassium Chloride) is + 715 kJ/mol. The enthalpies of hydration for Potassium and Chloride are -322 and -363 kJ/mol respectively. So, enthalpy of solution of $KCl \Delta H_{sol}=\Delta H_{lattice} - \Delta H_{hyd}$

$\Delta H_{sol}=715 kJ/mol -685 kJ/mol= 30 kJ/mol.$

But if you see table of enthalpy of solution, it is -17.22 kJ/mol for $KCl(s).$ Obviously, our experiment value 30 kJ/mol indicates dissolving $KCl$ into water is endothermic reaction.

I'm seeing slightly different numbers from different sources and I'm not sure why.
Either way, with your numbers it takes 715 kJ/mol to break the lattice, and 685 kJ/mol is released when we dissolve the ions.
The result is positive, meaning that it takes energy to dissolve $\ce{K Cl}$, which is indeed endothermic.

Should the temperature increase or decrease when $KCl$ dissolves?
My answer is temperature increase when $KCl$ dissolves.

It takes energy to dissolve the crystal, which must come from somewhere.
It means that the temperature will decrease instead of increase.

What types of interaction are present when $CaCl_2$ dissolves in ethanol($CH_3CH_2OH)$?
My answer: Ion-dipole
Correct.

What is the major type of interaction when toluene$C_7H_8$ dissolves in benzene$C_6H_6?$
Answer is unknown.

This page lists it as dispersion.

Which ion has a larger absolute value of enthalpy of hydration, $Na^+$ or $Ca^{2+}$?
My answer: $Ca^{2+}$

Which ion is larger, $Na^+$ or $Cl^-?$
My answer is $Cl^-$

You didn't explain why... but yeah, that looks correct to me.

When $KNO_3$ is dissolving in water, will the temperature decrease or increase?
My answer:Temperature will increase.

It appears that $\ce{K NO3}$ also has a positive enthalpy of solution.
It means that the temperature will decrease as well since it takes energy to dissolve.
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Thread 'Imaginary Pythagorus'
I posted this in the Lame Math thread, but it's got me thinking. Is there any validity to this? Or is it really just a mathematical trick? Naively, I see that i2 + plus 12 does equal zero2. But does this have a meaning? I know one can treat the imaginary number line as just another axis like the reals, but does that mean this does represent a triangle in the complex plane with a hypotenuse of length zero? Ibix offered a rendering of the diagram using what I assume is matrix* notation...
Back
Top