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Lattice energy, hydration energy, solution energy

Dhamnekar Winod

Active member
Nov 17, 2018
162
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?
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,485
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.