Some hydrated ionic solids when heated release their water of
crystallisation and dissolve in this water. When the solution is
cooled the solid recrystalizes and heat is released. One example is
calcium chloride hexahydrate and the process is described by the
equation
CaCl2.6H2O => CaCl2.2H2O +
4H2O delta H = +37.2kJ
Process is reversible.
Draw the energy level diagram for the above(forward)
reaction
It is the exothermic reaction yea? The forward reaction I mean.
Because according to the passage, heat is released only when
the solid recrystalizes. In the forward reaction, crystals are
being formed?
By the way which of the formula exist as the powder and which
one exists as the crystals? CaCl2.6H2O and CaCl2.2H2O ?
2
Calculate the energy absorbed when 100kg of calcium chlroide
hexahydrate undergoes the above reaction. Leave answer to nearest
kJ.
So to find the mass of CaCl2.6H2O, I have to get the Mr. Is that
is being calculated include the 6H2O? Referring to
value of 6..thanks
The heating of the hexahydrate form is of course endothermic
(hence "heating of"). The formation of the hexahydrate form is
exothermic, due to the interactions* between the water and the
ions.
The crystalline form refers to the hexahydrate.
Yes, the 100kg refers to the hexahydrate and hence the molar
mass includes the 6 water molecules per formula unit of the ionic
compound.
*Interestingly, not all of the "water of crystallization" has
direct ion-dipole or dative coordinate bonds with the ions.
It's somewhat more complex than what many students may think
:
http://en.wikipedia.org/wiki/Water_of_crystallization