Question

Suggest why CaO rather than MgO is used to dry ammonia.

Answer 1

Ca\(^{2+}\) is < elctronegative than Mg. Therefore, CaO is more ionic than MgO. and in turn means that the O in CaO is more like O\(^{2–}\). It can react better with the water.

Answer 2

I thought it was related to enthalpy change of solution. Calcium and Magnesium share the same charge density but are different compared to ionic radius, the charge density is spread out over a larger volume causing a weaker electrostatic forces of attraction in CaO than MgO and for values of enthalpy change of hydration CaO would be less exothermic as it has a larger ionic radius and so solubility is less in CaO than in MgO

Answer 3

Without diving into Physical Chemistry, you could argue that, and probably go a bit deeper. But for general purposes, I would go with the electropositivity. Unless you are in Physical organic chemistry, which this sort of sounds like a question for?

Answer 4

Thank you for your help :)

Answer 5

@abb0t is the difference in electronegativities related to the hydration spheres? larger Ca ion than Mg ion, so more water molecules are attracted to it, increasing the # absorbed?

Answer 6

Yes, good point, the number of water molecules in the total hydration sphere is determined by the charge density of the ion.
also, the ion-dipole attractions have an inductive effect on the :O:–H bonds of
the water molecules which can weaken the bonds in water allow H\(^+\)to be lost to solvent water.

Answer 7

Mg and Calcium share the same charge density. If its about electronegativity, Calcium is less electronegative than magnesium so would absorb less water.

Answer 8

they have the same charge, but the Ca ion has a much larger radius than Mg (114pm to 86pm) http://en.wikipedia.org/wiki/Ionic_radius#mediaviewer/File:Atomic_%26_ionic_radii.svg the larger ionic shell allows more water molecules around it

Answer 9

okay now i agree with that point but when i checked out the mark scheme it said that calcium has a greater affinity than magnesium. will greater affinity cause more water molecules to be absorbed