17.3 More enthalpy changes

Question 1

What solvents can ionic solids dissolve in

Answer 1

Polar solvents, as in order for them to dissolve, their lattice must be broken up

Question 2

How do ionic solids dissolve

Answer 2

In order for them to dissolve, their lattice needs to be broken up
Which requires an input of enthalpy- lattice enthalpy

The separate ions are then solvated by the solvent molecules, usually water
These cluster around the ions so the positive ions are surrounded by negative ends of water dipole molecules
And negative ions are surrounded by positive dipole of water molecule

This is called hydration when the solvent is water, so enthalpy of hydration

Question 3

How does the enthalpy of hydration shows the same trend as lattice enthalpy

Answer 3

It is more negative for more highly charged ions, and less negative for bigger ions

Question 4

What are the 3 processes of dissolving ionic compound in water

Answer 4

. Breaking ionic lattice to give separate gaseous ions (so lattice dissociation enthalpy is put in)
. Hydrating the positive ions (cations) , so the enthalpy of hydration is given out
. Hydrating the negative ions (anions), so the enthalpy of hydration is given out

Question 5

What would the equation for the enthalpy of solution of NaCl be

Answer 5

NaCl(s) +aq —> Na+(aq) + Cl-(aq)

So start with enthalpy change of lattice dissociation
Then do enthalpy of solution of the sodium ion
Then enthalpy of solution of chloride ion

Question 6

Usually, the lattice enthalpy formation enthalpy is very similar to the value calculated theoretically

However zinc selenide ZnSe has quite different values for both

Answer 6

Zn2+ ion is small and has a high charge density, whilst Se2- is large and has a high negative charge

So the Zn2+ ions can approach closely to the electron clouds of Se2- and distort them by attracting them towards it

Se2- is easy to distort because its outer shell is quite far from its nucleus and its double charge means there is plenty of charge to distort.

Overall, this means there are more electrons than expected concentrated between the Zn and se nuclei
So there is a degree of electron sharing so se2- is said to be polarised

Question 7

What factors increase polarisation

Answer 7

High charge, small size for the positive ion

Large size, high charge for negative ion

So there is some covalent character when this occurs, so it is not a pure ionic bond
So that is why the value differs from the theoretical value