resist
Badges: 0
Rep:
?
#1
Report Thread starter 5 years ago
#1
I'm currently revising CHEM5 and I seem to have hit some conflicting information in my text book.

According to the book experimental lattice enthalpies are higher than theoretical values when there is partial covalent bonding, i.e when the purely ionic model doesn't fit. It goes on to say that the covalent character creates stronger bonding.

However a few pages later in the chapter on Period 3 Oxides my book says "Al2O3 has a lower melting point than you might expect because the 3+ ions distort the oxygen's electron cloud making the bonds partially covalent".

This is completely conflicting with the pure ionic model is it not? Can anyone help me out here?
0
reply
charco
Badges: 17
Rep:
?
#2
Report 5 years ago
#2
(Original post by resist)
I'm currently revising CHEM5 and I seem to have hit some conflicting information in my text book.

According to the book experimental lattice enthalpies are higher than theoretical values when there is partial covalent bonding, i.e when the purely ionic model doesn't fit. It goes on to say that the covalent character creates stronger bonding.

However a few pages later in the chapter on Period 3 Oxides my book says "Al2O3 has a lower melting point than you might expect because the 3+ ions distort the oxygen's electron cloud making the bonds partially covalent".

This is completely conflicting with the pure ionic model is it not? Can anyone help me out here?
Yes, it is a confusing issue.

A degree of covalency leads to weaker bonding in general. However there are cases where the ionic model appears to break down (as far as lattice enthalpy goes) and there is stronger bonding.

My rationale is that there are two forms of covalent bonding:

1. one that creates individual molecules
2. one that creates a giant lattice

It is easy to say that the lower m.p. of lithium chloride is due to a degree of covalency, but that the higher experimental lattice enthalpy of AgCl (cf the theoretical) is due to some extra bonding not envisaged by the theory.

Occasionally chemistry is like this. Make the observation and then adjust the theory to fit ...
0
reply
resist
Badges: 0
Rep:
?
#3
Report Thread starter 5 years ago
#3
Thanks for your help. I guess I'll have to watch out for any questions on this in the exam and think them through carefully.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Sheffield Hallam University
    Get into Teaching in South Yorkshire Undergraduate
    Wed, 26 Feb '20
  • The University of Law
    Solicitor Series: Assessing Trainee Skills – LPC, GDL and MA Law - London Moorgate campus Postgraduate
    Wed, 26 Feb '20
  • University of East Anglia
    PGCE Open day Postgraduate
    Sat, 29 Feb '20

People at uni: do initiations (like heavy drinking) put you off joining sports societies?

Yes (422)
67.41%
No (204)
32.59%

Watched Threads

View All