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I am solving question on lattice energy and I have lots of doubt about it. First,how to compare lattice energy between 2 compounds belonging to different groups and periods.I have also doubt between some compounds. 1.Between $AlF_3$ and $MgO$(In $AlF_3$ product of ionic charge is 3×1 and in MgO it is 2×2.So,according to it MgO have high lattice energy but $AlF_3$ have highest 2.Between $KF$ and $LiF$ (According to Fajan's rule $KF$ is more ionic than $LiF$ and lattice energy depends directly on ionic character .But,value of lattice energy of $KF$ is 808 and $LiF$ is 1030.How,is this possible?? So,can you please explain me how to compare lattice energy in different compound and which factor determines more size of cation and anion or charge on ions.Please clear all the cases as I have great doubt in all these.

I am solving question on lattice energy and I have lots of doubt about it.
First,how to compare lattice energy between 2 compounds belonging to different groups and periods.I have also doubt between some compounds.
1.Between $AlF_3$ and $MgO$(In $AlF_3$ product of ionic charge is 3×1 and in MgO it is 2×2.So,according to it MgO have high lattice energy but $AlF_3$ have highest
2.Between $KF$ and $LiF$ (According to Fajan's rule $KF$ is more ionic than $LiF$ and lattice energy depends directly on ionic character .But,value of lattice energy of $KF$ is 808 and $LiF$ is 1030.How,is this possible??
So,can you please explain me how to compare lattice energy in different compound and which factor determines more size of cation and anion or charge on ions.Please clear all the cases as I have great doubt in all these.

Grade:12th pass

2 Answers

Pooja
askIITians Faculty 477 Points
5 years ago
Dear student,
Lattice energy – ultimately decide how closely are cations and anions attracted to each other in a crystal.
For your 1. The cationic charge on Al is 3+ while that on Mg is 2+
Greater the charge on cations greater is the lattice energy.

For your 2. The size of F- is small and this forms a stronger bond with the smaller sized Li+ as compared to K+
Rajdeep
231 Points
5 years ago
HELLO THERE!
 
Lattice energy is the energy released when an ion forms a crystal lattice.
 
Lattice energy depends upon Cationic charge and size.
Lattice energy is directly proportional to charge and inversely proportional to size (of cation).
 
We know that these are related. If the charge on cation is more, the size of the cation is less (e.g. Al3+ is smaller than Na+ and hence has more Lattice energy). Hence, for your first question, you got your answer. Al3+ has higher positive charge density than Mg2+, so lattice energy of AlF is more.
 
For your second question: If two compounds are given, whose cationic charge is the same, you have to compare with size. We know, that Li+ is much smaller than K+, and as mentioned earlier, lattice energy is inversely proportional to size, so smaller cation has more lattice energy. Thus, LiF > KF.

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