Question

Two atoms with charges of +1 and -1 are separated by a distance of 0.5 angstroms. What would happen to the lattice energy between ions if the distance increases to 2 angstroms?

A. The lattice energy decreases by a factor of 4.
B. The lattice energy increases by a factor of 16.
C. The lattice energy decreases by a factor of 16.
D. The lattice energy increases by a factor of 4.

Answer 1

I always have trouble with these because one never knows how negative numbers are counted when they increase or decrease. In addition the definition of lattice energy depends upon which book is used. Here is my reasoning. But first, the change is from 0.5 to 2 which is squared distance or 0.25 to 4 which is a factor of 16. So the answer must be B or C.

If I have a MCl molecule(crystal), it takes energy to break up the crystal lattice. Let's call that value 1000 J and we must ADD it to break the crystal lattice. The lattice energy, U, is - crystal energy so U = -1000 J. Now, if we separate MCl by a factor or 4, it will be easier to break the crystal; let's say that is +500. Now U = -500. If U changed from -1000 to -500 that is a less negative number so i say it increases. I would choose B.

Answer 2

the answer was c in case you were wondering

Answer 3

Between CsCl and BaCl2 compare lattice energy

Answer 4

If anyone wanted to know how to solve it for next time you need the equation for the force of attraction. It is (F1 * F2)/(r^2)

In this problem F1 is +1 F2 is -1 and r= 0.5 and then 2.
Plug the values in,(r= 0.5) you should get F= -4

Plug the values in the second time, this time with r= 2
You should get -0.25

Compare the 2 answers and you should see that the lattice energy decreases by a factor of 16.