"If you don't do it, I'll". Why does that sentence feel so awkward? [duplicate]

Possible Duplicate:
Is there some rule against ending a sentence with the contraction “it’s”?

Earlier today while writing a very informal email, I expressed:

If you don't do it, I will.

Upon looking at it, I realized that "I will" could be contracted to "I'll", however, this is completely unheard of and feels quite awkward. I am now quite curious, is there a reason why ending a sentence with "I'll" feels (or is) so inappropriate?

I understand that the use of contractions is generally an informal use of language so rules of usage can be a bit loose. Therefore, this is a case that to me doesn't make logical sense and I'm a bit confused. Why does it feel wrong when an "I'll" is placed at at that end of a sentence, even though such an expression is informal and not, technically, an incorrect usage of the contraction "I'll?"

Solution 1:

Contractions can only be used in English when the thing they're contracting does not have any sentence-level stress. (Sentences have stress in them much like words do.) In this sentence, the word I carries a heavy stress:

If she doesn't do it, I will.

Because of this, I cannot contract with any of its neighbors. The same thing explains why we don't use contractions in sentences of the following sort:

Do not go in there.

I would go to the store, but I have to study.