When do you need to balance equations?
1 Answer
Do you mean "why is it sometimes necessary to balance equations"?
Well, an example of an equation that is unbalanced to begin with is:
You can plainly see that there are two nitrogens on the left but only one on the right; that is enough to say it is unbalanced.
What this really means is that the reaction doesn't properly describe what happens in the reaction process. It omits some information unintentionally---how did one hydrogen get "created", and how did one nitrogen get "destroyed"?
The answer is that it doesn't make sense if it's not balanced, because neither of those atoms actually get created OR destroyed. Instead, they basically get cleaved via a catalyst and recombine in a different configuration.
To balance it, you need to double the ammonia on the right.
As a consequence, you have six hydrogens on the right, and so you need to triple the hydrogen count on the left. Finally you get:
