안녕하세요 :) 

강의 잘듣고 있다니 감사합니다. 

1. 전자 갯수를 맞춰주는 것이 redox reaction 의 가장 큰 특징입니다 

losing e- = oxidation

gaining e- = reduction 

이기 때문에 electron 을 주고 받는 것이 redox reaction 이죠 

우리가 일반적으로 하는 balance 는 mass conservation 을 시키는 것이 핵심인데 

redox reaction 은 mass conservation & charge conservation 을 시켜야 합니다 

그래서 반드시 양쪽의 charge 를 맞춰줘야 합니다. 

To be more specific in your question equation, 

5Fe2+ --> 5Fe 

We have +10 in the left side, but no charge in the right side, 

that means they are not balanced, 

to make them balanced in terms of charges, 

you need 10- more in the left side, right? 

just algebra. 

but the meaning is this, Fe2+ should gain 2 electrons to form Fe solid (neutral charge) 

and we have 5Fe2+, that requires 10e- overall ( 2e- each * 5 Fe) 

thus, we put 10- in the left side, 

but you know what, 

just algebra doing, or thinking of principles which has gained or lost how many electrons 

have no differences 

so choose either way that is easier to you 

and the purpose of this balancing is to make sure that the charge of everything else in conserved throughout the reaction. 

Nothing can lose electrons if there's none to gain it. 

and for Na+ and NO3-

they are just always soluble ions. :) 

You should remember that rule #1: group 1A, ammonium, nitrate containing compounds are always soluble. 

I hope it helps!