Question

How do you balance the following:

KI->K + I2
Na + H2O -> H2 + NaOH
Ch4 + O2 -> CO2 + H2O

Answer 1

These simpler equations can be balanced by trial and error. For example, here is how you go about it.
Na + H2O ==> H2 + NaOH.

I look at the equation and i see Na is ok, O is ok, but H has 2 atoms on the left and 3 on the right. Whatever I do to balanced H will mess up Na and O but so be it.
Since I need more H on the left, and 1 doesn't work as a coefficient, we'll try 2.
Na + 2H2O ==> H2 + NaOH
Now I have 4 H on the left and I can make 4 H on the right by placing a 2 in front of NaOH; like so.
Na + 2H2O ==> H2 + 2NaOH.

Now, I still have 4 H on left and 4 on the right. I also have 2 O on the left and 2 on the right. I see I have 2 Na on the right and I can balance Na by placing a 2 in front of Na on the left like so.
2Na + 2H2O ==> H2 + 2NaOH.
Now we check it to make sure we are right.
2Na on left; 2 on the right.
4 H on the left; 4 on the right.
2 O on the left; 2 on the right.
OK. Good. It balances.

I'll leave the others for you.