The Ostwald process for the commercial production of nitric acid from ammonia an

Question

The Ostwald process for the commercial production of nitric acid from ammonia and oxygen involves the following steps:

4NH3(g)+5O2(g)------->4NO(g)+6H2O(g) -908 kj/mol (delta H)

2NO(g)+O2(g)----------->2NO2(g) -112 kj/mol

3NO2(g)+H2O(l)-------->2HNO3(aq)+NO(g) -140 kj/mol

Write the overall equation for the production of nitric acid by the Ostwald process by combining the preceding equations (Water is also a product).

So i know the answer is this...

"Multiply the equations so that the coefficients of the N-products become the same as the N-educts of the following line. 4NH3(g)+5O2(g)------->4NO(g)+6H2O(g) -908 kj/mol (delta H) (x3)

2NO(g)+O2(g)----------->2NO2(g) -112 kj/mol (x 6)

3NO2(g)+H2O(l)-------->2HNO3(aq)+NO(g) -140 kj/mol (x 4)

12NH3(g)+15O2(g)------->12NO(g)+18H2O

12NO(g)+6O2(g)----------->12NO2(g)

12NO2(g)+4H2O(l)-------->8HNO3(aq)+4NO..

Add left and right sides of the 3 equations, 12NH3(g)+15O2(g) + 12NO(g)+6O2(g) + 12NO2(g)+4H2O(l)-->12NO(g)+18H2O(g) +12NO2(g) +8HNO3(aq)+4NO(g)

Add same types and remove duplicates 12 NH3(g) + 21 O2(g) --> 4 NO(aq) + 14 H2O(aq) + 8 HNO3(aq)"

What i want to know is why do we multiply the equations like that? Like how did the person who solved it know to multiply thoese eqautions by 3, 6, and 4? We cant just simply add the eqautiosn together without multiplying them? Or? I understand eveything else, besides the step in which each eqaution is mutlipled by a number... Please help!!! Thank you!

Explanation / Answer

It is because when you add equations of a process you always need to remove the intermediates, in this case the intermediates are NO and NO2 because they are produced and consumed on the process. The problem here is that there is no a posible multiplication in which you can eliminate both intermediates, so the NO appears at the end.

I think that is the reason. I hope it helps!

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