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)

The overall reaction is strongly exothermic."

What i want to know is why do we multiply the equations like that? Am i missing a concept? Because why can we just not add the eqautions together without multiplying them like how the correct asnwer is?

Answer 1

Solution :-

We multiply equations and add them to get the overall equation because the enthalpy change associated with the overall reaction is same as the total eneergy change association with the three separate reactions.

Therefore we multiply equations by proper numbers to get the reactant and products gets balanced on the opposite side of the equation and then add them together to get the overall reaction equation so that we can find out the enthalpy change associated with overall reaction.

Following image shows the calculation for the enthalpy change of the overall reaction.