Question

What is the entropy change for the third reaction below?

Answer 1

Given that  

N2(g) + O2(g) → 2NO(g), ∆H = 180 kJ  ------1

2NO2(g) → 2NO(g) + O2(g), ∆H = 112 kJ -----2

N2(g) + 2O2(g) → 2NO2(g)    ∆H = ? kJ  -----3  

To calculate ∆H of equation 3 use  Hess's law reverse the equation 2 and then add it with equation 1 as follows:

N2(g) + O2(g) --> 2NO(g) ................. ∆H = 180 kJ  

2NO(g) + O2(g --> 2NO2(g) ............. ∆H = -112 kJ  

------------------- --------------------- ----------------------------  

N2(g) + 2O2(g) --> + 2NO2(g)   ∆H = 68 kJ  

Hence the correct is D

Answer 2

292 kJ
Explanation: the summ of the first and the second reaction.