Consider the reaction: 4HCl(g) + O2(g)2H2O(g) + 2Cl2(g) Using standard absolute entropies at 298K, calculate the entropy change for the system when 2.23 moles of HCl(g) react at standard conditions. S°system = J/K

Explanation:

The chemical equation for the reaction is given as:

the entropy change in the system can be calculated as follows:

From the tables; the entropy values where obtained.

∴

i.e the entropy change in the system when 4 moles of HCl is used = -128.9 J/K

∴

when 2.23 moles of HCl is used, Then,

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.