Topic: Hess\' Law, H o ,S 0 ,G 0 Part A Given the following data and using Hess’
ID: 927869 • Letter: T
Question
Topic: Hess' Law, Ho,S0,G0
Part A
Given the following data and using Hess’ Law, Calculate change in Enthalpy for the reaction
Overall reaction : NO(g) + O (g) ------> NO2(g)
2O3(g) ------> 3O2(g) H = - 427 kJ
O2(g) ------> 2O(g)H = 495 kJ
NO(g) + O3(g) ------> NO2 (g) + O2 (g) H = -199 kJ
Part B
Calculate Ho,S0,G0 for the formation of sulfur trioxide from oxygen gas and sulfur dioxide ;
To get any credit you must write the balanced equation and write the #, unit, substance for the calculations
Substance
Enthalpy value (Kj/mole)
Entropy value
(J/K-mole)
Sulfur dioxide gas
-297
248
Sulfur trioxide gas
-396
257
Oxygen gas
0
205
Substance
Enthalpy value (Kj/mole)
Entropy value
(J/K-mole)
Sulfur dioxide gas
-297
248
Sulfur trioxide gas
-396
257
Oxygen gas
0
205
Explanation / Answer
Target reaction : NO(g) + O (g) ------> NO2(g)
Given reactions are to be arranged so that we get the above target reaction and at the same time
same algebraic action is done on the H values
---------------------------------------------------------------------------------------
-1/2 { 2O3(g) ------> 3O2(g) } -1/2 ( H = - 427 kJ)
-1/2 { O2(g) ------> 2O(g) } -1/2( H = 495 kJ)
1x { NO(g) + O3(g) ------> NO2 (g) + O2 (g) } 1( H = -199 kJ)
Hreaction = -1/2 ( - 427 kJ) - 1/2( 495 kJ) +1(-199 kJ) = -233 KJ
---------------------------------------------------------------
B part
SO2(g) + 1/2O2(g) ---------------------> SO3(g)
Ho = Hoproducts-Horeactants =-396 - (-297 ) = -99KJ = -99000Joules
S0 = soproducts-soreactants =257 - (248 + 1/2(205)) = -93.5 Joules
No temperature mentioned so calculating it at 300K
G0 = Ho - Tso = -99000 - 300x(-93.5) = -70950 joules
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