using a table from online or a chemistry textbook, calculate the delta H rxn and
ID: 826999 • Letter: U
Question
using a table from online or a chemistry textbook, calculate the delta H rxn and delta S rxn for the following reactions: ( I have listed all of the delta H and S values from the book here)
a.) 2 CH4 --> C2H4 + 2H2
CH4: delta H = -74.6, delta S = 186.3
C2H4: dela H = 52.4, delta S = 219.3
H2: delta H = 0, delta S 130.7
b.) 7.5 O2 (g) + C6H6 (l) --> 6 CO2 (g) + 3 H2O (g)
O2: delta H = 0, delta S = 205.2
C6H6: delta H = 49.1, delta S = 173.4
CO2: delta H = -393.5, delta S = 213.8
H2O: delta H =-285.8, delta S = 70.0
the calculate the delta G for each reaction above at 25 and 100 degree C and tell if its spontaneous at that temperature or not.
Explanation / Answer
(a) 2 CH4 => C2H4 + 2 H2
Delta Hrxn = Delta Hf(products) - Delta Hf(reactants)
= Delta Hf(C2H4) + 2 x Delta Hf(H2) - 2 x Delta Hf(CH4)
= 52.4 + 2 x 0 - 2 x (-74.6) = 201.6 kJ
Delta Srxn = S(products) - S(reactants)
= S(C2H4) + 2 x S(H2) - 2 x S(CH4)
= 219.3 + 2 x 130.7 - 2 x 186.3 = 108.1 J/K
When T = 25 deg C = 298.15 K
Delta Gxn = Delta Hrxn - T x Delta Srxn
= 201.6 - 298.15 x (108.1/1000) = 169 kJ
Since Delta Grnx > 0 => reaction is not spontaneous at 25 deg C
When T = 100 deg C = 373.15 K
Delta Gxn = Delta Hrxn - T x Delta Srxn
= 201.6 - 373.15 x (108.1/1000) = 161 kJ
Since Delta Grnx > 0 => reaction is not spontaneous at 100 deg C
(b) 7.5 O2 (g) + C6H6 (l) => 6 CO2 (g) + 3 H2O (g)
Delta Hrxn = Delta Hf(products) - Delta Hf(reactants)
= 6 x Delta Hf(CO2) + 3 x Delta Hf(H2O) - 7.5 x Delta Hf(O2) - Delta Hf(C6H6)
= 6 x (-393.5) + 3 x (-285.8) - 7.5 x 0 - 49.1 = -3267.5 kJ
Delta Srxn = S(products) - S(reactants)
= 6 x S(CO2) + 3 x S(H2O) - 7.5 x S(O2) - S(C6H6)
= 6 x 213.8 + 3 x 70.0 - 7.5 x 205.2 - 173.4 = -219.6 J/K
When T = 25 deg C = 298.15 K
Delta Gxn = Delta Hrxn - T x Delta Srxn
= -3267.5 - 298.15 x (-219.6/1000) = -3202 kJ
Since Delta Grnx < 0 => reaction is spontaneous at 25 deg C
When T = 100 deg C = 373.15 K
Delta Gxn = Delta Hrxn - T x Delta Srxn
= -3267.5 - 373.15 x (-219.6/1000) = -3186 kJ
Since Delta Grnx < 0 => reaction is spontaneous at 100 deg C
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