Question

• Like in thermochemistry, we define standard Gibbs energy of formation of compound G_f⁰ just like how we define standard enthalpy of formation, H_f⁰
• Knowing compounds Gibbs energy of formation allows one to determine reaction Gibbs energy change conveniently

1. Which of the following reaction gives the enthalpy of formation of the compound G_f⁰ ? Give the reasons if the reaction does not correspond to formation reaction.

• Cs+O2g→CO2g ∆Grxn°=-394.4 kJ
• 2 H2g+O2g→2 H2Og   ∆Grxn°=-457.1 kJ
• NO(g) + ½ O₂(g)  NO₂ (g)                ∆Grxn°=-35.4 kJ

Answer 1

free energy of formation is the free energy calculated
when 1 mol product is formed from its constituent
elements in their standard state.Hence when balanced reaction
shows formation of 1 mole product from it elements
in standard state, ∆Grxn°= ∆Gf

In the given set

2 H2g+O2g→ 2 H2Og 2 H2g+O2g→2 H2Og ∆Grxn°=-457.1 kJ=-457.1 kJ/mol
Here 2 mol product is formed : hence ∆Grxn° not same as ∆Gf of H2O

NO(g) + ½ O2(g) --> NO2 (g) ∆Grxn°=-35.4 kJ

Here product is formed from some other compound ,not
from constituent elements : hence ∆Grxn° not same as ∆Gf of NO2

Cs+O2g→CO2g ∆Grxn°=-394.4 kJ

Here 1 mol CO2 formed from its constituent elements
C and O2 ,both in their standard state .So for this
reaction
∆Grxn° = ∆Gf(CO2)
Answer : option A
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