What is the standard Gibbs free energy for this reaction? In kJ/mol At what temperature T_eq do the forward and reverse rusting reactions occur in equilibrium? In Kelvin?

The chemical reaction that causes iron to rust in air is given by
4Fe + 3O₂ → 2Fe₂O₃
in which
ΔHrxn⁰ =-1648.4 kJ/mol
ΔSrxn⁰ = -543.7 J/mol*K

1 Answer
Michael
May 2, 2016

#DeltaG^@=-1486" ""kJ/mol"#

#"T"=3032" ""K"#

Explanation:

#Delta"G"^@=Delta"H"^@-"T"Delta"S"^@#

Under standard conditions #"T" =298"K"#

#:.Delta"G"^@=-1648.4xx10^(3)-(298xx-543.7)#

#Delta"G"^@=-1486" ""kJ/mol"#

At equilibrium #Delta"G""=0#

#:.0=Delta"H"^@-"T"Delta"S"^@#

  • assuming the enthalpy and entropy changes don't change much with temperature we can use the standard values.

#:."T"=(Delta"H"^@)/(Delta"S"^@)#

#"T"=(-1648.4xx10^3)/(-543.7)" ""K"#

#"T"=3032" ""K"#

Strictly speaking, that is not the equation for rusting. This requires both air and water and the product, rust, is hydrated iron(III) oxide #sf(Fe_2O_3.xH_2O)#.

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