Calculate the ΔH o for the reaction: Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) ΔH o = kJ/mol Substance ΔH o f (kJ/mol) ΔG o f (kJ/mol) S o (J/K·mol) Fe(s) 0 0 27.2 Fe2+(aq) −87.86 −84.9 −113.39 Fe3+(aq) −47.7 −10.5 −293.3 FeCl3(s) −400 −334 142.3 FeO(s) −272.0 −255.2 60.8 Fe2O3(s) −822.2 −741.0 90.0 Fe(OH)2(s) −568.19 −483.55 79.5 Fe(OH)3(s) −824.25 ? ? H(g) 218.2 203.2 114.6 H2(g) 0 0 131.0 H+(aq) 0 0 0 OH−(aq) −229.94 −157.30 −10.5

ΔHr = -86.73 kJ/mol

Explanation:

Using Hess's law, you can calculate ΔH of any reaction using ΔH°f of products and reactants involed in the reaction.

Hess law: ∑nΔH°f products - ∑nΔH°f reactants = ΔHr

-Where n are moles of reaction-

For the reaction:

Fe³⁺(aq) + 3 OH⁻(aq) → Fe(OH)₃(s)

Hess law is:

ΔHr = ΔH°f Fe(OH)₃ - ΔH°f Fe³⁺ - 3×ΔH°f OH⁻

Where:

ΔH°f Fe(OH)₃: −824.25 kJ/mol

ΔH°f Fe³⁺: −47.7 kJ/mol

ΔH°f OH⁻: −229.94 kJ/mol

Replacing:

ΔHr = −824.25 kJ/mol - (−47.7 kJ/mol) - (3×-229.94 kJ/mol)

ΔHr = -86.73 kJ/mol

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