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Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

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BuyFindarrow_forward

Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

Using values of ΔfH° and S° to find ΔrH° and ΔrS°, calculate the free energy change, ΔrG°, for the formation of 2 mol of NH3(g) from the elements at standard conditions and 25 °C.

N 2 ( g ) + 3   H 2 ( g ) 2   N H 3 ( g )

Interpretation Introduction

Interpretation:

The standard free energy change for the formation of methane should be calculated using ΔrHoandS values.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and entropy by the following expression,

  ΔGo= ΔHo-TΔSo

Here, ΔHo is the change in enthalpy, ΔSo is the change in entropy and T is the temperature.

Explanation

The standard free energy change for the formation of ammonia at 298 K is calculated as follows,

Given: N2(g)+3H2(g)2NH3(g)

The Appendix L was referred for the values of standard entropies and enthalpies.

N2(g)+3H2(g)2NH3(g)ΔfH°(kJ/mol)00- 45.90So( J/K×mol)+191.56+130.7+192.77

  ΔrH°=fH°(products)-fH°(reactants)

  ΔrH°=[(2 mol NH3(g)/mol-rxn)ΔfH°[NH3(g)]-[(1 mol N2(g)/mol-rxn)ΔfH°[N2(g)]+(3 mol H2(g)/mol-rxn)ΔfH°[H2(g)]] ] 

  ΔrH°=[(2 mol NH3(g)/mol-rxn)(- 45.90 kJ/mol)-[(1 mol C(graphite)/mol-rxn)(0)+(3 mol H2(g)/mol-rxn)(0)] ]= - 91

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