Question

Calculate standard Gibbs energy change for the reaction 4NH3+5O2-4NO+6H2O

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Answer 1

Definition:-

Standard Gibbs free energy of formation of a substance is defined as the free energy change in which their is the formation of 1 mole from its constituent elements in their standard state.

ΔG° = ΣΔG°(products) - ΣΔG(reactants)

Important Point:-

Also it is to be noted that the standard Gibbs free energy of formation of all free elements is assured to be zero.

Now in the given equation,

$\sf4NH_{3} + 5O_{2} \rightarrow 4NO + 6H_{2}O$

$\sf{ \delta \: G \degree = \sum \delta \: G \degree(products) - \sum \delta \: G \: \degree(reactants)} \\ \\ \sf{ \delta \: G \degree = 4 mol \times 4 \delta \: G \degree \: NO(g) + 6 \times \sum \delta \: G \degree \: H_{2}O(g) - 4 \times \sum \delta \: G \degree \: NH_{3} +5mol \times \sum \delta \: G \degree \: O_{2}(g)} \\ \\ \sf{ \delta \: G \degree = 4 \times 86.7 + 6 \times ( - 237.2) - 4mol \times ( - 16.8 KJ/mol) + 5 \times 0} \\ \\ \sf \delta \: G \degree = 346.8 - 1423.2 + 67.2 \\ \\ \sf{ \delta \: G \degree= - 1009.2 KJ}$

Therefore:-

The standard Gibbs free energy is -1009.2 KJ.

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