Is the reaction to produce zinc from zinc sulfide spontaneous under standard conditions? Coupledreaction: ZnS(s) + H2 (g)  Zn(s) + H2S(g) ΔG°=? The free energy of the following two reactions are well known. Calculate ΔG° for the coupled reaction above. Rxn1: Zn(s) + S(s)  ZnS(s) ΔG1°= -201.3kJ Rxn2: S(s) + H2(g)  H2S(g) ΔG2°= -33.4kJ

The reaction to produce zinc from zinc sulfide is not spontaneous under standard conditions.

Explanation:

Hess's law states: "When one reaction can be expressed as the algebraic sum of others, its heat of reaction is equal to the same algebraic sum of the partial heats of the partial reactions". So Hess's Law is an indirect method of calculating the heat of reaction or enthalpy of reaction when the chemical reaction occurs in one or more than one stage.

Taking into account that ΔG is a state function, which only depends on the initial and final states, its variation in a reaction is calculated by adding the free energies of the reactants and products involved in it when both are in the normal state. That is, at the pressure of 1natm if it is gases or at the concentration of 1 mol / L for substances in liquid solution.

The sum of the fitted equations should give the problem equation. So, if in a "data" reaction a substance is as a reactant and in the reaction that you must obtain is as a product, you must turn the "data" reaction and ΔG will change its sign. In this case, this happen with the reaction 1 (Rxn1):

Rxn1: ZnS (s) → Zn (s) + S (s)          ΔG1°= 201.3 kJ

Rxn2: S (s) + H₂ (g) → H₂S (g)        ΔG2°= -33.4kJ

Adding both reactions (taking into account that certain substances appear sometimes as a reagent and others as a product, so they are totally eliminated if they appear in the same quantities) you get:

ZnS (s) + H₂ (g) → Zn (s) + H₂S (g)

Adding algebraically ΔG1° and ΔG2° you get:

ΔG°= ΔG1° + ΔG2°

ΔG°= 201.3 kJ - 33.4 kJ

ΔG°= 167.9 kJ

If a chemical reaction proceeds with a ΔG <0 the process is  spontaneous. If, on the other hand, ΔG> 0, the reaction is not spontaneous.

Since ΔG>0, the reaction to produce zinc from zinc sulfide is not spontaneous under standard conditions.

i do not get it   at all