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I was doing some questions on equilbrium. I got stuck in this particular question.

Identify which of the following reactions are reversible.

$$ \begin{align} \ce{Fe + S &-> FeS }\label{rxn:1}\tag{1}\\ \ce{NaCl + AgNO3 &-> AgCl + NaNO3}\label{rxn:2}\tag{2}\\ \ce{H2SO4 + Ba(OH)2 &-> BaSO4 + 2 H2O}\label{rxn:3}\tag{3}\\ \ce{H2 +I2 &-> 2HI }\label{rxn:4}\tag{4}\ \end{align} $$

I know that \eqref{rxn:2} and \eqref{rxn:3} cannot be reversible because one of the products is precipitated. I could not reach a conclusion between \eqref{rxn:1} and \eqref{rxn:4} (although I know that \eqref{rxn:4} is reversible). Is there a way to determine without actual experiments whether a reaction is reversible or not?

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Strictly speaking, the answer depends on conditions and, as a consequence, states of aggregation.

Reaction (1) can be reversible if iron(II) sulfide is subsequently heated above $\pu{700 °C}$ in vacuum to prevent oxidation of the elements [1, p. 422]:

$$\ce{FeS(s) ->[\pu{700 °C}][vac] Fe(s) + S(g)}\tag{1a}$$

Reactions \eqref{rxn:2a} and \eqref{rxn:3a} are indeed irreversible if carried in aqueous solution — as you already mentioned, a poorly soluble salt is formed in both cases:

$$ \begin{align} \ce{NaCl(aq) + AgNO3(aq) &-> AgCl(s) + NaNO3(aq)}\label{rxn:2a}\tag{2a}\\ \ce{H2SO4(aq) + Ba(OH)2(aq) &-> BaSO4(s) + 2H2O(l)}\label{rxn:3a}\tag{3a} \end{align} $$

Reaction \eqref{rxn:4a} is a canonical example of a reversible reaction used to study equilibrium in a gas phase:

$$\ce{H2(g) + I2(g) <=> 2 HI(g)}\label{rxn:4a}\tag{4a}$$

I'd say it's impossible to tell in advance what reaction is reversible or not without conducting an actual experiment or at least estimating free Gibbs energy.

References

  1. R. A. Lidin, V. A. Molochko, and L. L. Andreeva, Reactivity of Inorganic Substances, 3rd ed.; Khimia: Moscow, 2000. (in Russian)
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