# Reversibility of a reaction

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?

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)