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While doing certain laboratory tests for qualitative identification of cations, I found that on passing $\ce{H2S}$ in the solution, the temperature of the solution increased considerably. When the temperature decreased due to addition of $\ce{NH4Cl}$, the solution again attained room temperature in passing $\ce{H2S}$.

This rouses the question: is the dissolution of $\ce{H2S}$ in water exothermic? Or does it react with other components of solution to make an exothermic reaction?

The Wikipedia article stated only this:

Hydrogen sulphide is slightly soluble in water and acts as a weak acid (pKa = 6.9 in 0.01–0.1 mol/litre solutions at 18 °C), giving the hydrosulfide ion $\ce{HS^−}$. Hydrogen sulphide and its solutions are colourless.

The full experiment can be found here.

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Koschel et al. (2013, p. 14486) report that

dissolution of gaseous hydrogen sulfide in aqueous phase is exothermic at conditions where $\ce{H_2S}$ is gaseous, i.e., at a temperature of 323 K and pressure 2 MPa, the saturation pressure $p_\text{sat}(\mathrm{H_2S})$ being 3.5 MPa. The dissolution process is endothermic at the medium and highest temperature where hydrogen sulfide is liquid and supercritical, respectively, the critical parameters of $\ce{H2S}$ being $T_c = 373.4$ and $p_c = 8.94$ MPa.

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