Is H2S a Strong or Weak Acid? (And Why?)

Hydrogen sulfide (H2S) is a weak acid. 1 It does not completely dissociate in water, meaning only a small portion of H2S molecules ionize into hydrogen ions (H+) and sulfide ions (S)2-. As a weak acid, it has a lower concentration of H+ ions and a higher pH compared to strong acids.

Well, this was just a simple answer. But there are few more things to know about this topic which will make your concept super clear.

So let’s dive right into it.

Key Takeaways: Is H2S a Strong or Weak Acid?

  • Hydrogen sulfide (H2S) is a weak acid because it only partially dissociates in water, resulting in a lower concentration of hydrogen ions (H+) compared to strong acids.
  • The dissociation of H2S differs from strong acids as it is a reversible reaction with limited ion concentration and conductivity in water.
  • Applications of H2S based on its weak acidic nature include environmental monitoring, analytical chemistry, chemical synthesis, metal sulfide precipitation, biological research, wastewater treatment, and oil and gas industry safety.

Why is H2S a weak acid?

Hydrogen sulfide (H2S) is a weak acid because it does not fully dissociate in water to produce a high concentration of hydrogen ions (H+) as strong acids do. Acids are substances that can donate protons (H+ ions), and their strength is determined by the extent of their dissociation in water.

When H2S is dissolved in water, it undergoes a partial dissociation reaction:

H2S (aq) ⇌ H+ (aq) + HS- (aq)

In this equilibrium reaction, only a small percentage of H2S molecules dissociate into hydrogen ions (H+) and hydrosulfide ions (HS-). The reverse reaction, where H+ and HS- combine to form H2S, also occurs.

In contrast, strong acids, like hydrochloric acid (HCl) or sulfuric acid (H2SO4), undergo nearly complete dissociation in water, resulting in a high concentration of H+ ions. 2

The weak acidic nature of H2S is due to the strength of its chemical bond between the hydrogen and sulfur atoms. The hydrogen-sulfur bond in H2S is relatively strong, which makes it more challenging for the hydrogen atom to dissociate from the molecule in water.

Additionally, H2S is less polar than some other acids, and its molecular structure also influences its limited tendency to donate protons.

Overall, H2S behaves as a weak acid because of its partial dissociation in water and the relative stability of its molecular structure.

How does the dissociation of H2S differ from that of a strong acid?

Here is a comparison of the dissociation of hydrogen sulfide (H2S) with that of a strong acid (e.g., hydrochloric acid, HCl).

AspectHydrogen Sulfide (H2S)Strong Acid (e.g., HCl)
Molecular FormulaH2SHCl
Strength as an AcidWeakStrong
Dissociation ReactionH2S (aq) ⇌ H+ (aq) + HS- (aq)HCl (aq) → H+ (aq) + Cl- (aq)
Degree of DissociationPartial dissociation 3 4Complete dissociation 5
Ion Concentration in WaterLow concentration of H+ and HS- ionsHigh concentration of H+ and Cl- ions
pH in WaterSlightly acidic (pH < 7) 6Highly acidic (pH << 7) 7
Conductivity in WaterLower electrical conductivityHigher electrical conductivity 8
Reaction ExtentReversibleIrreversible
Acid Strength DefinitionAcid dissociation constant (Ka) is smallAcid dissociation constant (Ka) is very large

Applications of H2S based on its weak acidic nature

The weak acidic nature of hydrogen sulfide (H2S) gives rise to several important applications in various fields. Here are some notable applications:

  1. Environmental Monitoring: H2S reacts with metal ions in water to form metal sulfides, aiding in water quality assessment. 9
  2. Analytical Chemistry: H2S serves as a reducing agent to identify metal cations through the precipitation of metal sulfides in qualitative analysis. 10
  3. Chemical Synthesis: H2S is used as a sulfur source in the synthesis of organic sulfur compounds and sulfur-containing chemicals. 11
  4. Metal Sulfide Precipitation: H2S is employed to precipitate metal sulfides, aiding in metallurgical processes and hydrometallurgy. 12
  5. Biological and Medical Research: H2S plays a signaling role in physiological processes, and its interactions with biomolecules are studied for potential therapeutic applications. 13
  6. Wastewater Treatment: H2S removes metals from wastewater through the formation of metal sulfides, aiding in purification. 14

Further reading

Is HClO a Strong or Weak Acid?
Is Ba(OH)2 a Strong or Weak Base?
Is KOH (Potassium Hydroxide) a Strong or Weak Base?
Is NH3 (Ammonia) a Strong Base or Weak Base?
Is NaOH (Sodium Hydroxide) a Strong or Weak Base?

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  1. Hydrogen sulfide – Wikipedia. (2014, December 1). Hydrogen Sulfide – Wikipedia.
  2. 2. Strong and Weak Acids. (2013, October 3). Chemistry LibreTexts.
  3. Weak acid-base equilibria (article) | Khan Academy. (n.d.). Khan Academy.
  6. Colloids.
  9. Sadegh-Vaziri, R., & Babler, M. U. (2019, December 5). Removal of Hydrogen Sulfide with Metal Oxides in Packed Bed Reactors—A Review from a Modeling Perspective with Practical Implications. Applied Sciences, 9(24), 5316.
  10. Sun, J., Li, L., Zhou, G., Wang, X., Zhang, L., Liu, Y., Yang, J., Lü, X., & Jiang, F. (2018, March 16). Biological Sulfur Reduction To Generate H2S As a Reducing Agent To Achieve Simultaneous Catalytic Removal of SO2 and NO and Sulfur Recovery from Flue Gas. Environmental Science & Technology, 52(8), 4754–4762.
  11. Abdollahi, M., & Hosseini, A. (2014). Hydrogen Sulfide. Encyclopedia of Toxicology, 971–974.
  12. Al-Tarazi, M., Bert M. Heesink, A., & Versteeg, G. F. (2004, February). Precipitation of metal sulphides using gaseous hydrogen sulphide: mathematical modelling. Chemical Engineering Science, 59(3), 567–579.
  13. Sen, N. (2017, February). Functional and Molecular Insights of Hydrogen Sulfide Signaling and Protein Sulfhydration. Journal of Molecular Biology, 429(4), 543–561.
  14. Pohl, A. (2020, September 28). Removal of Heavy Metal Ions from Water and Wastewaters by Sulfur-Containing Precipitation Agents. Water, Air, & Soil Pollution, 231(10).

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