Predict the acid strengths of the following compounds: H2O, H2S, and H2Se.
H2S > H2Se > H2O
H2O > H2Se > H2S
H2Se > H2S > H2O
H2Se > H2O > H2S
H2S > H2O > H2Se
H2O > H2S > H2Se
The Correct Answer and Explanation is :
The correct order of acid strengths for the compounds H₂O, H₂S, and H₂Se is: H₂Se > H₂S > H₂O.
This trend can be explained by examining the periodic properties of the elements involved, particularly focusing on bond strength and atomic size.
Bond Strength and Atomic Size:
As we move down Group 16 in the periodic table from oxygen (O) to sulfur (S) to selenium (Se), the atomic radius increases. This increase in atomic size leads to longer and weaker bonds between hydrogen and the central atom in each molecule. Specifically, the H–O bond in H₂O is stronger than the H–S bond in H₂S, which in turn is stronger than the H–Se bond in H₂Se. Weaker bonds are more easily broken, facilitating the release of a proton (H⁺), which is the defining characteristic of acid strength. Therefore, H₂Se, with the weakest H–Se bond, is the strongest acid among the three, followed by H₂S, and then H₂O.
Electronegativity Considerations:
While electronegativity generally decreases down a group, which might suggest a decrease in acidity, in this case, the bond strength factor predominates. The decreasing bond dissociation energy down the group has a more significant impact on the acid strength than the electronegativity differences. Thus, the trend of increasing acid strength from H₂O to H₂Se is primarily due to the weakening H–X bonds as the central atom’s size increases.
Conjugate Base Stability:
Another factor influencing acid strength is the stability of the conjugate base formed after deprotonation. Larger atoms like selenium can better stabilize the negative charge due to their larger electron cloud, making the conjugate base more stable. This increased stability of the conjugate base further contributes to the higher acidity of H₂Se compared to H₂S and H₂O.
In summary, the increasing acid strength from H₂O to H₂Se is primarily attributed to the decreasing bond strength between hydrogen and the central atom as we move down Group 16, coupled with the enhanced stability of the conjugate bases formed.