Comparing the Acidity of H2S and H2O: The Role of Hydrogen Bonding

In summary: The hydrogen bond is important for boiling point and vaporization enthalpy, but it is not solely responsible for pH.
  • #1
Vineeth T
31
0
Which is more acidic H2S or H2O?And why?
 
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  • #2
Vineeth T said:
Which is more acidic H2S or H2O?And why?

How acidity is defined?

Which acid-base theory you should consider here? :smile:
 
  • #3
Vineeth T said:
Which is more acidic H2S or H2O?And why?
According to here:
evans.harvard.edu/pdf/evans_pka_table.pdf

H2S is more acid.
 
  • #4
We should remember that to argue that something is more acidic because it produces more hydronium ions in solution or that it is due to a lower pKa, while absolutely true, do not answer the OP's question of "WHY".
 
  • #5
chemisttree said:
We should remember that to argue that something is more acidic because it produces more hydronium ions in solution or that it is due to a lower pKa, while absolutely true, do not answer the OP's question of "WHY".
Of course, but if you tell me it's not a homework, I can answer that question too :smile:
 
  • #6
If you use the Bronsted-Lowry definition, you can say that the conjugate bases of both molecules are OH- and SH-. As oxygen is more electronegative than sulfur, it will attract electrons more than sulfur does. This means that oxygen will be charged more than sulfur, and it will want to receive a proton more than sulfur. Hence, OH- (hydroxide) is a stronger base than SH- (hydrsulfide), which implies that H2S is a stronger acid than H2O. This also holds with the Lewis definition of an acid.
 
  • #7
Millennial said:
If you use the Bronsted-Lowry definition, you can say that the conjugate bases of both molecules are OH- and SH-. As oxygen is more electronegative than sulfur, it will attract electrons more than sulfur does. This means that oxygen will be charged more than sulfur, and it will want to receive a proton more than sulfur. Hence, OH- (hydroxide) is a stronger base than SH- (hydrsulfide), which implies that H2S is a stronger acid than H2O. This also holds with the Lewis definition of an acid.
I don't understand this explanation: since oxygen is more electronegative than sulfur, it should retain the extra electron more strongly than sulfur! The reason H2S is more acid than H2O is in the different bond enthalpies of S-H and O-H.
 
  • #8
lightarrow said:
I don't understand this explanation: since oxygen is more electronegative than sulfur, it should retain the extra electron more strongly than sulfur! The reason H2S is more acid than H2O is in the different bond enthalpies of S-H and O-H.

Yes, it will want to retain the electron more, and it will hog it towards itself. Oxygen will be more negatively charged than sulfur when compounded with hydrogen, so it will want to receive a proton more than sulfur (protons will be attracted more towards oxygen).
 
  • #9
I'm surprised that nobody has mentioned the role hydrogen bonding plays in the comparison of these two compounds and the pH effects that difference leads to.
 
  • #10
Millennial said:
Yes, it will want to retain the electron more, and it will hog it towards itself. Oxygen will be more negatively charged than sulfur when compounded with hydrogen, so it will want to receive a proton more than sulfur (protons will be attracted more towards oxygen).
But the bond between H+ and OH- is not ionic, is covalent; it means that the charge on the ions doesn't count very much, what counts is the capability of forming a covalent bond between one of the lone pairs on the oxygen and the proton, that is the possibility for the oxygen to *give* a pair of electrons to the H+.
 
  • #11
chemisttree said:
I'm surprised that nobody has mentioned the role hydrogen bonding plays in the comparison of these two compounds and the pH effects that difference leads to.
I thought the hydrogen bond were more responsible of properties like boiling point and vaporization enthalpy, but I'd like to know your explanation.
 

What is relative acidic strength?

Relative acidic strength is a measure of the ability of an acid to donate a proton (H+) in a chemical reaction. It is compared to other acids on a scale to determine which is stronger or weaker.

What factors affect relative acidic strength?

The main factors that affect relative acidic strength include the stability of the acid's conjugate base, the strength of the bond between the acid's hydrogen atom and the rest of the molecule, and the electronegativity of the atoms in the molecule.

How is relative acidic strength measured?

Relative acidic strength is measured using a pH scale, which ranges from 0 to 14. The lower the pH, the stronger the acid. It can also be measured using pKa values, which represent the acidity of a compound on a logarithmic scale.

What is the difference between strong and weak acids?

Strong acids have a high relative acidic strength and completely dissociate in water, meaning they donate all of their protons in a reaction. Weak acids, on the other hand, have a lower relative acidic strength and only partially dissociate in water, meaning they donate some but not all of their protons in a reaction.

How does relative acidic strength impact chemical reactions?

The relative acidic strength of a compound can determine the direction and speed of a chemical reaction. Strong acids can react more quickly and completely, while weak acids may not be as reactive. Additionally, relative acidic strength can impact the stability and properties of products formed in a reaction.

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