3. The same study reported that the following reaction also tends to go forward:
ID: 1036792 • Letter: 3
Question
3. The same study reported that the following reaction also tends to go forward: AsH3 HS Which is the stronger acid, H2S or AsH3? Is this in agreement with the general trends for binary acids? This is an interesting example in that both the trends across the periodic table and down the periodic table are involved. Which has a greater influence, the trend across the table or the trend down the table? Does this fit with what you know about the relative acidities of binary acids of the second row (CH4, NH3, OH2, etc) and the halogen group (HF, HCl, HBr, etc.)? 4. Estimate the pK, values for HNO and HNO2Explanation / Answer
3) The study reports that the reaction moves in the forward direction. H2S and AsH3 are both acids. It is common knowledge that an acid-base reaction proceeds in the direction of the weak acid, i.e, a strong acid reacts with a base to form a weak acid spontaneously or easily. Since, the reaction goes in the forward direction; H2S is a stronger acid as compared to AsH3.
The acidity of a binary acid depends on the electronegativity of the heteroatom (atom other than H in the binary acid). The more easily is the H lost from the binary acid, the more acidic is the compound. Since H2S is a stronger acid, H2S loses H+ easily to form HS-. This facile loss of H+ can be explained in terms of the electronegativity of S and As atoms. S is more electronegative than As atom; consequently, S atom in HS- is better able to accommodate the negative charge, thereby yielding the more stabilized base, HS-. As is less electronegative and hence, less able to accommodate the negative charge in AsH2-.
The trend is in line with the binary acids CH4, NH3 and H2O. OH- is the stronger base and hence, the proton is easily lost from H2O. CH4 barely loses a proton to form CH3-. The same trend of electronegativity of the central atom works well in the halogen acids HF, HCl, HBr, etc. HCl is the strongest acid due to the strong electronegativity of Cl- (check the note below). HF is the weakest acid in the group (this is an exception).
Note: HF is expected to be the strongest acid due to the highest electronegativity of F atm. However, note that F is the smallest atom and an increased electron density in F- leads to electron-electron repulsion, thereby, making F- a weak base and consequently, HF is a weak acid.
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