In phosphoric acid, why is it relatively easy to remove the first proton, considerably harder to remove the second proton, and extremely difficult to remove the third proton in chemical reactions?
I'm not sure about this one is it because each time a proton is removed, more negative charge is built up and the molecule is attracted more to the remaining protons, thus making them less acidic?
That's a great answer.
Yes, you are correct. The acidity of a compound is determined by its ability to donate a proton (H+). In the case of phosphoric acid (H3PO4), it has three acidic protons, known as the first, second, and third protons.
The first proton is relatively easy to remove because it is not strongly attracted to the rest of the molecule. When the first proton is removed, the resulting negatively charged species (H2PO4-) is stabilized by resonance, where the negative charge can be delocalized over multiple oxygen atoms. This resonance stabilization reduces the electron density on the remaining protons, making it less acidic.
Removing the second proton is considerably harder because now there is one less oxygen atom available for resonance stabilization. The negatively charged species (HPO42-) resulting from the removal of the second proton is less stable and has a higher electron density on the remaining proton, making it less likely to dissociate.
Finally, removing the third proton is extremely difficult because now there are no more oxygen atoms available for resonance stabilization. The negatively charged species (PO43-) resulting from the removal of the third proton is highly unstable and exhibits very strong electrostatic repulsion between the three negative charges. This makes it extremely difficult to remove the third proton, and as a result, phosphoric acid is predominantly considered a triprotic acid.
Yes, you are correct! The acidity of an acid depends on its ability to donate protons (H+ ions). In the case of phosphoric acid (H3PO4), it has three acidic protons. When the first proton is removed, the resulting species is called a dihydrogen phosphate ion (H2PO4-).
To understand why it is relatively easy to remove the first proton and progressively harder to remove the second and third ones, we need to consider the concept of electronegativity and the stability of the resulting ions.
When the first proton is removed, the negative charge of the remaining dihydrogen phosphate ion (H2PO4-) is distributed over three oxygen atoms. Oxygen is more electronegative than phosphorus, which means it has a stronger pull on electrons. The negative charge is stabilized and delocalized over the three oxygen atoms, making it relatively stable. The delocalization of the negative charge contributes to the acidity of the first proton since it can be easily removed.
However, when the second proton is removed, the resulting species is a monohydrogen phosphate ion (HPO42-). In this case, only two oxygen atoms can stabilize the negative charge. Since the negative charge is concentrated on fewer atoms, it is less stable compared to the dihydrogen phosphate ion. Thus, it requires a higher amount of energy to remove the second proton, making it harder.
Finally, removing the third proton from phosphoric acid forms a phosphate ion (PO43-). In this ion, only one oxygen atom can stabilize the negative charge. The negative charge is highly concentrated on a single atom, which makes it very unstable. Consequently, removing the third proton requires a significant amount of energy and is thus extremely difficult.
In summary, as each proton is removed from phosphoric acid, the negative charge becomes more concentrated on fewer atoms, leading to decreased stability and increased difficulty in removing subsequent protons.