Question

How does the acid strength of an oxyacid depend on (a) the electronegativity of the central atom; (b) the number of nonprotonated oxygen atoms in the molecule?

Short answer

The acid strength of an oxyacid depends on the electronegativity of the central atom and the number of nonprotonated oxygen atoms in the molecule. As the electronegativity of the central atom increases, the acid strength also increases due to the enhanced stability of the resulting oxyanion. Similarly, a greater number of nonprotonated oxygen atoms in the molecule increases the acid strength as it results in stronger resonance stabilization of the oxyanion.

Step-by-step solution

(Step 1: Understanding oxyacids)

An oxyacid is an acid that contains a central atom, hydrogen atoms, and oxygen atoms. The central atom is usually a nonmetal. The acid strength of an oxyacid depends on the stability of the resulting anion formed when the hydrogen atom is lost as a proton (\(H^+\)).

(Step 2: Electronegativity of the central atom)

The electronegativity of the central atom plays a significant role in determining the acid strength of an oxyacid. An element with high electronegativity will have a strong affinity for electrons, making it more difficult for the hydrogen to dissociate. In general, as the electronegativity of the central atom increases, the stability of the oxyanion increases, which in turn increases the acid strength of the oxyacid. This is because a more electronegative central atom pulls electrons towards itself, making the O-H bond more polar, and thus more likely for the hydrogen to dissociate as a proton (\(H^+\)).

(Step 3: Number of nonprotonated oxygen atoms)

The number of nonprotonated oxygen atoms (oxygen atoms not directly bonded to hydrogen) also affects the strength of an oxyacid. These oxygen atoms have lone pairs of electrons and are capable of resonance stabilization, which stabilizes the oxyanion formed by the dissociation of hydrogen as a proton. So, in general, as the number of nonprotonated oxygen atoms bonded to the central atom increases, the acid strength of the oxyacid also increases. This is because the resonance stabilization of the oxyanion becomes stronger, making hydrogen dissociation more favorable. In summary, the acid strength of an oxyacid is influenced by the electronegativity of the central atom and the number of nonprotonated oxygen atoms in the molecule. Higher electronegativity of the central atom and a greater number of nonprotonated oxygen atoms both lead to a more stable oxyanion, which in turn increases the acid strength of the oxyacid.

Key concepts

Electronegativity and Oxyacid Strength

Electronegativity can be considered a central actor on the chemical stage when discussing oxyacid strength. When a central atom has high electronegativity, it has a strong pull on shared electrons. This tug-of-war over electrons affects the stability of the bonds within the acid, particularly the bond between hydrogen and oxygen. Picture it like a tightly held rope in a game of tug-of-war; the stronger the pull (electronegativity), the more likely it is that the rope (the hydrogen) will snap away.

Considering that electronegative atoms attract electrons more strongly, they make the bond between the oxygen and hydrogen atoms in an oxyacid more polar. This polarization means that the oxygen holds onto the shared electrons more tightly, and the hydrogen - being more positively charged - can be released as a proton (\(H^+\)) with relative ease. The end result? A stronger oxyacid, as it can more readily donate hydrogen ions to its surrounding environment, the hallmark of a strong acid.

Nonprotonated Oxygen Atoms Impact

Now, when we count the nonprotonated oxygen atoms in an oxyacid, we're essentially counting its potential for stability once it has given up a hydrogen atom. These oxygen atoms have unshared pairs of electrons, which are like a financial safety net; they provide 'resonance stabilization' to the remaining oxyanion, the anion left after the acid has donated its proton.

Imagine a trampoline with several springs; each spring represents a nonprotonated oxygen atom. The more springs there are, the more support—stabilization—there is for the person (oxyanion) bouncing. More nonprotonated oxygens mean the negative charge left behind can be spread out more efficiently, leading to a happier (more stable) oxyanion, and hence, a stronger oxyacid. This framework of oxygens allows the molecule to maintain a lower energy state, which is always preferred in the natural world.

Resonance Stabilization in Oxyacids

Resonance stabilization is somewhat like a support system within a molecule that helps distribute stress. For oxyacids, this system comes into play when one of the hydrogen atoms has left the scene as a proton (\(H^+\)). The negative charge that remains needs to be managed, and resonance lets this charge 'dance' across the structure, shared between the nonprotonated oxygen atoms through alternate bonding arrangements.

Think of resonance as a choreographed group dance rather than a solo performance. The entire group (molecule) benefits from the movement (distribution of charge), enhancing the stability of the molecule. This molecular 'dance' is a key player in oxyacid strength. The better the group can distribute the stress, the stronger and more stable the oxyacid becomes, as a stable anion correlates with a greater ability to donate protons.

Oxyanion Stability and Acid Strength

The stability of an oxyanion post-proton donation is a telling sign of oxyacid strength. A stable oxyanion doesn't hold onto its proton very tightly, which means it's more willing to part with it. On the contrary, an unstable oxyanion clings to its proton because it's not as comfortable with the negative charge that results from losing it.

The stability of oxyanions is influenced by the factors we've already discussed: electronegativity and resonance stabilization via nonprotonated oxygen atoms. A stable oxyanion is the end goal for a strong oxyacid, as the more stable the oxyanion, the more acidic the parent compound. Simply put, a stable oxyanion means a stronger oxyacid, capable of successfully competing in the chemical race to donate protons.