Oxyacids - Online Tutor, Practice Problems & Exam Prep

Oxyacids are covalent compounds consisting of hydrogen ions bonded to polyatomic ions containing oxygen. The strength of an oxyacid is determined by the oxyacid strength rule, which states that if a neutral oxyacid has two or more oxygen atoms than hydrogen atoms, it is considered a strong acid, with the exception of oxalic acid and iodic acid due to their central elements' low electronegativities. When comparing oxyacids, the number of remaining oxygen atoms after subtracting hydrogen atoms is crucial: more remaining oxygens indicate a stronger acid. If the number of oxygens is the same, the higher electronegativity of the central non-hydrogen element makes the oxyacid more acidic. Understanding these principles helps in identifying and comparing the strength of different oxyacids.

Oxyacid Formation

Hey everyone, so here we're going to say that Oxy acids represent covalent compounds containing the hydrogen ion bonded to a polyatomic ion containing oxygen. So here if we want to take a look at two typical types of oxy acids, let's say we're reacting H+ with your nitrite ion which is NO2-. The numbers and the charges are the same, so when they combine, this gives me HNO2 which is nitrous acid.

It is a covalent compound, begins with a hydrogen and it is composed of a polyatomic ion containing oxygen. Now let's say that we react H+ with are sulfite ion. The numbers in the charges are different when this happens they crisscross one comes here, 2 comes here. Don't worry about the charges, we just cross cross the numbers.

This gives us H2SO3 which is sulfurous acid. This is also an example of an oxy acid. So just remember, an oxy acid is just on acid with the presence of oxygen.

Oxyacids Example

Which of the following represents the possible structure of an OXY acid? So if we take a look at the first one, the first one is sodium nitrate. There is no H⁺ ion involved within the structure, so it can't represent an acid, let alone an oxy acid.

For the next one, it's covalent. It has hydrogen in the beginning, so it is an acid. It has the presence of oxygen, so it is an oxy acid here. This one is called formic acid.

By the way, CC is not an acid because in this one it represents an ionic base. It is a positive metal cation potassium ion connected to hydroxide ion which is a basic anion. OK, so this is a base and then find the last one.

It is covalent, only nonmetals together. It starts with a hydrogen, so it is an acid, but it has no oxygen, so it is a binary acid, not an oxy acid. So the only one that's an oxy acid here is option B.

Oxyacid Strength Rule

Now that you know how to identify an oxy acid, let's talk about how do we identify if an oxy acid or is weak or strong O here. To do that we rely on the oxy acid strength rule and it says if a neutral oxy acid has two or more oxygens than hydrogens, than the oxy acid is a strong acid.

Now let's take a look at these three oxy acids before us. So first we have HCl3 here. How many oxygen does it have? It has three. How many hydrogens does it have? It has one. When you subtract them, you have two oxygens left, so we've met the requirement of having at least two oxygens more than hydrogen's. So this is strong.

Next, HOCN is an oxy acid. We're going to say here we have one oxygen, one hydrogen. We have no oxygens remaining. We don't meet the requirement of having at least two more oxygens and hydrogens, so this is weak. And then find the last one. We have two oxygens, we have 7 + 1 = 8 hydrogens. We definitely don't have more oxygens than hydrogens. So this is weak.

Now of course there will be exceptions. Here are two exceptions and the good thing is it's just these two. We have oxalic acid which is H2C2O4 and we have iota acid which is HIO3. If you did the math, we'd have two more oxygens than hydrogens, but they're both still considered weak oxy acids and that's because they're non hydrogen elements. In this case carbon and iodine have low electronegativities, so this is just the simplest way to think about why there are exceptions to the rule.

All right, so just remember if a neutral OXY acid has two or more oxygen's than hydrogens, then it can be a strong acid. The exceptions to this are oxcolic acid and ionic acid. So just remember to do the math and you could determine which is weak and which is strong.

Oxyacids Example

Which of the following represents a strong oxy acid? Remember, based on our rule, we're going to say if a neutral oxy acid has two or more oxygens than hydrogens, then it is a strong acid. These out because D is not neutral, it has a charge. So we have to look at ABC and E.

When we do the math, we're going to have two oxygens here. -1 Hydrogen gives us only one oxygen left, doesn't meet the requirement here. We have two oxygens, we have 4 hydrogens. We definitely don't have any oxygens left. This one here we have one oxygen, one hydrogen. Comes out to 0. Doesn't work.

East is our answer because here we have 3 oxygens -1 hydrogen. We have the minimum requirement of two oxygens more than hydrogen, so this would be our strong oxy acid. This is bromic acid, right? So just keep in mind the rule that we talked about when it comes to oxyacid strength.

Remember our two exceptions of oxalic acid and iota acid. If you can remember that, you'll know how to identify a weak oxy acid versus a strong oxy acid.

Comparing Oxyacid Strength

So up to this point, we've learned how to identify Oxy acids. We've learned to determine which one is weak and which one is strong and now we're going to go over comparing them to one another. We're going to say when comparing the strengths of Oxy acids, it's all in the oxygens. We're going to say, do they have different numbers of oxygen remaining or do they have the same number of oxygens remaining?

Well, here if they have a different number of oxygens remaining, we're going to say that more oxygens remaining equals more acidic. If we take a look here, we have as our examples, we're going to hypo Iotis acid versus Iota's acid. When you do the math, this one has nothing left. And this one here has one oxygen left. Iotus acid has more remaining oxygens, therefore it's stronger. So this would be our stronger oxy acid.

Now if the difference between oxygen and hydrogens are the same, then we're going to say here the higher the electronegativity of the central element then the more acidic. If you were to figure out the remainder of oxygen's left for both of them you get the same result one oxygen left. So then you have to look at the non hydrogen element. Here we're comparing sulfur versus selenium. They're both in Group 6A, so we have oxygen, sulfur, selenium.

Remember, as we head up a group, your electronegativity is increasing, so sulfur is more electronegative than selenium. Therefore H2S3, which is sulfurous acid, is more acidic. So again, the way you're able to compare different oxy acids to each other, it's all in the number of oxygens remaining. Do they have different number of oxygens remaining, or do they have the same number of oxygens remaining? Knowing that is key to comparing them to one another.

Oxyacids Example

Rank the following oxyacids in terms of increasing acidity. Alright, so since they're all oxy acids, we're going to first look at the difference in number of oxygens and hydrogens for each one. So here we do 3 oxygens -1 hydrogen. So we have two oxygens left here. This has three oxygens left. When we do 3 -, 1, that's two oxygens left here. This would have three oxygens left, and this would have 0 oxygens left.

When you do number of oxygens minus number of hydrogens, we're doing it in terms of increasing acidity, so we want to do weakest first, up to strongest here. Option five will be the weakest since it has the least number of oxygens remaining. Next, we look at the ones that have two oxygens remaining apiece. Now, how do we determine which one is stronger?

Remember when the number of the difference between the number of oxygens and hydrogens is the same then we rely on the electronegativity of the non hydrogen element, Here chlorine versus bromine. The more electronegative the non hydrogen element, the more acidic the oxy acid. Here, chlorine is more electronegative than bromine, so that means that three would be less acidic than one.

And then we do the same thing with two and four. Again, the number of the difference between number of oxygens and hydrogens is the same. It's three. So look at the electronegativity of the nonhydrogen element. Again, it's chlorine versus bromine. Chlorine is more electronegative, therefore HCL₄, which is perchloric acid, is stronger. So that would mean two and then four would be the strongest.

So this would be the correct order in terms of increasing acidity when we're going from the weakest acid to the strongest oxy acid from the list provided.

Here’s what students ask on this topic:

What is an oxyacid?

An oxyacid, also known as an oxoacid, is a type of acid that contains hydrogen, oxygen, and at least one other element, with the hydrogen being bound to the oxygen in a hydroxyl group (-OH). The central element, often a nonmetal or a metalloid, is bonded to the oxygen, and it's this element that determines the properties and name of the acid. The general formula for an oxyacid is $H_{x}E{O}_y$, where E represents the central element. A simple example of an oxyacid is sulfuric acid ($H_2{\mathrm{SO}}_4$), where sulfur is the central element bonded to four oxygen atoms, two of which are also bonded to hydrogen atoms. The strength of an oxyacid is influenced by the electronegativity of the central atom and the number of oxygen atoms bonded to it; more oxygen atoms usually lead to a stronger acid due to the increased ability to stabilize the negative charge that results when the acid donates a proton ($H^{+}$).

Which factors affect the relative acidity of an oxyacid?

The relative acidity of an oxyacid, which is an acid containing hydrogen, oxygen, and another element (usually a nonmetal), is influenced by several factors:

1. Electronegativity of the Central Atom: The more electronegative the central atom (the atom bonded to the oxygen), the stronger the oxyacid. This is because a more electronegative atom can stabilize the negative charge on the oxygen after the hydrogen ion (proton) is lost.
2. Number of Oxygen Atoms: As the number of oxygen atoms bonded to the central atom increases, the acidity typically increases. This is due to the inductive effect, where the additional oxygen atoms pull electron density away from the O-H bond, making it easier to lose the hydrogen ion.
3. Oxygen-Hydrogen Bond Strength: The strength of the O-H bond also affects acidity. Weaker O-H bonds make it easier for the acid to donate a proton, thus increasing acidity.
4. Stability of the Conjugate Base: If the conjugate base (the species that remains after the acid donates a proton) is stable, the oxyacid will be stronger. Resonance stabilization of the conjugate base, for example, can make the oxyacid more acidic.
5. Central Atom's Oxidation State: A higher oxidation state of the central atom generally increases the acidity of the oxyacid.

Which factor is the most important in determining the strength of an oxyacid?

The strength of an oxyacid, which is an acid containing hydrogen, oxygen, and another element, is primarily determined by the electronegativity of the central atom and the number of oxygen atoms bonded to it. The more electronegative the central atom, the more it pulls electron density towards itself, weakening the H-O bond and making it easier for the acid to donate a proton (H⁺), which is the defining characteristic of an acid's strength.

Additionally, the number of oxygen atoms is crucial because oxygen is highly electronegative. More oxygen atoms increase the electron-withdrawing effect through the inductive effect, further weakening the H-O bond and enhancing the acid's ability to donate a proton.

In summary, an oxyacid with a highly electronegative central atom and a greater number of oxygen atoms will generally be a stronger acid.