Acid Strength - Online Tutor, Practice Problems & Exam Prep

Binary acids consist of an H⁺ ion bonded to an electronegative element, while oxyacids include an H⁺ ion, a nonmetal, and oxygen. The strength of binary acids is influenced by electronegativity and atomic radius, with stronger acids having higher electronegativity or larger atomic size. Oxyacids are strong if they have two or more oxygens than hydrogens. Exceptions include oxalic and iodic acids, which are weak despite meeting criteria. Understanding these concepts is crucial for grasping acid-base reactions and their classifications.

Binary Acid Strength

When it comes to binary acids, recall that binary acids are just when we have an H⁺ ion connected to an electronegative element. Those electronegative elements typically are nitrogen, phosphorus, sulfur, selenium, tellurium, or a halogen. We're going to say here, this is true for any acid whether it's a binary acid or an oxyacid. We're going to say here that strong acids are considered strong electrolytes. That means they completely ionize in water.

We show this by using a single arrow going in the forward direction. This illustrates that we make 100% of the ions here as products. Here we have hydrochloric acid which represents a strong binary acid. When thrown into water, it completely dissolves into H⁺ ion and Cl^-. What's really happening in actuality though is that the HCl is donating an H⁺ ion to the water and water itself is becoming H₃O⁺ and we're left with Cl^-.

But H₃O⁺ and H⁺ are seen as being the same thing. They both represent the hydronium ion. But here we're just showing for simplicity's sake that HCl is breaking down into H⁺ and Cl^-. Weak acids are considered to be weak electrolytes. Meaning they do not completely ionize when thrown into water.

Here, we have hydrofluoric acid which represents a weak binary acid. Here because it is weak, it does not completely ionize so we don't have a solid arrow going forward. Instead, we have double arrows to show that an equilibrium has been established. As a result, we're going to make much less than 100% of these following ions as products. Again, we have our acid which donates an H⁺ over.

That's how we wind up with F^- ion and H₃O⁺. Now that we've got down the fundamentals of what is a strong acid versus a weak acid, click on to the next video and see how exactly do we gauge the strength of any typical binary acid.

Binary Acid Strength

Here we can say that the strength of a binary acid is based on the electronegativity, which I'll abbreviate as en, or size, which is really the atomic radius, which I'll abbreviate as ar, of the nonmetal. Now remember, we're looking at a periodic table. As we head towards the top right corner of the periodic table, the electronegativity will increase and our atomic radius will decrease. Now when it comes to the periodic table, we don't give electronegative values to the noble gases. Fluorine is the most electronegative element on the periodic table with a value of 4.0.

Now, for elements in the same period or row, then we look at their electronegativity. We're going to say here the greater the electronegativity, then the more acidic the acid. So let's say for example I'm comparing CH₄, BH₃ and HF. If we look here, B, C, and F, they're all in the same row or period with one another, and we would say here the greater the electronegativity the more acidic. Fluorine we just said is the most electronegative element on the periodic table, so HF would be the most acidic out of these three options.

Now if they are in the same group or column, then instead we look at their size. We're going to say the bigger or the greater the atomic radius, then the more acidic. So let's say we were comparing HCl, HBr, and HI. So Cl, Br, and I. As we go up the group the atomic radius gets smaller so you become less acidic.

I is larger than Br and Cl because it's lower down the group. Since it's larger it's more acidic, so HI would be the strongest acid out of these 3. Alright, so if they're in the same row we look at electronegativity, if they're in the same group then we look at size. But let's say they're not in the same group or row, what do we do then? Well, if they're separated by only one row from each other, then we look at their electronegativities.

Alright. So how do we do something like that? Let's say we're looking here at NH₃ versus H₂S. So N is here, S is here. They're not in the same row and they're not in the same group.

We look at electronegativity. Nitrogen is more electronegative than sulfur. Okay? If you don't recall that, make sure you go back and take a look at my video dealing with the periodic trend of electronegativity. So as a result of this, NH₃ should be a little bit more acidic than H₂S.

Now, let's say that they're separated by more than one row from each other. If they're separated by more than one row from each other, then we're going to say here that we look at size. Okay? So let's say we're comparing NH₃ versus H₂Te. They're not separated by one row from each other.

This is in row 2, this is all the way down here in row 4. Size is more important since tellurium is lower down than nitrogen is, it's larger and therefore H₂Te would be more acidic. Alright, so just keep these trends in mind when trying to compare different binary acids to one another.

Binary Acid Strength

Which is the weakest acid from the following? So here we have hydrosulfuric acid, hydroselenic acid, and hydrotelluric acid. If we take a look at the periodic table, you'll notice that sulfur, selenium, and tellurium are all in the same group. They're all in group 6A. Now, since they're all in group 6A, we can take a look at their atomic size or radius.

And remember, what do we say? As you head down your group, your atomic size increases and that causes your acidity to also increase. So based on these three, tellurium would be the largest and therefore H₂Te would be the strongest acid. But here they're not asking for the strongest acid, they're asking for the weakest acid. Since sulfur is highest up, sulfur would be the smallest out of these three elements.

Therefore, hydrosulfuric acid would be the weakest acid out of all these options. So here our final answer would be option a.

Binary Acid Strength

Which of the following acids would be classified as the strongest? Here we have CH₄, NH₃, H₂O, HF, and pH 3. So here we look on the periodic table, we have carbon, nitrogen, oxygen, fluorine, and phosphorus. Remember, when they're in the same period or row we look at electronegativity. The more electronegative the nonmetal, the more acidic.

Fluorine is more electronegative than carbon, nitrogen, or oxygen, so these would not be the most acidic. Now we have fluorine versus phosphorus. When they're separated by just one row from each other then we can still look at electronegativity. Fluorine is more electronegative than phosphorus, so HF would be more acidic than pH 3. So, option b is out and our final answer would be HF.

So here the answer is option d.

Oxyacid Strength

Now, recall that an oxyacid is composed of an H⁺ ion connected to a nonmetal plus oxygen. That nonmetal in oxygen is usually in the form of a polyatomic ion. We're going to say the strength of oxyacids is based on the number of oxygens contained within the acid or the electronegativity of the nonmetal. Now here, if we don't have a K_a value and we need to determine if an oxyacid is weak or strong, then we can just follow this one simple rule.

Now we're going to say if my oxyacid has 2 or more oxygens than hydrogens, then my oxyacid is classified as a strong acid. Again, we can use this rule whenever we don't know what the K_a value is for a particular oxyacid. But if we have a strong oxyacid, remember a strong oxyacid would have a K_a value that is much greater than 1. It would have a pKa value that is negative. That would classify it as a strong oxyacid.

But if you don't have those pieces of information, we rely on this rule here. Now here we're looking at this first oxyacid. This is chloric acid. It has hydrogen, a nonmetal, and oxygen. That's why it's classified as an oxyacid.

Here we have 3 oxygens involved and 1 hydrogen. So, 3 oxygens and 1 hydrogen. When we subtract those, you need a minimum of 2 oxygens remaining. Here we have a minimum of 2 oxygens remaining so this would be a strong oxyacid. Next, HOCN.

This is also an oxyacid. It has hydrogen, oxygen, and nonmetals. Here we're going to look at the number of oxygens which is 1, number of hydrogens which is 1. So at the end, we have nothing left. Here, this would be a weak oxyacid because it hasn't met the requirement of having a minimum of 2 oxygens remaining.

Then finally here we have HC₄H₇O₂. Here we have 2 oxygens and we have 8 hydrogens. We need a minimum of 2 oxygens remaining. Here, we don't have any oxygens remaining. We have an excess of hydrogens.

This is classified as a weak oxyacid. Again, if you don't have a K_a value or pKa value associated with your oxyacid, you can rely on this rule. Later on, we'll look at the exceptions that reside in terms of certain oxyacids, that break this one rule. For now, on to the next video as we continue by comparing different oxyacids to one another.

Oxyacid Strength

When comparing the strengths of different oxyacids, remember that if they have a different number of oxygens, then the higher the remaining oxygens, the more acidic. So let's say we're comparing nitric acid versus nitrous acid. In this case, when we subtract the number of oxygen by hydrogens we have 2 oxygens left for nitric acid, while nitrous acid would have one oxygen left. Since nitric acid has more remaining oxygens, it's more acidic. But let's say if they have the same number of oxygens remaining, then we'd say the more electronegative the nonmetal, the more acidic.

So let's say, for instance, we're looking here at perbromic acid versus per iodic acid. They both have the same number of oxygens remaining, 3. We look at electronegativity, which remember, as we're heading towards the top right corner of the periodic table, will increase. Bromine is higher up than iodine, so bromine is more electronegative, and therefore perbromic acid is more acidic than per iodic acid. Now, of course, with rules, sometimes we run into exceptions.

So here we have 3 different illustrations. For the first one, we have oxalic acid which is H₂C₂O₄. When we work out the math, we still have 2 oxygens remaining. And when we do the same thing with iodic acid, we still have 2 oxygens remaining, which would tell us they both should be strong oxyacids, but there are exceptions to that rule. The electronegativity of their carbon and iodine is pretty low compared to other nonmetals. This interferes with their acidity and this is the easiest way to think about this exception.

Because of that, oxalic acid and iodic acid are weak oxyacids. Now, this last one doesn't necessarily represent only this compound, but it represents a class of compounds it belongs to, which is your amphoteric species. Remember, amphoteric species can be acidic or basic. For the group of amphoteric species that are not strong oxyacids, even when following the math, they will have a hydrogen and they will have a negative charge—hydrogen because it can be acidic, negative charge because they have the ability to be basic.

So here, it doesn't matter that we do the math and it fits the criteria to be a strong oxyacid. It just won't be because it's amphoteric. So remember, our 3 exceptions are oxalic acid, iodic acid, and amphoteric species. Just keep these rules in mind when you're trying to compare the strength of different types of oxyacids.

Oxyacid Strength

So when it comes to our oxy acids, remember we have our rule: when we have 2 or more oxygens and hydrogens, it's classified as an oxyacid. Now, there are exceptions to this rule. Here we have 3 major exceptions. We have here oxalic acid, we have here iodic acid, and then this here is just the placeholder that represents amphoteric species. Now, here when we do the math we have 4 oxygens, 2 hydrogens, so we have 2 oxygens left, and the same thing here: we're going to have 2 oxygens left.

Now based on our previous rule, we would think that oxalic acid and iodic acid would be classified as strong oxy acids, but in fact, they are weak. They are weak because the electronegativity of the carbon and iodine here are not significant. They are not electronegative elements; therefore, overall the strength of these oxy acids is weak. Okay. So both of these would be weak oxy acids that have ka values less than 1.

Now amphoteric species can act as acids or bases, acids because they contain hydrogen, and bases because they have that negative charge there. These are not the only ones, remember we saw in previous pages we had bisulfite, bicarbonate, dihydrogen phosphate, hydrogen phosphate as well. These amphoteric species, because they can act as acids or bases, we can't say that they are definitively strong oxy acids. So we would say that here, they would also be weak. Now just recall the amphoteric species that I said that were acidic and the ones that I said that were basic, to help guide you determine what the true nature of that amphoteric species is.

So remember, guys, we have the rules for oxy acids, it's just that these 3 are exceptions to that rule. Now that we've seen that, try the example that's left at the bottom of the page and see if you can get the correct answer. Once you do, come back and take a look at how I approach that same question.

Oxyacid Strength

Write the following oxyacids in terms of increasing acidity. The best approach for this question is to look at the difference between the number of oxygens and hydrogens for each of the following. For the first one, we have nitric acid, where there are 3 oxygens. We'll subtract 1 hydrogen that we have, so we have 2 oxygens left.

For the next one, we have 2 oxygens and it looks like we have 6 hydrogens, so we're actually left with 4 hydrogens.

The next one has 3 oxygens, 2 hydrogens, so we have 1 oxygen left. And then for the last one, which is chloric acid, we have 3 oxygens, minus 1 hydrogen we have 2 oxygens left. In oxyacids, the more oxygens you have relative to hydrogens, the stronger the acid will be. We want to arrange them in terms of increasing acidity, starting with the weakest one. The weakest one would be option B since it doesn't have any oxygens remaining.

Next, we have option C, and now for options A and D, they both have the same number of oxygens remaining after deducting the number of hydrogens. To determine which one is stronger, we look at the electronegativity of the non-metal. In this case, it's nitrogen versus chlorine. Remember, looking at your periodic table, as you head towards the top right corner—or, more specifically, towards fluorine—the electronegativity increases.

Fluorine, being the most electronegative, has an electronegativity value of 4.0. The next highest on the periodic table is oxygen, at 3.5. Then we have chlorine, with around 3.0 electronegativity, making it the third most electronegative element on the periodic table and more electronegative than nitrogen. Therefore, option A would be next, with option D being the most acidic out of these four options. Thus, the order of increasing acidity is B as the least acidic, followed by C, then A, and D as the most acidic.

Here’s what students ask on this topic:

What is the difference between strong and weak acids?

Strong acids completely ionize in water, meaning they dissociate into their ions 100%. This is represented by a single arrow in chemical equations. Examples include HCl and HNO₃. Weak acids, on the other hand, do not fully ionize in water and establish an equilibrium between the undissociated acid and its ions, shown by double arrows. Examples include HF and CH₃COOH. Understanding the ionization behavior helps in predicting the acid's reactivity and strength.

How does electronegativity affect the strength of binary acids?

In binary acids, the strength is influenced by the electronegativity of the nonmetal element bonded to the H⁺ ion. For elements in the same period, higher electronegativity results in a stronger acid. For example, HF is stronger than CH₄ because fluorine is more electronegative than carbon. This increased electronegativity stabilizes the negative charge on the conjugate base, making the acid stronger.

What role does atomic radius play in determining acid strength?

Atomic radius affects the strength of binary acids within the same group. Larger atomic radius means a stronger acid. For instance, HI is stronger than HBr and HCl because iodine has a larger atomic radius than bromine and chlorine. The larger size allows for easier dissociation of the H⁺ ion, increasing the acid's strength.

How do you determine the strength of oxyacids?

The strength of oxyacids depends on the number of oxygen atoms and the electronegativity of the central nonmetal. If an oxyacid has two or more oxygens than hydrogens, it is generally strong. For example, HNO₃ (nitric acid) is strong because it has more oxygens than hydrogens. Additionally, if two oxyacids have the same number of oxygens, the one with the more electronegative central atom is stronger.

What are the exceptions to the rule for determining oxyacid strength?

Exceptions to the rule include oxalic acid (H₂C₂O₄) and iodic acid (HIO₃), which are weak despite having two or more oxygens than hydrogens. This is due to the low electronegativity of carbon and iodine. Amphoteric species, which can act as both acids and bases, are also exceptions. These species have a hydrogen and a negative charge, making them weak oxyacids regardless of the oxygen count.