The APsolute RecAP: Chemistry Edition

The APsolute RecAP: Chemistry Edition - Episode 62: Molecular Structure and Acid Strength

Episode Summary

All hydrohalic acids, but hydrofluoric acid, are strong acids. Why is that?

Episode Notes

All hydrohalic acids, but hydrofluoric acid, are strong acids. Why is that (0:31)? Strong acids fully dissociate and have a large Ka, weak acids only dissociate to a small percentage (1:20). When looking at strength, we are comparing the stability of the conjugate acid/base pair partners (2:00). For binary acids across a period (2:37) as well as down a group (3:09) electronegativity determines acid strength. For oxyacids, we can compare acids with different numbers of oxygen atoms, which affects the inductive effect, as well as resonance structures (4:58). We can also discuss acid strength across a period (6:44) and down a group (7:34). Bases are proton acceptors. Common bases are conjugate bases of weak acids, like carboxylic acids, and amines (7:53).

Question of the Day: Acid A has a Ka = 3.5 x 10-8, acid B has a Ka = 1.2 x 10-2. Which acid is HClO, which one is HClO2?

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Episode Transcription

Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Molecular Structure and Acid Strength 

Lets Zoom out: 

Unit 8 - Acids and Bases

Topics - 8.6 Molecular Structure of Acids and Bases

Big idea - Structure and Properties

“Get the F out!” - sorry, I don’t mean you! I mean Fluorine! You might be wondering why I’m starting this episode like this? Hopefully, it sticks with you and helps you remember that the hydrohalic acids, the acids formed by hydrogen and one of the halogens, are strong acids - except for Fluorine! That’s why we ask fluorine to get out! But why is fluorine not a strong acid? Let’s take a closer look at how the molecular structure affects the strength of acids and bases. 

Let’s zoom in: 

We’ll start with a quick recap: what is a strong acid or base? What is a weak acid or base? Generally speaking, strong acids and bases fully dissociate, the equilibrium constant K is very large and the equilibrium lies on the side of the products. Strong acids have a low pH while strong bases have a high pH. On the other hand, weak acids and bases only dissociate to a small percentage. Therefore, the acid pH are comparably high - as in “closer to 7” and the pH of weak bases are between 7 and 9. To add some AP chemistry terminology to it, we can take a look at the conjugate acids and bases. The conjugate base to a strong acid has a negligible basicity, meaning, their conjugate bases do not have a tendency to react as bases and are more stable. Similarly, the conjugate acids of a strong base are weak or have negligible acidity, which means they are more stable as conjugate acids and don’t react. This is a key point in understanding today’s episode: we are comparing the stability of the conjugate acid/base pair partners! The more stable, the weaker the strength! 

When looking at acids, we can put them in two groups: binary acids and oxyacids, also called oxoacids. Binary acids, as the “bi” indicates, are acids made of two elements, like our hydrohalic acids. Oxyacids are acids that contain a central atom bonded to one or more oxygens. The oxygens can be bonded to a hydrogen. Examples for these are nitric acid, HNO3, or sulfuric acid, H2SO4. 

Let’s talk about binary acids first. We can compare acid strength across a period as well as down a group. Bond length, bond strength and electronegativity will affect the strength of the acid. The strength of acids increases across a period, like when we compare CH4, NH3, H2O and HF. Why? Because the difference in electronegativity increases, the bond gets more polar, the acid dissociates more easily and the conjugate base is stronger as well as more stable. Acid strength increases down a group, for example the elements in group 7. As we go down a group, the bond length increases, the bond strength decreases and therefore the acid dissociates more easily, the conjugate base is more stable and the acid is stronger. 

What about oxyacids? There are several factors that affect the strength of an acid, first being the inductive effect. This effect describes that electrons of adjacent bonds can be more attracted by more electronegative atoms. That means that a highly electronegative atom can affect the bond polarity beyond its bonding partner. It is especially important when  comparing the acid strength of the same element with different numbers of oxygen, for example nitric acid and nitrous acid. Nitric acid is a strong acid while nitrous acid is a weak acid. In nitric acid the central nitrogen is bonded to three highly electronegative oxygens, one of which is bonded to hydrogen. The difference in polarity between the oxygen and the hydrogen is enhanced by the presence of the other oxygen - the inductive effect. The more additional oxygens, the more polar the OH bond is and therefore, the hydrogen is lost more easily.  This makes the acids stronger. We can also look at the conjugate base of nitric acid, the nitrate ion and resonance structure. The negative partial charge can spread out more evenly due to three possible resonance structures compared to the nitrite ion. Having more resonance structures and being able to spread out the negative charges more, increases the stability of the conjugate base and therefore makes the base weaker - and the acid stronger. In summary, we have two arguments: 1) more oxygens lead to a greater inductive effect, which makes the molecule more ionizable. 2) even distribution through more possible resonance structures thanks to the additional oxygen stabilizes the conjugate base.  

When looking at oxyacids, I can also discuss the acid strength across a period as well as down a group. For example, across a period we can compare phosphoric acid, H3PO4, sulfuric acid, H2SO4 and perchloric acid, HClO4. These acids all have four oxygens, but they are bonded to central atoms with increasing electronegativity. Keep in mind, this is like a tug of war. Chlorine tugs on the shared electrons with oxygen, and so oxygen tugs harder on the shared electrons between itself and hydrogen. This makes the OH bond more polar and the hydrogen is lost more easily, which makes the acid stronger. This results in oxyacid strength increasing across a period. We can use the same argument down a group: If you compare hypochlorous acid, HClO, with hypoiodous acid, HIO, iodine is less electronegative. Therefore HClO will be the stronger acid, polarizing the OH bond more and losing the hydrogen more easily. 

We’ve talked a lot about acids, but let's take a brief look at bases. Bases are proton acceptors. How easily a base accepts or attracts a proton depends on th e lone pair as well as the stability of the ion formed. Common examples for bases other than hydroxides are the conjugate bases of weak acids, like carboxylic acids like acetic acid. Acetic acid has a fairly stable OH bond because carbon is not very electronegative and only dissociates a small percentage. The conjugate base, acetate, is therefore a stronger base. Amines, like ammonia, can act as acids or bases. As a base, they accept protons using their lone pairs. 

To recap: 

To understand the strength of acids and bases we have to consider the stability of their conjugates as well as the inductive effect, electronegativity and resonance. For binary acids, acid strength increases down a group due to the increased bond length and decreased bond strength. Acid strength also increases across a period due to the greater difference in electronegativity increasing the bond polarity. For oxyacids, a higher number of oxygens increases the acid strength due to the inductive effect and resonance stabilizing the conjugate base. When comparing oxyacids down a group, the acid strength decreases as the electronegativity of the central atom decreases, reducing the inductive effect and making the OH bond less polar. Acidic strength increases across a period as the electronegativity increases, weakening the OH bond. Conjugate bases of carboxylic acids and amines accept protons more easily and are therefore bases. 

Coming up next on the APsolute RecAP Chemistry Edition: Unit 8 Selected FRQs

Today’s Question of the day is about acid strength: acid A has a Ka = 3.5 x 10-8, acid B has a Ka = 1.2 x 10-2. Which acid is HClO, which one is HClO2?

A. A = HClO, B = HClO2 because A is the weaker acid. 
B. A = HClO, B = HClO2 because A is the stronger acid.
C. A = HClO2, B = HClO because A is the weaker acid.
D. A = HClO2, B = HClO because A is the stronger acid.