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Course: AP®︎/College Chemistry > Unit 8
Lesson 4: Molecular structure of acids and basesFactors affecting acid strength
The relative strength of an acid can be predicted based on its chemical structure. In general, an acid is stronger when the H–A bond is more polar. Acidity is also greater when the H–A bond is weaker and when the conjugate base, A⁻, is more stable. Created by Jay.
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I thought as bond polarity increases difference in en when bond is made between the two elements shouldn't bond strength also increase?(6 votes)
How does having two resonance structures make the molecule more stable? Wouldn't a molecule with only one possible structure be more stable because it doesn't have to delocalize the charge and create half-bonds? What does being stable mean?(1 vote)
Being stable in a chemistry context means matter has minimal energy and in unreactive, compared to unstable particles which have higher energy and eagerly react with other particles.
Molecules with nonzero formal charges on the atoms have more potential energy compared to molecules with no formal charges (0 formal charge) on their atoms. A negative formal charge for example means there is an excess of electrons which means greater electron-electron repulsions which translates to greater potential energy and therefore less stability. If a molecule is able to distribute that excess formal charge through resonance structures, it means that each atom has less potential energy concentrated onto itself.
For the acetate ion at the end, the resonance structures distribute that -1 formal charge to the two oxygen atoms so in the resonance hybrid they each have a -1/2 charge instead (less charge is more stable).
Hope that helps.(4 votes)
wouldn't a higher bond polarity make the overall bond strength stronger? so the acid would be weaker like for HF the bond is very polar but HF is a weak acid(1 vote)
Bond strength doesn't really depend on the polarity of the bond. It more so greatly depends on other factors such as bond dissociation energy, number of bonds, and bond length.
HF is a weak acid for a different reason (I also questioned why it was a weak acid when I took Gen Chem in college). It's mainly due to the size of F, and the electronegativity of F. Fluorine is relatively speaking a very small atom, yet it is so electronegative and wants electrons desperately. Because F is small, the F- ion has a high charge density, which is what leads it to not be stable (recall that a low charge density is more stable and a high charge density is less stable). This idea paired with how electronegative F is leads HF to not be a strong acid. It doesn't dissolve well because F needs the help of H to share some of the charge density (even though F wants H's electrons. Talk about a double standard..)(2 votes)
I thought that acetic acid and other organic acids are generally regarded as weak acids.(0 votes)- They are weak acids, but in this video they're just comparing weak acids to other weak acids. Being referred to as stronger in this case is relative, i.e. being considered the strongest of the weak acids.
Hope that helps.(4 votes)
Video transcript
- [Instructor] Factors
that affect asset strength include bond polarity, bond strength, and conjugate base stability. Let's think about a generic acid, HA, that donates a proton to water to form the hydronium ion H3O plus, and the conjugate base
to HA, which is A minus. First let's consider the polarity of the bond between H and A. If A is more electronegative than H, A withdraws electron density. So we could draw an arrow
pointing towards the A, right, as the electrons and the bond between them are pulled closer to the A. As the electronegativity of A increases, there's an increase in
the polarity of the bond. As the bond polarity increases, more electron density is
drawn away from the H, which makes it easier for
HA to donate a proton. Therefore, in general, an increase in the
polarity of the HA bond, means an increase in the
strength of the acid. Next, let's think about the
factor of bond strength. And let's consider the strength
of the bond between H and A. The weaker the bond, the more easily the proton is donated. Therefore, in general, a
decrease in the bond strength, means an increase in the
strength of the acid. The stability of the conjugate base, can also affect the strength of the acid. The more stable the conjugate base, the more likely the asset
is to donate a proton. So if you think about that for HA, the conjugate base is A minus. And the more stable A minus is, the more likely HA will
donate a proton in solution. Therefore, in general, the more stable the conjugate
base, the stronger the acid. So let's go ahead and write here an increase in the stability
of the conjugate base, means an increase in the
strength of the acid. Even though acid strength is usually due to all
three of these factors, bond polarity, bond strength, and conjugate base stability, when we look at examples, we're only gonna consider
one or two factors that are the main
contributors to acid strength. Let's look at the binary
acids from group 7A on the periodic tables,
that's hydrofluoric acid, hydrochloric acid, hydrobromic
acid, and hydro ionic acid. As we go down the group
from fluorine to chlorine, to bromine, to iodine, there's an increase in
the strength of the acid. So out of these four, hydro
ionic acid, is the strongest. The main factor determining the strength of the binary acids in
group 7A, is bond strength. Looking at values for bond enthalpy, allows us to figure out the
strengths of these bonds. For example, the HF bond, has a bond enthalpy of
567 kilojoules per mole. While the HI bond, has a bond enthalpy of
299 kilojoules per mole. The lower the value for the bond enthalpy, the easier it is to break the bond. And because bond enthalpy
decrease as we go down the group, that means there's a
decrease in bond strength. A decrease in bond strength, means it's easier for the
asset to donate a proton. Therefore, we see an increase
in the strength of the asset as we go down the group. Next, let's compare the
strengths of some oxyacids. These oxyacids all have
the general formula XOH, where X is a halogen. The acidic proton, is the proton that's directly
bonded to the oxygen. And if an oxyacid donates
its proton to water, that forms the hydronium ion H3O plus, and the conjugate base to the oxyacid. For these three oxyacids, the halogens are iodine,
bromine, and chlorine. And as we go from iodine to bromine, to chlorine in group 7A
on the periodic table, that's an increase in
the electro negativity. So chlorine is the most electronegative out of these three halogens. And as we go up the group in our halogens, there's an increase in
the strength of the acid. So hypochlorous acid is
the strongest of the three. The main factor determining
the strength of these oxyacids, is the bond polarity, which is affected by the
electronegativity of the halogen. So the polarity of this
oxygen, hydrogen bond, is affected by the
presence of the halogen. As the electronegativity
of the halogen increases, the halogen is able to
withdraw more electron density away from the right side of the molecule. That increases the
polarity of the OH bond, it makes it easier to donate this proton. Therefore, as the electronegativity of the halogen increases, the acidity of the oxyacid increases. This effect of the electronegative atom increasing the acidity, is often referred to as
the inductive effect. Let's compare hypochlorous
acid to two other oxyacids, and notice how I've
left the formal charges off of these acids just so we can focus on general structure. Notice how the acidic proton is directly bonded to the oxygen and all three of these oxyacids. And in all three of these oxyacids, the oxygen is directly
bonded to a chlorine. Notice what happens to the structure as we move to the right. Comparing chlorous acid
to hypochlorous acid, chlorous acid has an additional oxygen bonded to the chlorine. And looking at perchloric acid, instead of only one additional oxygen, there are three additional oxygens. So as we move to the right, we're increasing in the number of oxygens bonded to the chlorine. Oxygen is a very electronegative element. So as we move to the right, we're increasing in the number
of electronegative atoms in the acid. And as the number of
electronegative atoms increases, more electron density is pulled
away from the acidic proton, which increases the polarity
of the oxygen hydrogen bond. So bond polarity increases
as we move to the right, which predicts an increase
in the strength of the acid as we move to the right. And that's what we observe experimentally, perchloric acid is the
strongest of the three. In reality, all three factors affect the strength of the acid. However, for simplicity sake, we could just say that
increasing bond polarity, is the main factor for the
increasing acid strength in these oxyacids. Carboxylic acids are a group of assets that all contain a carboxyl group. A carboxylic group consists of carbon, oxygen, oxygen and hydrogen. So if we look at acetic acid, I'll circle the carboxyl
group on acetic acid, and I can also circle the
carboxyl group on formic acid. The acidic proton in the carboxylic acid, is the one that's directly
bonded to the oxygen in the carboxyl group. One reason why this proton is acidic, is because of the presence of this oxygen in the carboxyl group. This oxygen hydrogen bond
is already polarized, but the presence of another
electronegative atom, further increases the polarity
of the oxygen hydrogen bond. Increasing the bond polarity, makes it more likely to donate the proton, which increases the acidity. For carboxylic acids, it's also important to consider the stability of the conjugate base. When acetic acid donates its proton, it turns into its conjugate base, which is the acetate anion. Notice that the oxygen that used to be bonded to the proton, now has a negative formal charge on it. There are two possible
resonance structures that you can draw for the acetate anion. The first is with the negative charge on this oxygen. And then we could draw
another resonance structure with a negative charge
on the other oxygen. In reality, neither resonant structure is a perfect representation
of the acetate anion. And we need to think about a hybrid of these two resonant structures. In the hybrid, the negative charge isn't
on one of the oxygens, that one negative charge, is spread out or de-localized over two, over the two oxygens. So it's like one oxygen
has negative one half, and the other oxygen
has negative one half. This de-localization
of the negative charge, stabilizes the conjugate base. And the more stable the conjugate base, the stronger the acid. Therefore, the stability
of the conjugate base also affects the acidity
of the carboxylic acid. So because carboxylic acids have conjugate bases that
are resonant stabilized, carboxylic acids like
acetic acid and formic acid, are more acidic.