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Factors affecting acid strength

AP.Chem:
SAP‑9 (EU)
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SAP‑9.F (LO)
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SAP‑9.F.1 (EK)
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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  • blobby green style avatar for user dddangilanc
    I thought as bond polarity increases difference in en when bond is made between the two elements shouldn't bond strength also increase?
    (3 votes)
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  • leafers ultimate style avatar for user Nahom
    I thought that acetic acid and other organic acids are generally regarded as weak acids.
    (1 vote)
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    • leaf red style avatar for user Richard
      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.
      (1 vote)

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.