Why are more polarized bonds weaker (Acid dissociation)?

  • Thread starter yucheng
  • Start date
  • Tags
    Polarity
In summary, the weak acidity of dilute hydrofluoric acid is due to the high H-F bond strength, which overcomes the more negative enthalpy of hydration of the fluoride ion. This is often attributed to the high dissolution enthalpy of HF. However, when considering the dissociation of acids, the argument that H-F bond is stronger than H-I bond based on bond enthalpies may be inaccurate. This is because the hydration of ions must also be taken into account. In carboxylic acids, the polarity of O-H bonds is related to their strength, with electron donating groups causing the bond to be less polar and therefore stronger. This is known as the inductive effect.
  • #1
yucheng
232
57
Wikipedia says that:
The weak acidity in dilute solution is sometimes attributed to the high H—F bond strength, which combines with the high dissolution enthalpy of HF to outweigh the more negative enthalpy of hydration of the fluoride ion.
(https://en.wikipedia.org/wiki/Hydrofluoric_acid)
(the H-F bond is highly polar compared to the H-I bond)
(H-F bond is stronger than H-I bond)

It is a common argument to say that H-F bond is stronger than H-I bond on the basis of bond enthalpies:

$$\text{HX} -> \text{H(g)} + \text{X(g)}$$.

However, for dissociation of acids, we must actually consider (hence the electron affinity comes in)

$$\text{HX} -> \text{H}^+(g) + \text{X}^-(g)$$,

and then the hydration of the ions, no? Does this mean the argument given above is... inaccurate, though it works?
____________
For O-H bonds in carboxylic acids, electron donating group causes the O-H bond to be less polar hence stronger.
Carboxylic acids, https://en.wikipedia.org/wiki/Inductive_effect

Why Is polarity related to bond strength? Why are polar bonds weaker than non-polar bonds?
 
Last edited:
Chemistry news on Phys.org
  • #2
Polar bonds are weaker than non-polar bonds because the electrons in polar bonds are not evenly distributed. In a non-polar bond, the electrons are shared equally between the two atoms, creating a uniform electric field. In a polar bond, the electrons are attracted to one atom more than the other, causing an unequal distribution of charge and therefore an uneven electric field. This uneven electric field creates an instability, resulting in a weaker bond.
 

1. Why are more polarized bonds generally considered weaker in the context of acid dissociation?

In the context of acid dissociation, a bond's polarity refers to the uneven distribution of electron density between different atoms in a molecule. More polarized bonds are considered weaker because the electron density is more concentrated around the more electronegative atom, making the bond easier to break. This facilitates the release of a proton (H+), a key aspect of acid dissociation.

2. How does bond polarity affect the strength of an acid?

The strength of an acid is largely determined by how easily it can donate a proton. In molecules with highly polarized bonds, the bond between the hydrogen atom and its counterpart (often oxygen or another electronegative element) is weakened due to the unequal sharing of electrons. This weakening makes it easier for the acid to lose a hydrogen ion, thereby increasing its acidity.

3. What role does the electronegativity of atoms play in bond polarization and acid strength?

Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. In a bond between two atoms where one atom is significantly more electronegative than the other, the electrons will be more attracted to the more electronegative atom. This results in bond polarization. For acids, if the atom bonded to hydrogen is highly electronegative, it will pull the electron density away from the hydrogen, weakening the bond and thus enhancing the acid's ability to donate the hydrogen ion.

4. Can you explain why HF is a weak acid despite having a highly polarized H-F bond?

While it's true that HF has a highly polarized bond due to the high electronegativity of fluorine, the strength of an acid also depends on the stability of the conjugate base formed after dissociation. In the case of HF, the fluoride ion (F-) formed is highly stable and holds onto the electron pair strongly, resisting the release of the proton. This stability contributes to HF being a weak acid, as it does not dissociate easily in water.

5. Are there exceptions to the rule that more polarized bonds in acids lead to greater acid strength?

Yes, there are exceptions. While generally, more polarized bonds can lead to stronger acids due to easier proton donation, other factors such as the overall molecular structure, the solvent, and the stability of the conjugate base can influence acid strength. For example, certain organic acids may have polarized bonds but still act as weak acids due to the resonance stabilization of the conjugate base, which makes the release of the proton less favorable.

Similar threads

Replies
2
Views
2K
Replies
5
Views
3K
Replies
15
Views
35K
  • Biology and Medical
Replies
2
Views
3K
Replies
8
Views
16K
  • Biology and Chemistry Homework Help
Replies
1
Views
2K
Replies
1
Views
3K
  • Biology and Chemistry Homework Help
Replies
2
Views
2K
Back
Top