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.
 

Related to Why are more polarized bonds weaker (Acid dissociation)?

Why are more polarized bonds generally weaker in terms of acid dissociation?

More polarized bonds are weaker because the difference in electronegativity between the atoms creates a partial positive and negative charge. This uneven distribution of electron density weakens the bond, making it easier for the acid to dissociate and release a proton (H+).

How does bond polarity affect acid strength?

Bond polarity affects acid strength by influencing the ease with which the proton can be released. A more polarized bond is typically weaker, which means the proton can be released more readily, resulting in a stronger acid. Conversely, less polarized bonds are stronger, making it harder for the proton to dissociate, resulting in a weaker acid.

What role does electronegativity play in bond polarization and acid dissociation?

Electronegativity is the ability of an atom to attract electrons towards itself. In a bond, if one atom is significantly more electronegative than the other, the bond becomes polarized, with electrons being pulled towards the more electronegative atom. This polarization weakens the bond, facilitating acid dissociation as the proton is more easily released.

Can you provide an example of a highly polarized bond and its effect on acid dissociation?

A common example is the bond between hydrogen and chlorine in hydrochloric acid (HCl). Chlorine is much more electronegative than hydrogen, creating a highly polarized bond. This polarization weakens the bond, making it easy for HCl to dissociate in water, releasing a proton and forming a strong acid.

Are there any exceptions to the rule that more polarized bonds are weaker in terms of acid dissociation?

While the general trend is that more polarized bonds are weaker and thus result in stronger acids, there are exceptions. Factors such as bond length, bond strength, and the overall molecular structure can influence acid dissociation. For example, in some molecules, resonance stabilization or hydrogen bonding can affect the ease of proton release, deviating from the simple rule of bond polarity.

Similar threads

Replies
2
Views
2K
Replies
5
Views
4K
Replies
15
Views
36K
  • Biology and Medical
Replies
2
Views
3K
Replies
8
Views
16K
  • Biology and Chemistry Homework Help
Replies
1
Views
3K
Replies
1
Views
3K
  • Biology and Chemistry Homework Help
Replies
2
Views
2K
Back
Top