- Acid-base definitions
- Organic acid-base mechanisms
- Ka and acid strength
- Ka and pKa review
- Using a pKa table
- Using pKa values to predict the position of equilibrium
- Stabilization of a conjugate base: electronegativity
- Acid strength, anion size, and bond energy
- Stabilization of a conjugate base: resonance
- Stabilization of a conjugate base: induction
- Stabilization of a conjugate base: hybridization
- Stabilization of a conjugate base: solvation
How anion size and bond dissociation energies affect acid strength.
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- 1.What about astatine? Isn't that also a halogen?
2.What is the strongest acid in the world?(12 votes)
- 1. Astatine is in the group of the radioactive chemical element. Having one more layer than the iodine stop it from being an halogen.
2. The strongest acid in the world is the fluoroantimonic acid (HSbF6)(26 votes)
- What is pKa?(18 votes)
- pKₐ = -log(Kₐ).
Kₐ usually runs from 10⁻¹ to 10⁻¹⁴. so pKₐ runs from 1 to 14.
pKₐ gives no more information, but it is easier t think of numbers from i to 14 than to visualize those numbers with negative exponents.(12 votes)
- What does it mean by 'ion is stable'?I have heard about 'atom is stable.( I usually ask many silly questions!)(8 votes)
- When an ion is stable, that means it is relatively unreactive. In the context of acids and bases, that means the ion is relatively unreactive toward water in aqueous solution. For example, a strong acid such as HCl dissociates to form Cl- ions in solution. You could imagine if those ions were not very stable, they could react with H+ (or H3O+) in the water to reform HCl. However, since Cl- ions are (relatively) stable in water, that reaction doesn't occur much and we can consider HCl a strong acid that completely dissociates.(15 votes)
- what is pka value of an acid as in this video?(10 votes)
- The dissociation of an acid in water is an equilibrium reaction:
HX + H₂O ⇌ H₃O⁺ + X⁻, and Ka =[H⁺][X⁻]/[KX]
The pKa is the negative logarithm of the Ka value:
pKa = -log(Ka)(8 votes)
- what is the strongest base in the world?is it NaOH?(2 votes)
- NaOH is the strongest base that can exist in water, but there are much stronger bases.
I'm not sure about the strongest base, but Grignard reagents would certainly be among the contenders.
A Grignard reagent is effectively R⁻, ⁺MgX.
The pK_a for the reaction R-H ⇌ R⁻ + H⁺ is about 45 (extremely weak acid), so the conjugate base (R⁻) is extremely strong.(5 votes)
- I don't understand how HI and HBr can have different Ka's if they both dissociate completely. If you had two solutions of 0.1M HI and HBr wouldn't they have the same [H+] and pH?(3 votes)
- Yes, they both have the same pH in water. This is called the levelling effect.
But they aren't completely dissociated in nonaqueous solutions, for example in acidic solvents such as acetic acid.
Their Ka values in that solvent can then be extrapolated to aqueous solutions by comparison with moderately strong acids whose Ka values can be measured both in water and in that solvent.(3 votes)
- How do I relate pKa with pH or pOH?(3 votes)
- Use the Henderson-Hasselbalch equation.
If HA ⇌ H⁺ + OH⁻, pH = pKₐ + log([A⁻]/[KA]) and pH + pOH = 14.00.
For a base, B + H₂O ⇌ BH⁺ + OH⁻, and pOH = pKb + log ([BH⁺]/[B]).(3 votes)
- Why do strong acids have weak conjugate bases?(2 votes)
- If a strong acid didn’t have a weak conjugate base, then it couldn’t be 100% ionised, so it wouldn’t be a strong acid.
The conjugate base of a strong acid must be a weak base.
Strong acids, say HCl always exist only as H+ and Cl- in an aqueous solution, HCl doesn’t get reformed.
HCl (strong acid) is a very good H+ donor
Cl- (weak conjugate base) is a very poor H+ acceptor(5 votes)
- Why does acidity increase when the anion size is greater/the anion is more stable?(2 votes)
- Larger anions have a physically larger volume of space for the negative charge to be spread over.
Being able to spread charges over a larger area is a better situation in terms of potential energy, that’s why we say it’s more stable. More stable in chemistry generally means lower energy.
The acid itself also tends to form weak bonds to hydrogen because of the mismatch in size between the atoms, eg HI.(4 votes)
- What does stabilizing negative charge mean? Does it mean it decreases conjugate bases' strength to pull electrons close?(3 votes)
- Stabilising negative charge means that electrons are not concentrated but they are spread over a large volume.(1 vote)
- Let's say we have a binary acid, HX, where X is equal to a halogen. So X is equal to Fluorine, Chlorine, Bromine, or Iodine. If HX donates this proton, we're left with the conjugate base, which is X minus. We saw from the previous video the more stable the conjugate base, the more likely HX is to donate a proton. So the more stable the conjugate base, the stronger the acid. If we take a look at these four binary acids here, we have hydrofluoric acid with a pKa value of approximately positive three, hydrochloric acid with an approximate pKa of negative seven, hydrobromic acid at negative nine, and hydroiodic acid at -10. We know the lower the pKa value, the stronger the acid. So as we go down this way, we are decreasing in pKa values, and therefore, we are increasing in acid strength. So we're increasing in acidity. Therefore, hydroiodic acid is the strongest acid out of these four because hydroiodic acid has the lowest value for the pKa. If hydroiodic acid is our strongest acid, the conjugate base must be the most stable. So the conjugate base to hydroiodic acid would be the iodide anion, I minus. So here we have all of the different conjugate bases. We'd have the fluoride anion, which is the conjugate base to HF. We have the chloride anion, which is the conjugate base to HCl. We have the bromide anion, which is the conjugate base to HBr, and of course, again, the iodide anion, the conjugate base to HI. The iodide anion must be the most stable because HI is our strongest acid. So we can explain the stability of this conjugate base in terms of the size of the ion. Remember as you go down a group on the periodic table, you would increase in the size of the anion. So let me go ahead and write anion instead of ion here. So we increase in the size, in the size of the anion. So why does that help to stabilize the conjugate base? Well we need to think about this negative charge here. So we have a negative charge in the iodide anion, and we have this charge spread out over a large volume of space. That makes the anion more stable. So remember electrons repel each other, but if you can spread out the negative charge over a large amount of space, then you can better stabilize that negative charge. So this is more stable than, for example, the fluoride anion. The fluoride anion has a negative charge that's concentrated in a small volume of space. So that destabilizes this anion compared to the iodide anion. The iodide anion becomes the most stable, and therefore, HI is the most likely to donate a proton, and therefore, HI is our strongest acid out of these four. Notice this is different from the previous video where we talked about electronegativity. There we were comparing elements in the same period on the periodic table. So we were moving horizontally across our periodic table this way. And in that video, the fluoride anion was the most stable one because fluorine's our most electronegative element, and therefore, best able to stabilize a negative charge. But as you go down a group on the periodic table, your electronegativity decreases. So that can't be the dominant trend because if your electronegativity decreases as you go down, just thinking about electronegativity, that would predict HF to be the strongest acid, and that's not what we observe. So as you go down a group on the periodic table, it's the size of the anion that determines the stability of the conjugate base. So the larger the anion, the better it is to stabilize a negative charge, and therefore, the more stable the conjugate base. The more stable the conjugate base, the more likely HX is to donate a proton, and therefore, the stronger the acid. Another very important factor to think about is the strength of the bond. We've already said that hydroiodic acid is our strongest acid with the lowest pKa value. So this bond right here must be the easiest to break. If it's easy to break this bond, that makes it easy to donate this proton. So we can get an idea of the bond strengths for our binary acids by looking at bond association energy. So we could also call these bond energies or bond enthalpies. So remember bond dissociation energy measures the amount of energy that's needed to break a bond in the gaseous state. So if we look at our hydrogen halides and we think about our bonds, notice what happens to the bond energy. It's the hardest to break this bond, the bond between hydrogen and fluorine. This takes the most energy to break this bond, and as you go down, we see we decrease in bond dissociation energy. So it only takes 299 kilojoules per mole to break this bond between H and I. I should say these are approximate bond energies and you'll see several different values in different textbooks. So if the HI bond is the easiest to break, that means when you're thinking about the acids, this bond is the easiest to break, therefore, it's the most likely to donate a proton, and therefore, it has the lowest value for the pKa. Hydroiodic acid is the strongest out of these four binary acids.