Concerning discreptencies with what students learn - strengths and conjugates (1 Viewer)

[leehuan]

I apologise for a tiny amount of irrelevance as this worry first came out of helping people from other sources (seems like the source is filtered). But during my time spent on the chemistry section of the forum I haven't had problems with this so I'm posting it here for people's opinions.

So, people are beginning to tell me things such as NH4(+) is a STRONG acid as a result of the weak conjugate base NH3.
And things like CH3COO(-) is a STRONG base because the conjugate acid CH3COOH is weak.

I only agree with the latter part of both the above statements.

From what I was taught, and I have never lost marks for this, the conjugate of a weak acid/base is STILL a weak base/acid. This occurs because an acid/base is termed 'strong' when, in the scope of the HSC course, it fully ionises in solution (in reality it's like 90%+ or something).
This implies that the reaction Acid + Water -> Hydronium ion + Conjugate base can be written with the ONE-sided arrow.

As far as the HSC goes, the only strong acids relevant are the hydrogen halides (excluding HF), sulfuric acid and nitric acid. As for the bases, it's basically group I metal bases.

When you have a strong acid, it just so happens that it's conjugate base is EXTREMELY weak (e.g. the chloride ion Cl(-) is virtually neutral). This also occurs for bases (Na(+) being an example of a 'neutral cation').

Have any of you been taught otherwise? That is, that substances such as CH3COO(-) and NH4(+) are indeed, strong as well?

 

[RachelGreen]

Yeah, I was taught like that as well, that the relative strengths of conjugate acids and bases depend on the strength of the parent bases/acids.

Here's an extract from my notes that helped me to understand hydrolysis reactions and finding acidity/neutral/alkalinity of substance:

- CONJUGATE BASE of a STRONG acid is an EXTREMELY WEAK BASE. E.g. HCl completely ionises in water (complete reaction), meaning there is no reaction between the conjugate base Cl- and water in solution. As such, conjugate base of strong HCl acid, Cl-, is an extremely weak base. (HCl+H2O --> H3O+Cl-

(Since the base is extremely weak, it does not take part in reactions

- The CONJUGATE BASE of a WEAK ACID is a WEAK BASE (not extremely). E.g. ionisation of weak acetic acid (equilibrium). Means BOTH acetic acid and acetate reacts appreciably with water. CH3COOH- as a weak base accepts proton in solution (CH3COO- + H2O<---->CH3COOH+OH-)

(Note: conjugate weak base is not extremely weak compared to first dotpoint)

- CONJUGATE ACID of a WEAK BASE is a WEAK ACID e.g. ionisation of NH3, both basic NH3 and conjugate acid NH4+ reacts with water to SIGNIFICANT extents. The NH4+ accepts a proton in solution (NH4+ + H2O <----> NH3 + H3O+)

- The CONJUGATE ACID of a STRONG BASE is an EXTREMELY WEAK ACID. Therefore, conjugate acid don't take part in reactions.

E.g. strong base + weak acid --> basic salt + water
CH3COOH + NaOH --> CH3COOHNa + H2O
CH3COONa --> CH3COOH- + Na+

The sodium ion doesn't hydrolyse because it is the extremely weak acid of a strong base. However CH3COO- reacts by accepting proton since it is the weak conjugate base of a weak acid. CH3COO- + H2O <----> CH3COOH + OH-

Most of my friends don't understand why Na+ doesn't hydrolyse, but if it is a strong acid/base, the conjugate base/acid will be extremely weak and hence take no part in hydrolysis reaction.
Further e.g. NH3 (weak base) + HCl (strong acid) --> NH4+ + Cl-
Chloride ion doesn't hydrolyse because is the extremely weak conjugate base of a strong acid. However NH4+ reacts with water by donating proton because it is the weak conjugate acid of a weak base.

I've never heard that CH3COOH is a strong base due to weak conjugate acid CH3COO-. If the conjugate acid/base is weak, then its parent base/acid has to be weak as well. I hope this theory helps!