Weak/Strong acids - am frustrated! (1 Viewer)

[LuthienAdrianna]

OK, this is what my textbook says about weak acids:

~*~

Most acids are only partially ionised in water and are therefore classified as weak acids. The ionisation of the weak acid, ethanoic acid, can be represented by the following equations:

CH₃COOH (aq) <--> H⁺(aq) + CH₃COO^- (aq)

or

CH₃COOH (aq) + H₂O (l) <--> H₃O⁺ (aq) + CH₃COO^- (aq)


The reversible arrows indicate that an equilibrium is established between ethanoic acid molecules and hydrogen ions and ethanoate ions. In terms of the Bronsted-Lowry theory CH₃COOH is a weaker acid (proton donor) than H₃O⁺, and H₂O is a weaker base (proton acceptor) than CH₃COO^-. The reverse reaction is therefore favoured, so that at equilibrium only a small proportion of ethanoic acid molecules are ionised.

~*~

I don't get the bolded part. Could someone PLEASE explain this to me? Am feeling quite desperate.:worried:

 

[Nodice]

Consider it like this: The conjugate base of a weak acid is a strong base. The conjugate of a weak base it a strong acid.
The acetic acid is the weak acid, and so CH3COO- will be a strong base. H2O is a comparatively weak base, and so its conjugate, H3O+ will be a strong acid. Since the products are stronger acids/bases, they will want to react more and so the equilibrium is to the left.

 

[LuthienAdrianna]

I'm sorry... conjugate acids?

 

[pLuvia]

Acids and bases have conjugates to itself

Eg

CH₃COOH the conjugate base is CH₃COO^-
CH₃COO^- the conjugate acid is CH₃COOH

CH₃COOH (aq) + H₂O (l) <--> H₃O<sup>+ (aq) + CH₃COO- (aq)

Here acetic acid being a proton donor donated a H+ to H₂O hence why it has a negative charge on it</sup>

 

[LuthienAdrianna]

I see. So why does the reaction get pushed to the left just because the products are strong?

 

[Nodice]

The product ions react very easily to reform the acid and the water. They want to react more than the acetic acid and water.
Maybe an analogy will work? - Sodium is a more reactive metal than gold, and so it will react much easier with water than gold ~ consider sodium like the strong products, and gold as the weak reactants.

 

[Riviet]

Weak acids will only ionise partially, meaning that some of the acid will remain unreacted while the rest react with the water to form acetate ions and hydronium.

 

[LuthienAdrianna]

I get it now. Thanks!