[James A]

Hey guys,

a buffer solution is formed from the addition of propanoic acid to sodium hydroxide.

The reason why i need to form two equations is because the following question says :

When a further small amount of sodium hydroxide is added in the titration, the pH changes very little.

The answer talks about the OH- reacting with the H+ which is confusing.

Basically i need two equations which will help me understand this process. Cheers

[This Honest]

Write the ionic equation first, that might make it easier.
You don't really need two.

[hayjes02]

hiya,
a base is a mixture of weak acid (propanoic acid) and its conjugate base and H+ (as its dissociated).

ok sodium hydroxide is a base, so upon the addition of a base (OH-) it would react with the H+ from the dissociation of acid (in the buffer) forming water. this would mean that more acid has to dissociate to make up for the deficit in H+ ions,

do that make sense?
Hope it helps

[King_Arthur]

(Original post by James A)
Hey guys,

a buffer solution is formed from the addition of propanoic acid to sodium hydroxide.

The reason why i need to form two equations is because the following question says :

When a further small amount of sodium hydroxide is added in the titration, the pH changes very little.

The answer talks about the OH- reacting with the H+ which is confusing.

Basically i need two equations which will help me understand this process. Cheers

CH3CH2COOH + NaOH = CH3CH2COO-Na+ + H2O

THAT IS A WEAK ACID AND A SALT OF A WEAK ACID THUS A BUFFER SOLUTION

WHEN H+ ADDED
CH3CH2COO- + H+ = CH3CH2COOH

WHEN OH- ADDED
CH3CH2COOH + OH- = CH3CH2COO- + H2O

SO POTENTIAL pH CHANGE IS PREVENTED

[alkyone]

You only need 1 equation. The second one is simply hydrogen ions reacting with OH ions to form water. Propanoic acid dissociating is your buffer solution. Don't consider NaOH. it's a reversible reaction since propanoic acid dissociates partially to give hydrogen ions and CH3CH2COO- ions. adding MORE hydroxide ions remove hydrogen ions from the solution, so the equilibrium shifts from left to right to produce more of the H+ ions and restoring the pH.

[James A]

(Original post by hayjes02)
hiya,
a base is a mixture of weak acid (propanoic acid) and its conjugate base and H+ (as its dissociated).

ok sodium hydroxide is a base, so upon the addition of a base (OH-) it would react with the H+ from the dissociation of acid (in the buffer) forming water. this would mean that more acid has to dissociate to make up for the deficit in H+ ions,

do that make sense?
Hope it helps

Thanks, it does, but I'm struggling to figure out the equation of the dissociation of propanoic acid.

The correct way is:

CH3CH2COOH ⇌ CH3CH2COO- + H+

however i also stumble across books saying:

CH3CH2COOH + H20 ⇌ H30+ + CH3CH2COO-


why have they included the (h20) in this equation? I thought that when a weak acid dissociates, it needs H20 in the reaction to do so?

[alkyone]

(Original post by James A)
Thanks, it does, but I'm struggling to figure out the equation of the dissociation of propanoic acid.

The correct way is:

CH3CH2COOH ⇌ CH3CH2COO- + H+

however i also stumble across books saying:

CH3CH2COOH + H20 ⇌ H30+ + CH3CH2COO-


why have they included the (h20) in this equation? I thought that when a weak acid dissociates, it needs H20 in the reaction to do so?

You know in reality H+ ions don't exist, it's H3O ions. Something dissociates because it reacts with water. Whether you consider the role of water or not should make no difference though

[alkyone]

Oh and if that makes no sense, then think of HCl. In gaseous form it's just a gas. You test it with MOIST blue litmus paper. it has acidic properties ONLY when dissolved in water. That's because H+ ions are H3O+ ions really, but for simplicity we ignore the role of water

[King_Arthur]

(Original post by James A)
Thanks, it does, but I'm struggling to figure out the equation of the dissociation of propanoic acid.

The correct way is:

CH3CH2COOH ⇌ CH3CH2COO- + H+

however i also stumble across books saying:

CH3CH2COOH + H20 ⇌ H30+ + CH3CH2COO-


why have they included the (h20) in this equation? I thought that when a weak acid dissociates, it needs H20 in the reaction to do so?

CH3CH2COOH + H20 ⇌ H30+ + CH3CH2COO-

Kc = [ CH3CH2COO-][ H3O+]/ [CH3CH2COOH][H2O]


Kc*[H2O] = Ka

therefore Ka = [ CH3CH2COO-][ H3O+]/ [CH3CH2COOH]

H+ exist as H3O+
but in the equation always use H+