Setting up acids/ bases equation (1 Viewer)

[~Fire Jade~]

How do you know when you're doing a chem eqn whether to write
a) HA -> H+ + A-
or
b) A- + H2O -> HA + OH-

because i read it had somewhere to do with acids or bases or the strength of the acid......and i just got *reallly* confused.

Thanks.

 

[x jiim]

How do you know when you're doing a chem eqn whether to write
a) HA -> H+ + A-
or
b) A- + H2O -> HA + OH-

because i read it had somewhere to do with acids or bases or the strength of the acid......and i just got *reallly* confused.

Thanks.

From my somewhat limited knowledge of this topic, it looks like the first reaction you have is an acid ionising in water, and the second is the conjugate base of the acid reacting with water to produce the A- ion?

It should have to do with acids and bases- the acids will ionise in water to produce the hydronium ion, eg HCl ionising in water:
HCl + H2O --> H3O+ + Cl-

And bases will react to form a conjugate acid, and an OH- ion. I think. Eg, K2SO4 is a basic salt as
K2SO4 (s) <---> 2K+ (aq) + SO4 2- (aq)
SO4 2- (aq) + H2O (l) <---> HSO4 - (aq) + OH- (aq).

The strength of the acid/base just determines whether the reaction is complete or at equilibrium, ie whether the arrow is one way or not. Strong acids/bases ionise completely, so the it goes to the right.

I think that's it, can someone who's done this topic check for me? Hope that helps :]

 

[~Fire Jade~]

ehhh, well to be honest, i got lost halfway through the explanations~ but been looking at things for a bit more and i just need people to confirm whether my logic is right or wrong....
well HA -> H+ + A- is basically the same as HA + H20-> H3O+ + A-. These two "equations" are the same thing and used sor strong acid reactions~ so [H+]= [H3O+] in other words......
And A- + H2O -> HA + OH- is used for umm bases??? But yeh the arrows go both way and are at equilibrium....I just cant write it like that on this....

 

[adomad]

As X jim said, the first reaction shows the ionisation of and acid as H^+ is product (acids are proton donors). WEAK acids form STRONG conjugate bases. hence the unsatisfied STRONG conjugate base (A^-) then goes to water and asks for a hydrogen... due to how awesome water is, it gives the A^- an hydrogen ion (a proton) and the base then becomes HA and it is now happy.

the acids will ionise in water to produce the hydronium ion

for example... Acetic acid ( an incredibly weak acid)


since acetic acid (commonly know as vinegar) is weak,... it yields a strong conjugate base and the following takes place:


so in answer to your second post OP, they aren't the same thing as in the first "equation" it is the ionisation of the acid and the second is the further reaction of the conjugate base and water. i don't think it can be used for strong acids as they produce WEAK conjugate bases which are lazy and do nothing. For example:

and [H_3O^+]=[OH^_] only if an acid and a base of similar strengths (SA+SB and WB+WA) because if they are of different strengths, the end result will involve an inequality i.e.[H_3O^+]>[OH^_] (For SA+ WB)or [H_3O^+]<[OH^_](For SB+WA)
hmm...

this works

HA -> H+ + A- is basically the same as HA + H20-> H3O+ + A-

your just adding water

 

[~Fire Jade~]

Thanks that made sense ish. I needa mull over it a tad longer ==

 

[~Fire Jade~]

Another quick question:
HA + H20-> H3O+ + A- involves pka and ka
while
A- + H2O -> HA + OH- involves just pkb and kb

So if you want to work out the "ka" for the last eqn listed....then you have to convert it from ka? Thanks.