Buffers (2 Viewers)

[tlv6554]

I thought a buffer is made of a weak acid and its conjugate base but I found in the 2013 Sydney Tech trial that a buffer is made of a weak acid and a weak base. Any clarifications please?

 

[Mathew587]

I thought a buffer is made of a weak acid and its conjugate base but I found in the 2013 Sydney Tech trial that a buffer is made of a weak acid and a weak base. Any clarifications please?

A buffer is made of a weak acid and a weak base. I used to think the same considering the conjugate of a weak acid is a strong base but you have to remember that buffers are there to balance out and minimise changes in pH. A strong conjugate base acid wouldn't help just as it would be to strong.

 

[jazz519]

I'd have to say the answer they gave is wrong (we covered this stuff at uni again last semester, and the definition was exactly the same as the HSC one which states that a buffer is a mixture of a weak acid and its weak conjugate base, in equimolar concentrations. It resists changes in pH upon small additions of a strong acid or base). A weak acid such as HF, will give a conjugate base of F^- (still a weak base since the acid was weak), thus they are able to form a chemical equilibrium in HF + H2O ---> H3O^+ + F^. and thus a buffer

just having a random weak base and weak acid doesn't mean they will be able to form a buffer, as seen in HF and NaHCO3

 

[jazz519]

A buffer is made of a weak acid and a weak base. I used to think the same considering the conjugate of a weak acid is a strong base but you have to remember that buffers are there to balance out and minimise changes in pH. A strong conjugate base acid wouldn't help just as it would be to strong.

the conjugate of a weak acid is always a weak base (it is strong enough to hydrolyse water, but not strong enough that it will completely ionise, so thus forms an equilibrium and buffer as mentioned in the example above)

 

[Sp3ctre]

I think what the answers meant is a buffer can either be a weak acid and it's conjugate base OR a weak base and its conjugate acid.

 

[jazz519]

I think what the answers meant is a buffer can either be a weak acid and it's conjugate base OR a weak base and its conjugate acid.

Yeah probably (cause technically if its a weak acid then the conjugate base will have to be there and likewise for a weak base and its conjugate acid), but I think the question was just poorly worded. In HSC exams, they will probably be more explicit if thats the type of answer they want

 

[Mathew587]

the conjugate of a weak acid is always a weak base (it is strong enough to hydrolyse water, but not strong enough that it will completely ionise, so thus forms an equilibrium and buffer as mentioned in the example above)

wait what I thought it was the weaker the acid, the stronger the conjugate base?

 

[jazz519]

wait what I thought it was the weaker the acid, the stronger the conjugate base?

That is sort of true in terms of their relative strengths in equilibriums, but compared to like what we would define as a strong acid/base something that completely ionises, no where near.

If either one them is not a completely strong base or acid what you said doesn't really matter.

The 4 things you should remember are:

A strong acid such as HCl produces a very weak conjugate base (Cl^-)

A strong base such as NaOH produces a very weak conjugate acid (Na^+)

(Key word here is very weak, meaning that the strong acid or base has completely dissociated and the very weak conjugates are unable to be hydrolysed by water to form the strong acid or base again, therefore no buffer forms)

A weak acid such as HCO3^- produces a weak conjugate base (CO3^2-)

A weak base such as NH3 produces a weak conjugate acid (NH4^+)

(key word here is weak, but still strong enough to be hydrolysed by water thus able to form an equilibrium and buffer)

 

[Mathew587]

That is sort of true in terms of their relative strengths in equilibriums, but compared to like what we would define as a strong acid/base something that completely ionises, no where near.

If either one them is not a completely strong base or acid what you said doesn't really matter.

The 4 things you should remember are:

A strong acid such as HCl produces a very weak conjugate base (Cl^-)

A strong base such as NaOH produces a very weak conjugate acid (Na^+)

(Key word here is very weak, meaning that the strong acid or base has completely dissociated and the very weak conjugates are unable to be hydrolysed by water to form the strong acid or base again, therefore no buffer forms)

A weak acid such as HCO3^- produces a weak conjugate base (CO3^2-)

A weak base such as NH3 produces a weak conjugate acid (NH4^+)

(key word here is weak, but still strong enough to be hydrolysed by water thus able to form an equilibrium and buffer)

thanks for that

 

[jazz519]

thanks for that

No worries, good luck in your exams