pH titration curves and indicators

I'm in A2 chemistry in neutralisation, the following question is in the topic of neutralisation and the topics mentioned in the title of the post 🙂: is the end point equal to the equivalence point in a titration? And when are they not equal (as I'm guessing they're not when you add too little or too much indicator) 🙂

Reply 1

opopyldog

10

It's actually dependant on the type of indicator as there are quite a few! The end point is the point at which the indicator changes colour so if you have the wrong indicator it is less likely to match the equivilance point ( the point where there is neutralisation) However with the right indicator, they are more likely to be equal. Don't worry about learning indicators though as they give you that part if required by the question usually with their pka values :smile:

Reply 2

ChemistryWebsite

11

Original post by Freedom physics

I'm in A2 chemistry in neutralisation, the following question is in the topic of neutralisation and the topics mentioned in the title of the post 🙂: is the end point equal to the equivalence point in a titration? And when are they not equal (as I'm guessing they're not when you add too little or too much indicator) 🙂

Here's a link which may help you understand pH curves and the equivalence point. It also touches on use of indicators.
http://alevelchemistrytuition.co.uk/ph-curves-acid-base-titration/

Reply 3

Freedom physics

OP

12

Original post by godlypopo

It's actually dependant on the type of indicator as there are quite a few! The end point is the point at which the indicator changes colour so if you have the wrong indicator it is less likely to match the equivilance point ( the point where there is neutralisation) However with the right indicator, they are more likely to be equal. Don't worry about learning indicators though as they give you that part if required by the question usually with their pka values :smile:

Thank you so much! That's a really satisfying answer!! 😄

If i may, I'd also like to ask you this 🙂: at the end point, why is [HA] = [A⁻]? Where HA is the weak acid indicator and A⁻ is the conjugate base of it 🙂

Reply 4

opopyldog

10

Original post by Freedom physics

Thank you so much! That's a really satisfying answer!! 😄

If i may, I'd also like to ask you this 🙂: at the end point, why is [HA] = [A⁻]? Where HA is the weak acid indicator and A⁻ is the conjugate base of it 🙂

If the weak acid is also acting as an indicator, then it would make sense for the weak acid and the conjugate base to be of equal concentrations as they must be equivilant to eachother in order to neutralise. Due to the weak acid acting as an indicator, the equivilance point and end point should be the same if I'm reading the question right?

Original post by Freedom physics

Thank you so much! That's a really satisfying answer!! 😄

If i may, I'd also like to ask you this 🙂: at the end point, why is [HA] = [A⁻]? Where HA is the weak acid indicator and A⁻ is the conjugate base of it 🙂

Because that is the mid point of the weak acid which is the indicator. If both [HA] and [A-] have a colour then the mid-range colour is when they are in equal concentration.

Reply 6

opopyldog

10

^^^^^ much better worded

Reply 7

Freedom physics

OP

12

Original post by charco

Because that is the mid point of the weak acid which is the indicator. If both [HA] and [A-] have a colour then the mid-range colour is when they are in equal concentration.