Question

Draw a titration curve for both a strong acid titrated by a strong base

and a weak acid titrated by a strong base. Identify the equivalence point and the half equivalence point on these graphs. Define each of these points and tell what information these points can give you. Also tell what species are present at the beginning of the titration, at the half equivalence point, at the equivalence point and after the equivalence point. .

Draw a titration curve for a polyprotic acid and show these points on that curve as well and identify the species that will be present at the 1st and 2nd equivalence points, the 1st and 2nd half equivalence points, the beginning of the titration and after the second equivalence point. Explain how you will determine the concentration of the acid you are titrating using the titration curve.

Answer 1

Equivalence point indicates the exact neutralization ( i,e; neutral pH = 7 )

Half equivalence point is the point at which the pH of the acid is half of its initial pH.

in Strong acid vs Strong base case, the curve starts from the axis that means the pH is near to zero because of strong acid. Then the addition of alkali neutralize the acid & increase the pH gradually upto equivalence point. There after no more acid is present further addition of base increase the pH near to 14 because of strong Base.

Weak acid vs Strong base,

the curve doesn't starts from the axis. since the pH of weak acid itself low. then the addition of alkali neutralizes acid. Here, amount of alkali required for neutralization is less than that of strong acid neutralization. after equivalence point the pH suddenly increases to maximum.

in both curves,

in the beginning only Acid species present. while neutralization, acid and neutralization products ( i,e; water and salt) are there. At exact equivalence point only water and salt will be there. after that the base concentration increases.

for e.g;

First equivalence point is the neutralization of first H⁺ released from the acid. after complete this process acid would release the next proton then which will under go neutralization reaction with alkali. Then this will end up with second equivalence point. Then there is no more acidic species, therefore the increase in pH is due to the excess of Base.

And from the volume of base utilized for exact neutralization (equivalence point),concentration of base and the amount of acid taken for tiration are known values.

Then by simple relation V₁N₁ = V₂N₂

the concentration of acid can be calculated.