During an experiment,a student has added a drop of phenolphtalein solution in dilute sodium hydroxide solution.Then,he has poured hydrochloric acid to it dropwise.What would be his observation if he tested colourless solution with red and blue litmus paper?Identify the type of reaction involved.
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Answers
Phenolphthalein is one of the most common acid-base indicators used to determine the end point in
acid-base titrations. It is also the active ingredient in some laxatives. If excess base is present at the
end of an acid-base titration, the pink phenolphthalein color fades if the solution is allowed to stand for
a while. This slow fading is of no consequence to the titration. However the fading of
phenolphthalein in basic solution is an excellent example of second order reaction kinetics.
The structure of the colorless form of phenolphthalein (C20H14O4) is complex and not important to this
experiment. At a pH of 8 or lower, the structure of phenolphthalein will be abbreviated with the
formula H2P. In the pH range of 8-10 both acidic protons are rapidly removed from the colorless H2P
by NaOH, (OH -
). to give the pink P2- ion according to the equation:
H2P (aq) + 2 OH-
(aq) → P2-
(aq) + 2 H2O (l)
colorless pink
At a pH above 10 the pink color slowly fades as P2- reacts further with NaOH, (OH -
), to give the
colorless POH3- ion according to the equation:
P2-
(aq) + OH-
(aq) → POH3-
(aq)
pink colorless
The rate law for this second order reaction can be expressed as:
Rate = k2 [P2-]
m[NaOH]n
or Rate = k2[P2-]
m[NaOH]n
The diluted Naoh mixed with HCL will give an acidic solution as Naoh is diluted
This is why the phenolphthalein is colourless. Thus we know the solution is acidic and when we test it with red litmus paper it shows no change. When we test it with blue litmus paper, it turns red
The type of reaction involved is neutralisation