Ka Value Lab Report
The experiment is based on the fact that all acids in the experiment have a unique Ka value. However, the only difference between the possible acids is the number of chlorine atoms attached to the carbon atom that is not in the functional group. The reason for the number of chlorine atoms attached to the carbon atom affecting the Ka is that there is a large electronegativity difference between chlorine and carbon, pulling the electrons to tend to the chlorine side. This forms a domino effect where the electrons of each atom tend to the side of the chlorine atoms within the electron shell, including the oxygen atom attached by the potentially proton-becoming hydrogen atom. This intensify the electronegativity difference between oxygen and hydrogen which leads to the dissociation of the proton in acetic acid and contribute to the increased stability of the conjugate base, thus the chlorinated variations have greater KAs than non-chlorinated acetic acid. The more chlorine atoms attached to the carbon atom closer the the functional group, the greater the KA value. Hence, the four possible acids can be arranged in increasing acidity as acetic acid< chloroacetic acid< dichloroacetic acid< trichloroacetic acid.
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Note that the pH at the equivalent point of the titration between the acid and NaOH is larger than pH 7, unlike the titration between HCl and NaOH which equals pH 7.
This is caused by the reversible reaction between the conjugate base A- and water, forming the protonated form of the acid HA and hydroxide ion. The weaker the acid with smaller KA, the more significantly this happens. However, as there is nothing added to neutralize the hydroxide ion, the solution remains basic towards the end of the
titration.
Three acid-base indicators with colour changing range pH 0-2 were used to test for the unknown’s identity by comparing the colour of the unknown acid with the possible acid in the same concentration. As the pH of the acid falls within the colour changing range, the colour the acid exhibit is the mixture of the colour of the two forms of the indicator, with different propotion of the two forms of the indicator molecule. The more acidic solution will have a shade that is closer to the colour the indicator exhibit at the lower end of the pH as more of the protonated molecules with this colour exist in the solution. The one that shares the same shade with the unknown acid under the same concentration is the same acid as the unknown acid.
Note that the pH at the equivalent point of the titration between the acid and NaOH is larger than pH 7, unlike the titration between HCl and NaOH which equals pH 7.
This is caused by the reversible reaction between the conjugate base A- and water, forming the protonated form of the acid HA and hydroxide ion. The weaker the acid with smaller KA, the more significantly this happens. However, as there is nothing added to neutralize the hydroxide ion, the solution remains basic towards the end of the
titration.
Three acid-base indicators with colour changing range pH 0-2 were used to test for the unknown’s identity by comparing the colour of the unknown acid with the possible acid in the same concentration. As the pH of the acid falls within the colour changing range, the colour the acid exhibit is the mixture of the colour of the two forms of the indicator, with different propotion of the two forms of the indicator molecule. The more acidic solution will have a shade that is closer to the colour the indicator exhibit at the lower end of the pH as more of the protonated molecules with this colour exist in the solution. The one that shares the same shade with the unknown acid under the same concentration is the same acid as the unknown acid.