Titration of final reaction mixture against NaOH
No. of moles of CH3COOH in 1 cm3 of reaction mixture before heating
= [(density x volume) / molar mass] / volume
= [(1.05 gcm-3 x 15 cm3) / (12x2+1x4+16x2)]
= 8.75 x 10-3 mol
2 NaOH + H2SO4 Na2SO4 + 2 H2O
No. of moles of H+ from H2SO4 in 1 cm3 of reaction mixture before heating (8 drops)
= [(No. of moles of NaOH reacted / 2) x 2] / volume
= {[(15.1/1000) x 0.5] / 2} x 2 / 30
= 2.52 x 10-4 mol
= No. of moles of NaOH of 1 cm3 reacted after heating
= (7.8/1000) x 0.5
= 3.90 x 10-3 mol
Thus, no. of moles of H+ from CH3COOH in 1 cm3 of reaction mixture after heating
= 3.90 x 10-3 - 2.52 x 10-4 mol
= 3.65 x 10-3 mol
No. of moles of CH3COOH reacted in 1 cm3 of mixture
= 8.75 x 10-3 mol – 3.65 x 10-3
= 5.10 x 10-3 mol
No. of moles of CH3CH2CH2OH in 1 cm3 of reaction mixture after heating
= No. of moles of H+ from 1 cm3 of CH3COOH
= 3.65 x 10-3 mol
No. of moles of CH3COOCH2CH2CH3 in 1 cm3 of reaction mixture after heating
= No. of moles of H+ reacted in 1 cm3 of CH3COOH
= 5.10 x 10-3 mol
No. of moles of H2O in 1 cm3 of reaction mixture after heating
= No. of moles of CH3COOCH2CH2CH3
= 5.10 x 10-3 mol
Therefore, Kc =
(5.10 x 10-3 x 5.10 x 10-3)
(3.65 x 10-3 x 3.65 x 10-3)
= 1.95
When comparing with the theoretical value (Kc=3.00),
Percentage error = [(1.95 - 3.00) / 3.00] x 100%
= - 35%
- Discussion:
Sources of error
-
The concentration of the 8 drops of concentrated H2SO4 added to the reaction mixture before heating may have varied greatly comaparing to those titrated against NaOH afterwards. It is because sulphuric acid is a strong hygroscopic agent and hence its concentration will decrease continuously with time when in contact with the atmosphere.
-
The amount of the 8 drops of H2SO4 added to the reaction mixture before heating may have variation comaparing to those titrated against NaOH afterwards. It is because the each drop of solution dropping out of the dropper are different. Since the sulphuric acid used, is a concentrated strong acid, thus its amount may cause a large error during calculation of their respective concentration before and after heating.
- Questions:
-
The purpose of adding a small amount of conc. H2SO4 to the reaction mixture is to provide a catalyst to increase the rate of the slow process of esterification.
- The boiling chips act as anti-bumping granules to prevent the reaction mixture from vigorous boiling and hence spilling out of solutions.
- If the NaOH needed for the end-point of titration is constant, it indicates that the concentration of the reacting substances do not have further changes, meaning that the esterification process has completed and reached an equilibrium.
-
(i) The number of moles of NaOH required for complete neutralization with H2SO4 and also the CH3COOH in the reaction mixture will not be accurate, causing errors when estimating the number of moles of the 8 drops of the added H2SO4 and also that of the CH3COOH reacted. Hence, the equilibrium constant obtained will be inaccurate.
(ii) Since the actual volume of the reaction mixture was lower than the expected one, the concentration of the former would be higher than the latter. Since the volume after mixing the reactants is used to determine the concentration of the 4 reactants, the equilibrium constant obtained will be inaccurate.
- Conclusion:
The equilibrium constant (Kc) for the reaction
CH3COOH(l)+CH3CH2CH2OH(l) CH3COOCH2CH2CH3(l)+H2O(l)
is 1.95, with an percentage error of -35%