Quick answer: Carboxylic acids react with alcohols, in the presence of concentrated sulfuric acid, to form esters and water in a reversible equilibrium. This Chemistry 11-12 lab on esterification gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How far does the reaction between ethanoic acid and ethanol go? You will heat the two liquids with a catalyst until equilibrium is reached, then use a titration to find how much acid remains. From this you can calculate the equilibrium constant and test it by predicting the result of a second experiment.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.9
Aim
To measure the equilibrium constant Kc for the formation of ethyl ethanoate from ethanoic acid and ethanol by titrating the unreacted acid, and to predict the effect of changing the ethanol amount.
Equipment
- glacial ethanoic acid (5.7 cm³ = 0.100 mol) and ethanol (5.8 cm³ = 0.0995 mol, take as 0.100 mol)
- concentrated sulfuric acid, 5 drops
- 100 cm³ round-bottomed flask, condenser, water bath, anti-bumping granules
- 250 cm³ volumetric flask, 25.0 cm³ pipette, burette
- sodium hydroxide, 0.100 mol/dm³; phenolphthalein indicator
Safety
Concentrated sulfuric acid and glacial ethanoic acid are corrosive: wear goggles and gloves and add the acid with a dropper. Ethanol is flammable, so use an electric water bath, never a naked flame. Keep the condenser water flowing before heating.
Method
- Measure the acid and the ethanol into the flask, add 5 drops of concentrated sulfuric acid and the granules.
- Heat under reflux in the water bath for 45 minutes.
- Cool the flask, then transfer the contents to the 250 cm³ flask with distilled water washings and make up to the mark.
- Pipette 25.0 cm³ into a conical flask, add indicator and titrate with 0.100 mol/dm³ NaOH. Repeat to get concordant titres.
- Run a blank: titrate the same 5 drops of acid catalyst in 25.0 cm³ of water (diluted in the same ratio).
Results
Sample measurements (blank titre = 0.7 cm³):
| Titration | 1 | 2 | 3 |
|---|---|---|---|
| Titre of NaOH / cm³ | 34.1 | 33.9 | 34.0 |
Analysis questions
Question 1 Corrected titre
Calculate the mean titre and subtract the blank.Question 2 Acid left
Calculate the amount of ethanoic acid in the 25.0 cm³ portion and in the whole mixture.Question 3 Ester formed
Starting from 0.100 mol of each reactant, calculate the amounts of ester and water at equilibrium.Question 4 Calculate Kc
Calculate Kc from your results.Question 5 Quenching and blank
Explain why the mixture was diluted and cooled before titrating, and why the blank was run.Question 6 Prediction with more ethanol
Predict the titre in a repeat with 0.100 mol of acid and 0.200 mol of ethanol (Kc = 4.0). Use x for the amount of ester.Question 7 Errors and improvements
State two sources of error and one improvement.
