Practical - Finding The Equilibrium Constant Flashcards
What reaction are we finding Kc of?
Esterification between an alcohol and a carboxylic acid
Eg ethanol + ethanoic acid ——> Ethyl ethanoate
How do we prepare the equilibrium mixture?
Use burette, to add 10cm3 of ethanoic acid and 10cm3 of ethanol to a boiling tube
Add 2cm3 of dilute sulfuric acid catalyst
Once we completed esterification, what do we do?
Swirl the mixture in the boiling tube, place a bung then leave for a week
Making a solution with the equilibrium mixture
Transfer the contents in the boiling tube to a volumetric flask, wash out with distilled water
Transfer the washings and make up to the mark with distilled water
Stopper flask and invert multiple times to produce a uniform solution
How do we transfer 25 cm3 of the equilibrium mixture solution into a flask for titration?
By using a volumetric pipette
What conical flask do we need for the titration?
A large one (250cm3) because a lot of alkali may be added
What indicator is used for the titration?
Phenolphthalein
Colour change in this titration
Colourless in acid in conical flask ——> pale pink when neutralised
What do we Titrate?
Equilibrium mixture solution with sodium hydroxide
What does the sodium hydroxide from the burette react with?
Any remaining carboxylic acid and any remaining sulfuric acid catalyst
To find initial moles of alcohol, acid and carboxylic acid…
Based on volumes of each liquid added, use mass = density x volume to find mass of each added
Then divide mass by Mr to find moles
How can we find the initial moles of acid catalyst, without using densities?
By titration
The number of moles of sodium hydroxide added is equal to…
The number of moles of H+ in total, so both the carboxylic acid and the acid catalyst added together
Why does the pink colour fade after the titration in the mixture?
Because the addition of sodium hydroxide makes equilibrium shift to favour the backwards reaction, to produce more H+
How to find equilibrium moles?
By subtracting the moles at equilibrium calculated by titration (and scaled up to correct volume) from moles initially in the reaction mixture
How do we know that the reaction mixture had reached equilibrium?
Repeat but leave for longer
If Kc is the same then equilibrium was reached
