Titration evaluation questions

We have been told to carry out the following and then evaluate but I have some questions that I think might be in the evaluation. I would be ever so grateful.

We are reacting ethanol and ethanoic acid and determining the Kc by doing a titration with sodium hydroxide.

1. As we have added a catalyst how do we remove this from the working
2. Why would we use phenolphthalein and why does its go back colourless after the end point
3. Why is the preferred catalyst sulphuric acid
4. When transferring the equilibrium mixture from a tube to a flask we washing twice and add washings
5. Why would be make the solution up to 250cm3 using distilled water
6. Once the mixture is made up to 250cm3, why do we shake thoroughly
7. Why might two layers appear when shaking the mixture in with distilled water, plus why is it important to prevent this
8. Why would we rinse a pipette with the diluted equilibrium mixture before using the pipette to transfer 25cm3 of the mixture
9. Why would we rinse the burette with sodium hydroxide before filling it with sodium hydroxide and carrying out the titration
10. Why is it important to have a minimum of two concordant titres, and why should we ignore anomalies
11. What are the main errors from titrations?

(edited 10 years ago)

Reply 1

Wood123

Original post by KSmith15

Just out of interest are you on AQA, Edexcel, OCR A or OCR b salters?

Reply 2

KSmith15

Original post by Wood123

Just out of interest are you on AQA, Edexcel, OCR A or OCR b salters?

Aqa

Reply 3

Borek

These are good questions, tell us what you think the answers are and we will start from there.

Reply 4

KSmith15

Original post by Borek

These are good questions, tell us what you think the answers are and we will start from there.

I have tried to answer them :smile:

1. As we have added a catalyst how do we remove this from the workingDo we use the titration results asthe moles of acid at equilibrium and remove the moles of sulphuric acid
2. Why would we use phenolphthalein and why does its goback colourless after the end point. Pheonlphthalein changes colour between8.3 – 10. Does it go colorless because the OH- which make it pink, reactionwith H+ to make water which is colorless.
3. Why is the preferred catalyst sulphuric acid. Double H+?
4. When transferring the equilibrium mixture from a tube to a flask we washingtwice and add washings, to make sure all of the mixture is transferred because otherwiseit can affect reults.
5. Why would be make the solution up to 250cm3 using distilled water – no ions or salts,but not sure why its up to 250?
6. Once the mixture is made up to 250cm3, why do we shake thoroughly – to allow themixture to be distributed?
7. Why might two layers appear when shaking the mixture in with distilledwater, plus why is it important to prevent this – emulsion?
8. Why would we rinse a pipette with the diluted equilibrium mixture beforeusing the pipette to transfer 25cm3 of the mixture – is it to remove anychemical stains, but not sure why itsgot to be the equilibrium mixture rather than distilled water
9. Why would we rinse the burette with sodium hydroxide before filling it withsodium hydroxide and carrying out the titration – again to removestains, but not sure why it has to be the NaOH
10. Why is it important to have a minimum of two concordant titres, and whyshould we ignore anomalies – so results are more reliable and we canidentify anomalies. Ignore them as they are due to human error?
11. What are the main errors from titrations? – mixture left inequipment, the colour change is hard to identify, experimental error, humanerror – measuring at meniscus.

Reply 5

Borek

Original post by KSmith15

I have tried to answer them :smile:

Good.

(edited 10 years ago)

Reply 6

Pigster

Original post by KSmith15

3. Why is the preferred catalyst sulphuric acid

You shouldn't use conc. sulfuric!

Assuming the method used is to add it to ethanol and ethanoic acid, leave it to reach equilibrium, THEN dilute it so you can do multiple titrations (this assumes that even though the addition of excess water will shift the equilibrium to the LHS, it will happen so slowly that you can find the amount of acids, by titration, before the shift happens much).

Anyhoo, why not sulfuric? It is because it is a dehydrating agent, which means that any water released in the condensation reaction will be removed, moving the equilibrium to the RHS – affecting your calculated value of Kc.

Hydrochloric would be a better choice. Both acids work as catalysts since they are strong acids.