!! HL Chemistry 2015 - discussion, guesses, predictions, explosions, etc.

Niamhjkl

Fiona G wrote: »

Anyone know the purpose of adding sulfuric acid to the conical flask in the sodium thiosulfate/iodine titration? Can't find an explanation and they love throwing up those sort of questions.

Is it just to supply H+ for the reaction?

Yes, the use of the dilute sulfuric acid is to provide enough H+ ions for the complete reduction of the MnO4 ( excuse the large 4) to Mn^2+.
Best of luck

lc2015x

Last minute question lads! Is it too risky to leave out all of organic chemistry? I didn't attempt any of those questions in the mocks & still got myself a C1 (aiming for a B1 now) I know the other chapters in great detail. Should I practice organic questions for the next few hours or just leave it altogether and just have a look over the main topics? Thanks!

Deranged96

lc2015x wrote: »

Last minute question lads! Is it too risky to leave out all of organic chemistry? I didn't attempt any of those questions in the mocks & still got myself a C1 (aiming for a B1 now) I know the other chapters in great detail. Should I practice organic questions for the next few hours or just leave it altogether and just have a look over the main topics? Thanks!

If I were you anyway I'd go over heats of reactions (if you're including that in organic chem) and C2H2 experiment, prep of benzoic acid and the clove oil experiment.

lc2015x

Deranged96 wrote: »

If I were you anyway I'd go over heats of reactions (if you're including that in organic chem) and C2H2 experiment, prep of benzoic acid and the clove oil experiment.

have benzoic studied anyway, and as far as clove oil, I've the jist of it, thanks!
I kind of meant the theory here, bit late too study all the definitions, structures and mechanisms associated with organic?

Deranged96

lc2015x wrote: »

have benzoic studied anyway, and as far as clove oil, I've the jist of it, thanks!
I kind of meant the theory here, bit late too study all the definitions, structures and mechanisms associated with organic?

Well I don't know what your abilities are, but I wouldn't be able to learn enough theory to be able to attempt an extra question in the three or so hours that are left.

If you were going to study anything though I'd go addition mechanism

seinstein1

can anyone here pleasee send me some notes on the new benzoic acid experiment. i am desperate...

Dawn96

seinstein1 wrote: »

can anyone here pleasee send me some notes on the new benzoic acid experiment. i am desperate...

If you put 'preparation of benzoic acid' into google, the second result (Gill McMahon) has AMAZING notes on the experiment

Cheerios

seinstein1 wrote: »

can anyone here pleasee send me some notes on the new benzoic acid experiment. i am desperate...

Here are the notes my teacher gave us.

Procedure:
1. A known mass of phenylmethanol (benzyl alcohol), a saturated solution of potassium permanganate, and a solution of sodium carbonate are heated in a concial flask for approximately 20 minutes in a boiling water bath.

Observation:
- The purple colour changes to brown.
- A brown percipitate is formed.

Explanation:
- The potassium permanganate solution is saturated to ensure that sufficient is present to oxidise all the phenylmethanol completely to benzoic acid.
- The sodium carbonate ensures that the solution is alkaline as this oxidation reaction works best at an alkaline pH, i.e. the rate of reaction is greatest under alkaline conditions.
- As the reaction proceeds, the colour changes from purple (the colour of the manganese ion in an oxidation state of +7) to brown (the colour of the manganese ion in an oxidation state of +4), i.e. there is a reduction of Mn(VII) to Mn(IV) and the phenylmethanol is oxidised to benzoic acid.
- The brown percipitate is manganese dioxide (MnO2), which is not soluble in water.


Procedure:
2. The conical flask and its contents are cooled by running cold water over the outside of the conical flask.
3. In the fume cupboard, concentrated hydrochloric acid is added to the conical flask.
4. A glass rod is dipped in the solution and touched against a piece of damp blue litmus paper to check that the contents of the conical flask are acidic.

Explanation:
- A compound called sodium benzoate is produced as an intermediate in this reaction. It is necessary to add concentrated hydrochloric acid in order to convert sodium benzoate to benzoic acid. Since benzoic acid is only slightly soluble in cold water, this helps to percipitate out the benzoic acid. In addition, the hydrochloric acid neutralises any excess sodium carbonate added and the potassium hydroxide produced in the reaction. The third reason for adding the HCl is to provide an acidic medium to enable the Mn4+ ions in manganese dioxide to be reduced to soluble Mn2+ ions.


Procedure:
5. Using a dropper, sodium thiosulfate solution (Na2SO3) is added to the conical flask until the brown percipitate reacts fully with it.

Observation:
- Brown percipitate disappears.
- White crystals are now visible in the flask.

Explanation:
- Sodium sulfite is a reducing agent and reduces Mn4+ ions (insoluble) to Mn2+ ions (soluble). It is necessary to reduce the insoluble solid specks of brown manganese dioxide (MnO2) before filtering off the benzoic acid crystals as these brown specks would contaminate the benzoic acid.
- The half reactions are:
Mn4+ (insoluble) + 2e- ====> Mn2+ (soluble)
SO32- + H2O ====> SO42- + 2H+ + 2e-
- The white crystals of benzoic acid become visible as the brown percipitate disappears.


Procedure:
6. The conical flask is placed in ice.
7. The white crystals formed are filtered off using a Buchner flask , Buchner funnel and filter paper.
8. The conical flask is washed out with the clear liquid from the Buchner flask, i.e. with the liquid obtained by filtration (the filtrate).
9. The crystals are washed with ice-cold water.
10. The damp crystals are allowed to air dry overnight and then placed in a desiccator.
11. The mass of crystals formed is measured.

Explanation:
- Since benzoic acid has low solubility in cold water, the conical flask is placed in ice to maximise the yield of crystals.
- The crystals are filtered off using suction filtration with the aid of a Buchner flask and Buchner funnel since this type of filtration speedsu p the filtration process. In addition, it helps to dry the crystals.
- The conical flask is rinsed out with the filtrate from the Buchner flask to remove any crystals of benzoic acid that remain in the conical flask.
- The crystals are washed with ice-cold water to remove any soluble impurities that may be present on the filter paper or adhering to the crystals.

Sorry if there are any spelling mistakes I was tryna type fast. :P

seinstein1

Dawn96 wrote: »

If you put 'preparation of benzoic acid' into google, the second result (Gill McMahon) has AMAZING notes on the experiment [/QUOTE

seinstein1

Thanks a lot cheerios

Quarks

Best of luck lads