9.3.5 HSC Q Feedback

#1 Thing = ESTERIFICATION IS AN EQUILIBRIUM REACTION!!!!!!

#2 Thing = ESTERIFICATION IS A CONDENSATION REACTION!!!!!

#3 Thing = We made pentyl ethanoate – we used pentan-1-ol and ethanoic acid. The product smelled like banana. Please use this example rather than examples from other peoples notes, other books etc. NB if you choose an ester that ‘cannot’ be made in school it will be marked incorrect.

#4 Thing = Refluxing is heating with a vertical condenser. When justifying the need to reflux you MUST say i) why heating the esterification reactionis needed(to increase rate of reaction and also increase yield) and ii) why the vertical condenser is needed (to prevent escape of volatile ester).

2001

Q22) Justify means give reasons for the steps you took. So don’t just recount a method. Say what each thing was done – ie why heated, why reflux, why a catalyst.

You need a logical flow for esterification Q. 1) eqn and NB slow and low yield, 2) heat – explain why it increases yield (via LC) and rate (via EA), 3) NB problem with evaporation loss of ester (low bpt) 4) = describe refluxing = heat to high temp without loss of ester, 5) explain use of conc. Sulfuric aid (rate and yield!), 6) any other factors – anti-bumping, no naked flame, etc

NB the refluxing DOES NOT STOP evaporation. It just condenses the evaporated chemicals. Ie  It stops loss of ester due to evaporation.

 

2003

Q21b) Make sure your arrows actually touch the thing you are labelling or it will be considered an ‘incorrect guess’

Q21c) You have to say why the esterification needs to be heated, you have to say why the ester would be lost and how the reflux prevents this – but look at the marks(2) – ie simple to the point statements:

*Esterification needs to be heated to…rate (and yield as…)

* However esters are volatile because…and would escape…

*Refluxing (heating with…) allows the reaction to be heated without LOSS of volatile ester (ie the ester condenses back into the reaction flask). NB it is NOT so the ester doesn’t catch alight!

Q25) There are four trends on the graph i) alkanoic aids > alkanols, ii) alkanols > alkanes, iii) all bpt increase as MW increases, iv) bpt differences decrase as MW increases. You need to explain all four trends in terms of intermolecular bonds. Hints: alkanoic acids can form 2 hydrogen bonds per molecule, alkanols only 1, alkanes only form WDB. Increasing size of molecules increases the energy for them to vibrate and increases WDB. As alkanols and alkanoic acids become longer their polar functional groups become less significant.

2005

Q20) Balance your equations correctly. Identify conditions for each reaction – (eg what condition are required for fermentation, why is heating needed for esterification and why does this make refluxing necessary). Esterification is AN EQUILIBRIUM!!. Esterification is NOT an aqueous reaction. All reactants and products are liquids (no water added and sulfuric acid is a dehydrating agent). Actually say what distillation involves (at the very least say heating and separation by boiling point).

2007

Q23) If you are asked to draw products, draw the products only. If you draw the products and reactants 1) you are wasting valuable time, 2) You are telling the examiner that you don’t know the difference between reactants and products, and that’s just silly!

2009

Q16) I can’t believe that after all our work on esterification in class, experiment #17  and this far into the 9.3.5 HSC booklet that people are forgetting to say “reflux’ when describing the esterification procedure. Name the reactants (and the products) – be specific, show that you have actually done the experiment and actually know the names of the reactants and products in your experiment. Name the catalyst, identify the equipment by name (and / or a diagram). Say it is refluxed (ie heated with a vertical condenser). NO FLAMES – I don’t know what source a lot of you are getting this phrase “heat over a steady flame for 30min” from but stop – it’s rubbish. Safety – 18M sulfuric is being used – toxic, highly corrosive (ie goodbye skin and eyes) – so what are the most critical safety precautions?

2010

Q22c) – see 2007 Q23. They wanted see if students were still going to fall for the trap of drawing everything. Also – while there is propan-1-ol and butan-1-ol etc, there is ONLY methanol and ethanol (no 1’s) as there is only one isomer. Also prop is not easy to remember but surely you et that pent means five – so really that should be a clue that prop can’t be five….

2012

Q31c) Hydrogen bonds are responsible for giving alkanols a high boiling point compared to alkanes and esters. But the graph showed bpt vs MW. What makes propanol have a higher bpt than ethanol? It can’t be hydrogen bonding as they both have one hydrogen bond per molecule. It must be because as the MW (chain length) increases the…..increases.