HSC 2012-2015 Chemistry Marathon (archive) (7 Viewers)

[hawkrider]

re: HSC Chemistry Marathon Archive

By including relevant chemical equations in your answer, justify the procedure you used to prepare an ester in a school laboratory. 6 marks

shiet, dis Q looks tricky

but i'll try to answer it nonetheless - these are from my notes btw (so it might be a bit convoluted lol)

---

Argument

We conduct the procedure of preparing an ester because it allows us to identify if there any manifestation of such, which is usually detected (can be distilled first) by smell. Esterification is the reaction between an alkanol consisting OH functional group and an alkanoic acid containing the COOH functional group such that it produces an ester and water.

Procedure

1. Place 10mL of acetic acid, 20mL of 1-pentanol and 6-7 drops of concentrated sulfuric acid into a round bottom conical flask
2. Add 2 or 3 boiling chips
3. Set up a refluxing apparatus (with a heating mantle)
4. Begin to heat the flask with the mantle, the mixture should boil for about half an hour to reach an equilibrium
mixture of alcohol, acid, ester and water
5. Remove the heating source and allow the mixture to cool for approximately 10 minutes. Using a towel, hold
either side of the joint of the condenser and flask. Separate the condenser and flask by pulling apart and
twisting in one movement
6. Transfer the mixture to a separating funnel containing water of about the same volume as the mixture. Shake
the contents and allow the layers to separate. The lower layer contains water soluble acid and an alkanol acid
can be run off and discarded. Esters cannot hydrogen bond so have low water solubility
7. Add sodium carbonate to neutralise the H2SO4 in the separating funnel until no more reaction with the acid occurs. Add an equal amount of water to dissolve any salts. Shake, stand and let layers separate and then
discard the lower aqueous layer
8. Detect the ester (can be distilled first) by smell.

CH3COOH (l) + CH3OH (l) <--> CH3COOCH3 (l) + H2O (l) [note: the catalyst was H2SO4, but I obvs have no clue how to include this in latex or some crap lol)

Conclusion

As illustrated, this procedure is indeed able to verify the way in which an ester is produced to give off its fruity smell. The aroma of an ester is produced when an alcohol combines with a carboxylic group (carbon bonded with oxygen atoms).

 

[Crisium]

re: HSC Chemistry Marathon Archive

Method (The Pentyl Acetate/Ethanoate Ester):

1) Place 10 ml of 1-Pentanol, 12 ml of glacial acetic acid and 1 ml of concentrated sulphuric acid into the reaction flask.

2) Add a few boiling chips, which are not made of marble, and assemble the refluxing apparatus. Clamp the flask and condenser securely to a retort stand by using the bossheads.

3) Connect the tubing to the tap and the condenser, and turn the tap on so that a uniform flow of water is achieved.

4) Heat the mixture in a heating mantle for approximately 30 minutes and allow to cool for 5 minutes. Turn off the water, as the refluxing apparatus is no longer required.

5) Carefully remove the flask and pour the contents into a separating funnel containing 15 ml of water. Put the stopper on the funnel and shake it. Allow the layers to separate, and drain off and discard the lower aqueous layer.

6) Add approximately 15 ml of 0.1 mol L-1 sodium carbonate solution. Put the stopper on the funnel and shake it. Allow the layers to separate, and drain off and discard the lower aqueous layer. The remaining solution in the separating funnel is the ester.

7) Carefully smell the ester and describe the smell.

Safety:

Concentrated sulphuric acid and glacial acetic acid are corrosive, and so handle with care by avoiding contact with skin. If contact occurs, wash the affected area with plenty of water. 1-Pentanol and 1-Pentyl Ethanoate are flammable so avoid contact with naked flames <--- It is for this reason that a heating mantle is preferred over a bunsen burner.


Justification Stuff:

The boiling chips were added to the reaction flask so as to facilitate a slow and gentle heating. By placing distilled water in the separating funnel prior to adding the contents of the reaction flask, the purity of the ester was increased since all the reactants and products except for the ester are water soluble, hence they are immiscible with the ester. Furthermore, the ester is also less dense and floated on top, allowing the aqueous reactants to be drained off. Finally, the base was added to neutralise any acid remaining in the mixture. The high temperature allowed for a faster rate of reaction (Because of the particles gaining more kinetic energy and hence more collisions, etc. etc.), and thus equilibrium could be reached much faster. Moreover, esterification is an endothermic reaction, so by increasing the heat, equilibrium will encourage the forward reaction, hence increasing the yield of the ester. The addition of the sulphuric acid catalyst increased the rate of reaction, and thus equilibrium could be reached much faster. Moreover, as concentrated sulphuric acid is a powerful dehydrating agent, the concentration of the product water is reduced, and hence equilibrium will encourage the forward reaction, thus increasing the yield of the ester. Refluxing ensures that the condenser captures the volatile reactants and products (These are flammable and involve the alkanol, alkanoic acid and ester), returning them into the reaction mixture, and also prevents the build-up of pressure in a closed reaction vessel.

I can't LaTex like all you pros so yeah ...

I would have the normal equation and on the equilbrium yield arrow I would have Concentrated Sulphuric Acid on top, and if you're one of those people that likes to include conditions I would place "Refluxing" beneath the arrow.

I have way more notes of this but ceebs ...

I also would have made it a more integrated response

I would have included a diagram of the experimental set-up

^ If y'all think this is overboard welcome to what my school would expect from us for a 6 marker (They would probably ask for this much for a 3 - 4 marker :/ )

 

[hawkrider]

re: HSC Chemistry Marathon Archive

Method (The Pentyl Acetate/Ethanoate Ester):

1) Place 10 ml of 1-Pentanol, 12 ml of glacial acetic acid and 1 ml of concentrated sulphuric acid into the reaction flask.

2) Add a few boiling chips, which are not made of marble, and assemble the refluxing apparatus. Clamp the flask and condenser securely to a retort stand by using the bossheads.

3) Connect the tubing to the tap and the condenser, and turn the tap on so that a uniform flow of water is achieved.

4) Heat the mixture in a heating mantle for approximately 30 minutes and allow to cool for 5 minutes. Turn off the water, as the refluxing apparatus is no longer required.

5) Carefully remove the flask and pour the contents into a separating funnel containing 15 ml of water. Put the stopper on the funnel and shake it. Allow the layers to separate, and drain off and discard the lower aqueous layer.

6) Add approximately 15 ml of 0.1 mol L-1 sodium carbonate solution. Put the stopper on the funnel and shake it. Allow the layers to separate, and drain off and discard the lower aqueous layer. The remaining solution in the separating funnel is the ester.

7) Carefully smell the ester and describe the smell.

Safety:

Concentrated sulphuric acid and glacial acetic acid are corrosive, and so handle with care by avoiding contact with skin. If contact occurs, wash the affected area with plenty of water. 1-Pentanol and 1-Pentyl Ethanoate are flammable so avoid contact with naked flames <--- It is for this reason that a heating mantle is preferred over a bunsen burner.


Justification Stuff:

The boiling chips were added to the reaction flask so as to facilitate a slow and gentle heating. By placing distilled water in the separating funnel prior to adding the contents of the reaction flask, the purity of the ester was increased since all the reactants and products except for the ester are water soluble, hence they are immiscible with the ester. Furthermore, the ester is also less dense and floated on top, allowing the aqueous reactants to be drained off. Finally, the base was added to neutralise any acid remaining in the mixture. The high temperature allowed for a faster rate of reaction, and thus equilibrium could be reached much faster. Moreover, esterification is an endothermic reaction, so by increasing the heat, equilibrium will encourage the forward reaction, hence increasing the yield of the ester. The addition of the sulphuric acid catalyst increased the rate of reaction, and thus equilibrium could be reached much faster. Moreover, as concentrated sulphuric acid is a powerful dehydrating agent, the concentration of the product water is reduced, and hence equilibrium will encourage the forward reaction, thus increasing the yield of the ester. Refluxing ensures that the condenser captures the volatile reactants and products (These are flammable and involve the alkanol, alkanoic acid and ester), returning them into the reaction mixture, and also prevents the build-up of pressure in a closed reaction vessel.

I can't LaTex like all you pros so yeah ...

I would have the normal equation and on the equilbrium yield arrow I would have Concentrated Sulphuric Acid on top, and if you're one of those people that likes to include conditions I would place "Refluxing" beneath the arrow.

I have way more notes of this but ceebs ...

I also would have made it a more integrated response

I would have included a diagram of the experimental set-up

^ If y'all think this is overboard welcome to what my school would expect from us for a 6 marker (They would probably ask for this much for a 3 - 4 marker :/ )

oh fuck

when i look at my response, mine looks like a piece of shit

 

[Crisium]

re: HSC Chemistry Marathon Archive

oh fuck

when i look at my response, it looks like a piece of shit

Nah you're response was heaps good

You just needed to Justify (This seemed to be the key verb of the question) some of the steps in your method and you should be set for the 6 marks

 

[hawkrider]

re: HSC Chemistry Marathon Archive

Nah you're response was heaps good

You just needed to Justify (This seemed to be the key verb of the question) some of the steps in your method and you should be set for the 6 marks

yeah, the thing i didn't get was how the hell was i meant to address the justification bit on how esters are produced

am i supposed to talk about efficiency or some crap lol

 

[hawkrider]

re: HSC Chemistry Marathon Archive

alright, next one

Analytical data have shown that the pH of rain in developing countries is higher than the measurements obtained from industrialized areas such as Central Europe.

Suggest the causes of this difference and explain its impact on the marine environment. (4 marks)

 

[porcupinetree]

re: HSC Chemistry Marathon Archive

alright, next one

Analytical data have shown that the pH of rain in developing countries is higher than the measurements obtained from industrialized areas such as Central Europe.

Suggest the causes of this difference and explain its impact on the marine environment. (4 marks)

In industrialised regions, various non-metal oxides such as nitrous oxide and sulfur dioxide are released into the environment as pollution. Specifically, sulfur dioxide can ionise in rain to form sulfurous acid, a constituent of acid rain: SO2 (g) + H2O (l) --> HSO3 (aq). Alternatively, sulfur dioxide can combine with oxygen in the atmosphere (2SO2(g) + O2(g) --> 2SO3(g)) to form sulfur trioxide, which in turn can ionise in rain to form sulfuric acid: SO3 (g) + H2O (l) --> H2SO4 (aq). Hence pollution from industrial processes such as mining causes acid rain, lowering the rain's pH. Acid rain can have very detrimental effects on the environment, for example, marine organisms (which require a narrow pH range to live in) living in a body of water exposed to acid rain can be affected and killed.

 

[Crisium]

re: HSC Chemistry Marathon Archive

Outline how oxides of nitrogen and sulphur can be reduced/removed from the atmosphere. (3 Marks)

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

In industrialised regions, various non-metal oxides such as nitrous oxide and sulfur dioxide are released into the environment as pollution. Specifically, sulfur dioxide can ionise in rain to form sulfurous acid, a constituent of acid rain: SO2 (g) + H2O (l) --> HSO3 (aq). Alternatively, sulfur dioxide can combine with oxygen in the atmosphere (2SO2(g) + O2(g) --> 2SO3(g)) to form sulfur trioxide, which in turn can ionise in rain to form sulfuric acid: SO3 (g) + H2O (l) --> H2SO4 (aq). Hence pollution from industrial processes such as mining causes acid rain, lowering the rain's pH. Acid rain can have very detrimental effects on the environment, for example, marine organisms (which require a narrow pH range to live in) living in a body of water exposed to acid rain can be affected and killed.

You didn't mention the compound that contributes nearly all the proportion of acid rain.... I think that you could've mentioned something about developing countries not using as much fossil fuels as we do thus the pollution is less resulting in a higher pH. And tbh i don't think speaking about sulfur dioxide is good for this question since the main sources of sulfur dioxide are from the environment and one can argue that both the developing and developed countries can have the same maybe even more sulfur dioxide depending on the landscape. If the marker wants to be a douche he can mark you down for these

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Outline how oxides of nitrogen and sulphur can be reduced/removed from the atmosphere. (3 Marks)

Oxides of nitrogen and sulphur are dangerous in the atmosphere as they cause they production of acid rain and photo chemical smog.

The obvious reduction can be from using less fossil fuels and use naturally derived sources of energy such as cellulose. To reduce sulfur dioxide in the air use processes such as the frash process to extract sulfur without the release of . Put restrictions on the use of fossil fuels such as the carbon tax and use things that produce such as motors less AKA TURN SHIT OFF

 

[Crisium]

re: HSC Chemistry Marathon Archive

Oxides of nitrogen and sulphur are dangerous in the atmosphere as they cause they production of acid rain and photo chemical smog.

The obvious reduction can be from using less fossil fuels and use naturally derived sources of energy such as cellulose. To reduce sulfur dioxide in the air use processes such as the frash process to extract sulfur without the release of . Put restrictions on the use of fossil fuels such as the carbon tax and use things that produce such as motors less AKA TURN SHIT OFF

You could have mentioned scrubbers for reducing oxides of sulphur and catalytic converters in motor vehicles for reducing emissions of nitrogen dioxide

 

[Crisium]

re: HSC Chemistry Marathon Archive

Discuss the use of plasticisers in hard plastics such as PVC. (3 Marks) <--- If you've been watching the news lately then you should have no problem answering this

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

You could have mentioned scrubbers for reducing oxides of sulphur and catalytic converters in motor vehicles for reducing emissions of nitrogen dioxide

Thats industrial chemistry knowledge surely they didnt expect it in the marking criteria

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Discuss the use of plasticisers in hard plastics such as PVC. (3 Marks) <--- If you've been watching the news lately then you should have no problem answering this

A plasticiser is an addition to a plastic in order to make the plastic more flexible. In plastics such as PVC there is a large element such as chlorine such that the plastic becomes much more harder. However sometimes we may need to have PVC properties without a property such as hardness we add a plasticiser. Thus showing the uses of plasticisers in hard plastics
EDIT: we may need a soft PVC rather than a hard one

 

[Crisium]

re: HSC Chemistry Marathon Archive

Thats industrial chemistry knowledge surely they didnt expect it in the marking criteria

Nope

It's meant to be assumed from the Acidic Environment Module

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Identify a class of compounds that have been developed to replace CFC's and evaluate their effectiveness in reducing the level of ozone depletion. 4 Marks

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Identify a class of compounds that have been developed to replace CFC's and evaluate their effectiveness in reducing the level of ozone depletion. 4 Marks

Should this really be worth 4 marks ?

 

[porcupinetree]

re: HSC Chemistry Marathon Archive

Identify a class of compounds that have been developed to replace CFC's and evaluate their effectiveness in reducing the level of ozone depletion. 4 Marks

Ever since CFCs were phased out in the 80s and 90s by the Montreal Protocol, alternative chemicals have been developed to replace them: hydrofluorocarbons and hydrochlorofluorocarbons. The compounds in the latter group are generally about 10% as dangerous as CFCs in terms of ozone depletion potential, whereas HFCs virtually do not contribute at all to ozone depletion. Both the above classes of compounds are slightly more reactive than CFCs: not enough for them to be rendered ineffective for their uses, but enough so that when they are released into the atmosphere they will react before they reach the stratosphere where ozone depletion takes place. Studies have shown that since the implementation of these compounds, ozone levels have begun to decrease.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Amphiprotic substances have the ability to work as buffers in natural systems.
Explain why natural system requires buffers and using a specific example of an amphiprotic substance show how this can behave as a buffer. Including equations in your answer. 6 Marks (Lets get some long questions in this mofo)

 

[porcupinetree]

re: HSC Chemistry Marathon Archive

Amphiprotic substances have the ability to work as buffers in natural systems.
Explain why natural system requires buffers and using a specific example of an amphiprotic substance show how this can behave as a buffer. Including equations in your answer. 6 Marks (Lets get some long questions in this mofo)

Natural systems in which biological organisms thrive often require specific, narrow pH ranges in order for the organism(s) to survive; conditions outside of this range will cause harm (e.g. induce acidosis) and often death for organisms. Often in situations like these, a buffer is useful: a buffer is a substance which will lower the pH of the surroundings if the pH shifts upwards, and vice versa, and hence will generally maintain a relatively constant pH even when an acid or base is added. A buffer contains approximately equal amounts of a weak acid and its conjugate base at equilibrium, which could be represented as:
H2O + HA <--> A- + H3O
In human blood, the amphiprotic hydrogen carbonate ion acts as a buffer to maintain the blood's pH at around 7.4 in the following equilibrium:
H2CO3 (aq) + H2O (l) <--> H3O+ (aq) + HCO3- (aq).
By Le Chatelier's principle, if the blood's pH drops (i.e. H3O+ concentration increases), the equilibrium shifts to the left to form water and weaker acid, raising the pH. Conversely, if the blood's pH rises (i.e. H3O+ concentration decreases), the equilibrium shifts to the right to form the acidic ion hydronium and hydrogen carbonate, lowering the pH.