HSC 2012-2015 Chemistry Marathon (archive) (13 Viewers)

[SuchSmallHands]

re: HSC Chemistry Marathon Archive

Does your answer have to include carbonic acid, polluted water, le chateliers princible and a chemical equation ? not sure who Bronsted lowry is though

Clearly the 15ers aren't far at all into acidic, for them to have any chance answering these questions well the questions should really stay in POM.

Assess the environmental impact of the lead acid cell or dry cell (2 marks)

 

[Ekman]

re: HSC Chemistry Marathon Archive

Need to split it up and add an equilibrium, H20(l)+H20(l)<---> H30+(aq) +OH-(aq) .

I would personally say that the statement is valid, with reference to the equilibrium you stated, the H3O+ + OH- are of equal but very small concentration in the water solution. Plus having a pH of 7 doesn't mean that the substance isn't acidic or basic, it means that the substance is equally as acidic as it is basic. According to Bronsted-Lowry's theory of acids and bases, one of the H2O molecules acts like an acid and the other acts like a base, which creates a very strong acid and a very strong base. But overall, I would say that since they are of low and equal concentrations, they become negligible.

Personally I am not sure if I should state that the H3O+ and the OH- cancel each other out, since they are of equal proportion.

 

[Librah]

re: HSC Chemistry Marathon Archive

Even then, concentration is measured in moles per litre, so I still don't understand the problem

concentration is the amount of ions per litre of solution.

Might've been the way you defined "concentration" in general.

 

[InteGrand]

re: HSC Chemistry Marathon Archive

I don't think the statement can be valid, because you can have neutral solutions where the solvent isn't water, can't you? These solutions wouldn't be pure water.
Also, pH is dependant on temperature. Pure water only has pH 7 at room temperature I think.

 

[enigma_1]

re: HSC Chemistry Marathon Archive

Cellulose C6H10O5is derived from bio mass which is renewable as it is comes from every living and non living organism.
At the moment the material for production of petrochemicals is the cracking of hydrocarbons. Hydrocarbons come from crude oil which is non-renewable showing that one day the source of hydrocarbons is going to eventually run out. Scientists attempt to use other materials to produce substances like petrol, Polymers ect... which is not only environmentally friendly but also renewable. The only alteranative to this is the use of cellulose (which is a major component of biomass (more than 50% of it)).

ADVANTAGES
Cellulose is converted to glucose by the use of proteins enzymatic or acid digestionand then glucose is seperated into ethanol and carbon dioxide. [insert the equation for this and specifically state that the process is fermentation and quickly state the reaction conditions: anaerobic environment, 37 degrees celcius blood temperature, presense of yeast enzymes, phosphates and nitrates, suitable grain or fruit mashed into a pulp - produces 15% concentration of ethanol. After this the ethanol can undergo distillation to attain 95% concentration.] It can also be used to produce polymers which are biodegradable.
Cellulose is a renewable resource
- Cellulose contains the 5 basic carbon atoms joined together which can then be used as the basic starting structure for petrochemicals
- Cellulose is very abundant and inexpensive

DISADVANTAGES
The only barrier to the permanent use is the cost of this. (Using cellulose as a raw material to produce petrochemicals is currently not economically viable) The cellulose around us is not enough to industrially supply to a whole country. Many crops need to be planted in order to grow extra cellulose thus also reducing other produce.
- Conversion of cellulose to glucose via enzymatic digestion is costly and slow which reduces its effectiveness
- The process to drive the machinery and to heat the reaction mixture requires and energy input which is sourced from the combustion of fossil fuels anyway

ASSESSMENT: In the near future At present it is unlikely for cellulose to be used as a raw material for the production of materials. [WHY? Because at present it is not economically viable as a raw material despite it being a better alternative to fossil fuels. HOWEVER, Cellulose has potential to be used as a raw material for the production of petrochemicals and can only be fulfilled if an efficient technique is developed in the future to convert cellulose to glucose, through further research.

Ok decent, but missing quite a few things. I've just edited some stuff in your answer above for some stuff you could add in to make your answer more complete, but not all of it is necessary. Probably around 3/5 at the moment.

 

[Librah]

re: HSC Chemistry Marathon Archive

I would personally say that the statement is valid, with reference to the equilibrium you stated, the H3O+ + OH- are of equal but very small concentration in the water solution. Plus having a pH of 7 doesn't mean that the substance isn't acidic or basic, it means that the substance is equally as acidic as it is basic. According to Bronsted-Lowry's theory of acids and bases, one of the H2O molecules acts like an acid and the other acts like a base, which creates a very strong acid and a very strong base. But overall, I would say that since they are of low and equal concentrations, they become negligible.

Personally I am not sure if I should state that the H3O+ and the OH- cancel each other out, since they are of equal proportion.

The question is more to do with the nature of the equilibrium itself.

Maybe i should have asked something like this question first,

Is H20(l)+H20(l)<--> H30+(aq) +OH-(aq) an endothermic or exothermic reaction (in the forward direction or just state which side the heat term belongs)? If so, what happens when heat is introduced or removed from the solution?

Btw, for "assess the validity" questions like this, you need to justify/argue the extent to which the statement is true, not just straight out state whether it's valid or not, it may be partially true.

Clearly the 15ers aren't far at all into acidic, for them to have any chance answering these questions well the questions should really stay in POM.

Yer i mentioned to skip if you haven't learned yet in my OP, better just stick to very early AE or just POM for now.

 

[Librah]

re: HSC Chemistry Marathon Archive

Ok decent, but missing quite a few things. I've just edited some stuff in your answer above for some stuff you could add in to make your answer more complete, but not all of it is necessary. Probably around 3/5 at the moment.

Could mention purification by Zeolite units aswell. (replacement for the common distillation method)

I don't think the statement can be valid, because you can have neutral solutions where the solvent isn't water, can't you? These solutions wouldn't be pure water.
Also, pH is dependant on temperature. Pure water only has pH 7 at room temperature I think.

Only referring to water chemistry here, edited question.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

what is the concentration of an acid with pH of 2.6? 1 mark (Use Latex to show working)

 

[Ekman]

re: HSC Chemistry Marathon Archive

what is the concentration of a MONOPROTIC acid with pH of 2.6? 1 mark (Use Latex to show working)

10^-2.6 (keeping it in 2 sig figs)

 

[SuchSmallHands]

re: HSC Chemistry Marathon Archive

what is the hydronium ion concentration of an acid with pH of 2.6? 1 mark (Use Latex to show working)

This question wasn't quite complete. This is why I think we should stick to POM.

 

[Ekman]

re: HSC Chemistry Marathon Archive

This question wasn't quite complete. This is why I think we should stick to POM.

Exactly, it was too ambiguous. I assumed that the acid 100% ionized...

 

[Librah]

re: HSC Chemistry Marathon Archive

what is the concentration of an acid with pH of 2.6? 1 mark (Use Latex to show working)

Question is too ambiguous, do you mean the concentration of H30+(which is an acid) or the concentration of the original acid, if so, then you need to specify protism/acid strength.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Woops sorry ahahah i really want to do questions from acidic more than half way done

 

[hawkrider]

re: HSC Chemistry Marathon Archive

Woops sorry ahahah i really want to do questions from acidic more than half way done

Hey! I understand that you're eager, but like SSH before, please be considerate of the others. Not everyone is up to the stage of smashing the Acidic Environment yet. POW, is still the module we BoS'ers should focus on for now.

 

[Kaido]

re: HSC Chemistry Marathon Archive

throw the questions soccerdude, i'll answer it lol

 

[Ekman]

re: HSC Chemistry Marathon Archive

throw the questions soccerdude, i'll answer it lol

Alright:
What were the problems associated with Arrhenius's theory of Acids and Bases? 3 Marks

 

[SuchSmallHands]

Chemistry marathon 2015

Almost all of the 15ers on this thread have proven that they're not able to properly answer acidic environment questions yet. It seems redundant to be practicing questions when people are straight out making things up that aren't at all correct because they don't know the content. At HSC level an acid is defined by the Bronsted Lowry description. If people here 'don't know who Bronsted Lowry is' they don't know what an acid is in the context of the syllabus. It seems a little odd to want acidic environment questions in a thread where people don't even know what acids are. However, it's a 2015 thread so if that's what you guys prefer then go for it.

 

[siggy]

re: HSC Chemistry Marathon Archive

Assess the potential of cellulose in being used as a raw material for the production of petrochemicals. (5 marks)
Petrochemicals are currently mainly being produced from non-renewable resources / crude oil, which is estimated to run out within the next few decades. In addition to this not being sustainable the use of crude oil also means an increasing cost as supply runs lower, an unstable supply from producers and unclean source of petrochemicals. One alternative to this is cellulose is a naturally occurring condensation polymer made of Beta-Glucose monomers. The procedure to produce ethanol from cellulose is to first break it into glucose monomers by using enzymatic or strong acid digestion and then seperating the glucose into CO2 and ethanol. (C6H12O6(aq) > C2H6O(aq) + 2CO2(g))
It is seen as a possible replacement because it is abundant, it contains all the carbon-chain structures needed for the production of petrochemicals and is produced by plants, via photosynthesis in biomass (50% of biomass is cellulose) - hence its' abundance and also its' renewable nature, unlike crude oil. However, there are also multiple issues concerning the use of cellulose, this includes the fact that it is far more expensive than crude oil and also that the process of extracting and cracking cellulose into petrochemicals uses more oil than is received than petrochemicals received. Hence, currently cellulose is not a viable alternative due to its' cost, however, due to the unsuitability of crude oil it may be looked into further in the future and hence the issues may be overcome, however, it is very unlikely to run out unlike crude oil.

Assess the environmental impact of the lead acid cell or dry cell (2 marks)
The dry cell has very little environmental impact as the battery components are weakly acidic and non-toxic. However, the zinc shell is corroded over time by acidic NH4+, and if a high concentration of zinc ions escapes this can be deadly to vegetation. Rain quickly dilutes it though. The manganese (III) product (from the cathode) can be oxidised back to manganese(IV) which is insoluble and stable. In addition, the carbon and ammonium salts are harmless. However, since it can’t be recharged it is always discarded. It is non-spillable as well, however, is generally not recycled, however, it can be if handed in to the correct place. Hence, the dry cell is mostly environmentally friendly, however, reckless discarding of the cell can result in possible environmental impacts. (May have gone overboard for 2 marks...)

 

[InteGrand]

re: HSC Chemistry Marathon Archive

This is a general tip, try and include at least one chemical equation in each answer, because marking criteria can say you need chemical equation(s) in your answer even if it's not explicitly stated in the question. (This is because there's a syllabus dot point in the start of Production of Materials I think that says students learn to apply chemical equations wherever appropriate, or something like that.)

So spam equations. And if they say explicitly to include one, but you can't remember the states of matter, just guess the states, since it can be mathematically proven that this is on average the best thing to do (assuming you don't lose marks for incorrect states if the marking criteria says you need a chemical equation but no states required).

 

[Fiction]

re: HSC Chemistry Marathon Archive

Alright:
What were the problems associated with Arrhenius's theory of Acids and Bases? 3 Marks

Alright:
What were the problems associated with Arrhenius's theory of Acids and Bases? 3 Marks

In 1884 Arrhenius proposed the notion that acids were substances which produced hydrogen ions in solution while bases were substances which formed hydroxide ions in solution. This theory however does not recognise the role the solvent plays as it is only applicable to solvents such as water, which can react with acids to form hydronium ions.

Furthermore, acid-base reactions are able to occur outside solutions, which Arrhenius's theory does not recognise. This can be see in reactions such as HCl(g) + NH3(g) -> NH4CL (g)

The Bronsted-Lowry definition is able to overcome the shortocmings of the Arrhenius definition as it stipulates that all acids are proton donors and bases are proton acceptors, e.g HCl (aq) + H20 (l) --> H30+ + Cl- The water acts as a base hence identifies the role of the solvent. The bronsted-lowry definition is furthermore not restricted to acids and bases only acting in a solution.