HSC 2012-2015 Chemistry Marathon (archive) (9 Viewers)

[Ekman]

re: HSC Chemistry Marathon Archive

I have a feeling a similar question will be in the exam this year.. did it to refresh my memory haha

Ethanol is a versatile solvent that has many adv and disd.
Advantages-
- Ethanol as a renewable fuel: ethanol is produced from renewable sources (fermentation of biomass), whereas other fuels are produced from petroleum (C61206 -> 2C02 + 2H20 (on top of arrow 37C, yeast, airlock))
- Net Co2 stays constant: the amount of Co2 produced in heat of combustion (C2H5OH + 302 -> 2CO2 + 3H20 ) the same amount is required to grow crops for ethanol ( photosynthesis for ethanol crop growth: 6CO2 + 6H20 -> C61206 + 602)
-10-15% of ethanol safely added to petrol w/o any modifications and causing damage to engines. (wouldn't need to be distilled) = reduces costs

Disadvantages-
- existing car engines need to be modified if more than 10-15% ethanol is used, distillation is energy intensive = economic pressures + high costs involved
- ethanol has a lower heat of combustion than petrol: combustion of ethanol produces less energy than same amount of octane, car can travel further with same amount of octane thus ethanols expensive
- large areas of land is needed to grow crops for ethanol (later used for fermenting) this is a problem as land is needed to grow food crops and land clearance contributes to salinity, deforestation, soil erosion, impacts on living organisms/environment.

Overall, ethanol has great potential as an alternative car fuel as it is renewable and biodegradable than can be safely added to petrol (10-15%) w/o any modifications.

You could also mention for the advantages that the smaller carbon chain and the presence of the oxygen atom allows it to undergo complete combustion easier, hence it has greater fuel efficiency and does not produce harmful products as easily as octane.

Another one for the disadvantages is that ethanol can wear down engines faster due to the higher compression ratios when using ethanol in comparison to octane.

 

[Ekman]

re: HSC Chemistry Marathon Archive

Example of Saponification:

Glycerol + Stearic Acid ---> Glyceryl Tristearate + Water

Glyceryl Tristearate + Sodium Hydroxide ---> Sodium Stearate + Glycerol

So in a way, is glycerol being regenerated?

In my opinion, the second reaction is the saponification reaction since it is a hydrolysis reaction between a fatty ester and a base, producing glycerol and soap molecules

The first reaction is something that I overlook as it refers to the esterification reaction of a fatty acid with glycerol to produce a fatty ester and water. However we obtain fatty esters straight from animal fats hence the reason why I dont really consider the first reaction.

 

[Crisium]

re: HSC Chemistry Marathon Archive

In my opinion, the second reaction is the saponification reaction since it is a hydrolysis reaction between a fatty ester and a base, producing glycerol and soap molecules

The first reaction is something that I overlook as it refers to the esterification reaction of a fatty acid with glycerol to produce a fatty ester and water. However we obtain fatty esters straight from animal fats hence the reason why I dont really consider the first reaction.

Fair enough

Thanks and good luck with 4u

 

[Crisium]

re: HSC Chemistry Marathon Archive

You could also mention for the advantages that the smaller carbon chain and the presence of the oxygen atom allows it to undergo complete combustion easier, hence it has greater fuel efficiency and does not produce harmful products as easily as octane.

Another one for the disadvantages is that ethanol can wear down engines faster due to the higher compression ratios when using ethanol in comparison to octane.

I was typing up the same thing but my internet crashed

@anousha

The thing about ethanol being carbon neutral isn't entirely true due to the carbon dioxide released from the machinery involved in the process such as harvesting the crop, its transportation, etc.

The "on top of arrow part" you should only have the catalyst which is yeast and put in brackets the zymase enzyme

The reaction conditions such as 37 degrees, anaerobic environment, etc. go below the arrow/yield sign

Other than that it was a well structured answer

 

[hawkrider]

re: HSC Chemistry Marathon Archive

I have a feeling a similar question will be in the exam this year.. did it to refresh my memory haha

Ethanol is a versatile solvent that has many adv and disd.
Advantages-
- Ethanol as a renewable fuel: ethanol is produced from renewable sources (fermentation of biomass), whereas other fuels are produced from petroleum (C61206 -> 2C02 + 2H20 (on top of arrow 37C, yeast, airlock))
- Net Co2 stays constant: the amount of Co2 produced in heat of combustion (C2H5OH + 302 -> 2CO2 + 3H20 ) the same amount is required to grow crops for ethanol ( photosynthesis for ethanol crop growth: 6CO2 + 6H20 -> C61206 + 602)
-10-15% of ethanol safely added to petrol w/o any modifications and causing damage to engines. (wouldn't need to be distilled) = reduces costs

Disadvantages-
- existing car engines need to be modified if more than 10-15% ethanol is used, distillation is energy intensive = economic pressures + high costs involved
- ethanol has a lower heat of combustion than petrol: combustion of ethanol produces less energy than same amount of octane, car can travel further with same amount of octane thus ethanols expensive
- large areas of land is needed to grow crops for ethanol (later used for fermenting) this is a problem as land is needed to grow food crops and land clearance contributes to salinity, deforestation, soil erosion, impacts on living organisms/environment.

Overall, ethanol has great potential as an alternative car fuel as it is renewable and biodegradable than can be safely added to petrol (10-15%) w/o any modifications. However, more cost effective means of producing ethanol must be developed if it is used as an alternative car fuel.

Just added one bit to your answer to make it complete. Your judgement ideally has to have a "whilst A has shown this, it needs to address x,y,z to satisfy B" line of argument, as it allows the marker to deem your judgement as more critical rather than one-sided.

Otherwise, had the not bolded not been included, I would've given this 6/7 (it's still a solid response nonetheless).

 

[Crisium]

re: HSC Chemistry Marathon Archive

Just added one bit to your answer to make it complete, as your judgement ideally has to have a "whilst A has shown this, it needs to address x,y,z, to satisfy" line of argument, makes the marker think your assessment more critical as opposed to one-sided.

Otherwise, had the not bolded not been included, I would've given this 6/7 (it's still a solid response nonetheless).

The elusive hawkrider reveals himself

 

[Anousha]

re: HSC Chemistry Marathon Archive

Thanks for the responses guys, now I have a question

I got the answer right.. but I can't remember how, can someone please work this out showing each step clearly thanks.

A student measured the heat of combustion of 1-butanol, by heating 200g of water initially at 21.0 degrees. 0.750g of 1-butanol was burnt in the reaction. The heat of combustion of 1-butanol is 2077j/mol. Assuming that 50% of heat produced was lost to the environment, what would be the final temp of water ?

A) 33.6
B) 46.1
C) 57.2
D) 71.3
(degrees)

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Thanks for the responses guys, now I have a question

I got the answer right.. but I can't remember how, can someone please work this out showing each step clearly thanks.

A student measured the heat of combustion of 1-butanol, by heating 200g of water initially at 21.0 degrees. 0.750g of 1-butanol was burnt in the reaction. The heat of combustion of 1-butanol is 2077j/mol. Assuming that 50% of heat produced was lost to the environment, what would be the final temp of water ?

A) 33.6
B) 46.1
C) 57.2
D) 71.3
(degrees)

1) n=m/M
2) n(butanol) = 0.750/74.12

3) Molar heat of combustion (given in question) = Theoretical Delta-H / Number of moles (butanol)
4) Re-arrange: Theoretical Delta-H = Molar heat of combustion (given in question) * Number of moles (butanol)

5) However, experimental value of Delta-H is 50% of original (50% of heat is lost to environment)
6) Experimental Delta-H = (50/100) * Theoretical Delta-H

7)Experimental Delta-H (value you have just found in step (6)) = m (mass of water --> in question) * C (is on data sheet) * Delta-T

8) Delta-T = Experimental Delta-H (value you found in step (6)) / [m (mass of water --> in question) * C (is on data sheet))]

9) Delta-T + 21 = Final Temperature

be careful when doing these steps to ensure all your values are in the right units.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Discuss how buffers are used in natural systems with a named example. 4 MARKS

bump

 

[Crisium]

re: HSC Chemistry Marathon Archive

bump

Brief response

- Buffers are a solution of approximately equal amounts of a weak acid and its conjugate base that resist a change in pH when an acidic or basic substances are added.

- They occur naturally in lakes such as the hydrogen carbonate buffer which resists detrimental changes in pH caused by acid rain that affect aquatic organisms and the broader food chain.

- Equation of hydrogen carbonate acting as a buffer

- Explanation of what happens when and acid or base is added using Le Chatelier's Principle

 

[Anousha]

re: HSC Chemistry Marathon Archive

1) n=m/M
2) n(butanol) = 0.750/74.12

3) Molar heat of combustion (given in question) = Theoretical Delta-H / Number of moles (butanol)
4) Re-arrange: Theoretical Delta-H = Molar heat of combustion (given in question) * Number of moles (butanol)

5) However, experimental value of Delta-H is 50% of original (50% of heat is lost to environment)
6) Experimental Delta-H = (50/100) * Theoretical Delta-H

7)Experimental Delta-H (value you have just found in step (6)) = m (mass of water --> in question) * C (is on data sheet) * Delta-T

8) Delta-T = Experimental Delta-H (value you found in step (6)) / [m (mass of water --> in question) * C (is on data sheet))]

9) Delta-T + 21 = Final Temperature

be careful when doing these steps to ensure all your values are in the right units.

THANK YOU, you're a legend

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

I just realised Crisium's photo is basically Morgan from "The Walking Dead."

 

[Crisium]

re: HSC Chemistry Marathon Archive

I just realised Crisium's photo is basically Morgan from "The Walking Dead."

You watch the Walking Dead ...

Could have told me earlier m9

Have you watched any episodes from the other series - Fear the walking dead?

Its from the matrix btw

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

any quick way to do this??
dont really want to write equations for all of them and do n=m/M then see mole ratio to CO2 ---> and then n=V/Vm for all of them.

 

[SilentWaters]

re: HSC Chemistry Marathon Archive

Throwback Thursday - this was the paper I sat.

Think about the net ionic equations for each type of reaction:



In both cases, we have a 1:1 stoichiometry between the carbonate salt, and the carbon dioxide.

This means we can skip straight to the formula n = m/M: our goal is to maximise n_salt so we can maximise n_CO2. This means maximising M. Na happens to be higher than K on the period table (lower M) and clearly M_H < M_Na, so we would need NaHCO3 to yield a maximum volume of CO2.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Throwback Thursday - this was the paper I sat.

Think about the net ionic equations for each type of reaction:



In both cases, we have a 1:1 stoichiometry between the carbonate salt, and the carbon dioxide.

This means we can skip straight to the formula n = m/M: our goal is to maximise n_salt so we can maximise n_CO2. This means maximising M. Na happens to be higher than K on the period table (lower M) and clearly M_H < M_Na, so we would need NaHCO3 to yield a maximum volume of CO2.

Yeh, my brother (Who also did this paper last year) tried explaining it to me today but he jumped straight to the last step without telling me how they were all in a 1:1 ratio. thx.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

i dont get why concentrated sulfuric acid (is 100% molecular) reacts differently to dilute sulfuric

 

[sy37]

re: HSC Chemistry Marathon Archive

any quick way to do this??
dont really want to write equations for all of them and do n=m/M then see mole ratio to CO2 ---> and then n=V/Vm for all of them.

You don't have to calculate anything for any of them apart from writing the equations. By moles = mass / molar mass, the substance with the LOWEST molar mass produces the greatest moles of CO2 (as molar mass decreases, moles increases since it is a 1g constant..you want to maximise the moles of co2 since everything is a 1:1). Thus it is clearly D.

tl;dr lower the molar mass --> greater the CO2 produced assuming 1:1 ratios since 1g substance remains constant

 

[sy37]

re: HSC Chemistry Marathon Archive

i dont get why concentrated sulfuric acid (is 100% molecular) reacts differently to dilute sulfuric

Dilute sulfuric acid is already ionised so it can already attack metals eg steel containers. Concentrated sulfuric acid is all molecular, hence it reacts vigorously with water so using steel containers is ideal since it prevents moisture from entering and thus prevents ionisation which is highly exothermic etc

 

[Khan.Paki]

re: HSC Chemistry Marathon Archive

Does a buffer necessarily have to be a weak acid and its conjugate base? Or can it be a weak base and its conjugate acid? Since that's what I learnt.