HSC 2016 Chemistry Marathon (3 Viewers)

[leehuan]

Re: HSC Chemistry Marathon 2016

Hehe yeah I haven't done enough of Acidic Environment to answer leehuan's question

[HR][/HR]

Is there a faster way of answering this question? We haven't covered these kinds of questions at school

Try to avoid using exponential decay related concepts altogether in chemistry. Although obviously calculus has been avoided, it's nonetheless a mathematics topic and exponentiation is quite rarely necessary in chemistry. (Only time I've seen it required is for pH where an exponential formula is used to find [H3O+])
(Note: pH is a logarithmic scale)

InteGrand's pattern was more than sufficient.

 

[Shuuya]

Re: HSC Chemistry Marathon 2016

Thanks, that's so much easier!

 

[Shuuya]

Re: HSC Chemistry Marathon 2016

leehuan needs to perform an experiment to deduce the molar heat of combustion of various alkanols. He was given excess samples of ethanol, propan-1-ol, butan-1-ol and pentan-1-ol in respective spirit burners.
a) leehuan needs to perform this experiment in the laboratory. Suggest a suitable method for him to use. (3)
b) When arriving at the final results, he finds that his values are always lower than the theoretical value. Explain why this is inevitable and justify possible solutions to minimise the difference in results. (4)
c) Write down the equation for complete combustion of pentan-1-ol. (1)

a) Method:
1. Measure the initial mass of the spirit burner
2. Measure 100mL of water into a beaker and suspend it directly over the spirit burner
3. Measure the initial temperature of the water
4. Light the spirit burner
5. Allow the temperature of the water to rise by about 10°C. Record the final temperature reached by the water.
6. Measure the final mass of the spirit burner
7. Repeat steps 1-6 for the remaining alcohols

b) The experimental value will be lower than the theoretical value due to the scope of error in the experimental process. Heat from the combustion of the alcohol is lost to the surroundings, through the inevitable heating of the air. Incomplete combustion may have also occurred, meaning that all of the energy released was not absorbed by the water and was instead used to form carbon on the base of the beaker. In order to minimise the difference in results, a copper calorimeter could be used to allow an increased absorption of heat, as copper is a better conductor of heat than a glass beaker. The distance between the flame and beaker can also be minimised in order to reduced the effect of heat loss to surroundings.

c) C5H11OH(l) + 8O2(g) --> 5CO2(g)+ 6H2O(g)

 

[chanandlerbong]

Re: HSC Chemistry Marathon 2016

[deleted]

 

[chanandlerbong]

Re: HSC Chemistry Marathon 2016

a) Method:
1. Measure the initial mass of the spirit burner
2. Measure 100mL of water into a beaker and suspend it directly over the spirit burner
3. Measure the initial temperature of the water
4. Light the spirit burner
5. Allow the temperature of the water to rise by about 10°C. Record the final temperature reached by the water.
6. Measure the final mass of the spirit burner
7. Repeat steps 1-6 for the remaining alcohols

b) The experimental value will be lower than the theoretical value due to the scope of error in the experimental process. Heat from the combustion of the alcohol is lost to the surroundings, through the inevitable heating of the air. Incomplete combustion may have also occurred, meaning that all of the energy released was not absorbed by the water and was instead used to form carbon on the base of the beaker. In order to minimise the difference in results, a copper calorimeter could be used to allow an increased absorption of heat, as copper is a better conductor of heat than a glass beaker. The distance between the flame and beaker can also be minimised in order to reduced the effect of heat loss to surroundings.

c) C5H11OH(l) + 8O2(g) --> 5CO2(g)+ 6H2O(g)

______________________________________
I might be mistaken, but should the state of water in the products be liquid and not gas? If not, can anyone explain why its in a gaseous state. There has been a debate going on at my school, that the products of ethanol combustion for water, it should be liquid state, others have said it should be gas?

 

[leehuan]

Re: HSC Chemistry Marathon 2016

a) Method:
1. Measure the initial mass of the spirit burner
2. Measure 100mL of water into a beaker and suspend it directly over the spirit burner
3. Measure the initial temperature of the water
4. Light the spirit burner
5. Allow the temperature of the water to rise by about 10°C. Record the final temperature reached by the water.
6. Measure the final mass of the spirit burner
7. Repeat steps 1-6 for the remaining alcohols

b) The experimental value will be lower than the theoretical value due to the scope of error in the experimental process. Heat from the combustion of the alcohol is lost to the surroundings, through the inevitable heating of the air. Incomplete combustion may have also occurred, meaning that all of the energy released was not absorbed by the water and was instead used to form carbon on the base of the beaker. In order to minimise the difference in results, a copper calorimeter could be used to allow an increased absorption of heat, as copper is a better conductor of heat than a glass beaker. The distance between the flame and beaker can also be minimised in order to reduced the effect of heat loss to surroundings.

c) C5H11OH(l) + 8O2(g) --> 5CO2(g)+ 6H2O(g)

a) I'm reluctant to say 3 despite my gut instinct to give it because whenever I wrote this method I always wrote to do the calculation specifically as a last step. But from my understanding of chemistry this is sufficient for that 3, because my understanding of the chemistry course is that such details are typically addressed in the discussion.

b) Awesome. 4/4

Suggestion - Try putting the copper calorimeter INTO the method rather than the beaker. Believe it or not heat lost to the surroundings is the main reason why it's inevitable, because in a system where there's essentially unlimited supply of O2, incomplete combustion will be rare. For the fourth mark, instead of suggesting the calorimeter you could alternatively suggest putting a lid on the calorimeter with a hole for the thermometer and enclose the environment with aluminium foil in an attempt to trap the heat.

c)
Moles of C: LHS - 5, RHS - 5
Moles of O: LHS - 1 + 16 = 17, RHS - 10 + 6 = 16 !!!
Moles of H: LHS - 12, RHS = 6

Oops!

Correct answer: 2 C5H11OH(l) +15 O2(g) = 10 CO2(g) + 12 H2O(g) [Source: WolframAlpha]

 

[leehuan]

Re: HSC Chemistry Marathon 2016

______________________________________
I might be mistaken, but should the state of water in the products be liquid and not gas? If not, can anyone explain why its in a gaseous state. There has been a debate going on at my school, that the products of ethanol combustion for water, it should be liquid state, others have said it should be gas?

Immediately after the reaction it's a vapour. You don't get water pouring out of a cigarette or burning ethanol or something.

After I set ethanol on fire at school by the time it was done I had pretty much nothing but a few baby drops of water

 

[leehuan]

Re: HSC Chemistry Marathon 2016

(A) Tritium (H-3) has been used as a radioactive tracer in studies of water movement. What is a radioactive tracer? [1]

I don't think the HSC expects that specific language for the radioisotope however.

 

[Shuuya]

Re: HSC Chemistry Marathon 2016

a) I'm reluctant to say 3 despite my gut instinct to give it because whenever I wrote this method I always wrote to do the calculation specifically as a last step. But from my understanding of chemistry this is sufficient for that 3, because my understanding of the chemistry course is that such details are typically addressed in the discussion.

b) Awesome. 4/4

Suggestion - Try putting the copper calorimeter INTO the method rather than the beaker. Believe it or not heat lost to the surroundings is the main reason why it's inevitable, because in a system where there's essentially unlimited supply of O2, incomplete combustion will be rare. For the fourth mark, instead of suggesting the calorimeter you could alternatively suggest putting a lid on the calorimeter with a hole for the thermometer and enclose the environment with aluminium foil in an attempt to trap the heat.

c)
Moles of C: LHS - 5, RHS - 5
Moles of O: LHS - 1 + 16 = 17, RHS - 10 + 6 = 16 !!!
Moles of H: LHS - 12, RHS = 6

Oops!

Correct answer: 2 C5H11OH(l) +15 O2(g) = 10 CO2(g) + 12 H2O(g) [Source: WolframAlpha]

So awkward, I always forget about the Oxygen in the alcohol

 

[helloimyellow]

Re: HSC Chemistry Marathon 2016

______________________________________
I might be mistaken, but should the state of water in the products be liquid and not gas? If not, can anyone explain why its in a gaseous state. There has been a debate going on at my school, that the products of ethanol combustion for water, it should be liquid state, others have said it should be gas?

Yeah like leehuan said it's water vapour. However, keep in mind that for molar heat of combustion, it's complete combustion with products being liquid water and CO2 gas (Conquering Chemistry p36).

 

[leehuan]

Re: HSC Chemistry Marathon 2016

[/B]

So awkward, I always forget about the Oxygen in the alcohol

I think the trend is that for odd number carbons you need two moles whereas for even number you only need 1

 

[chanandlerbong]

Re: HSC Chemistry Marathon 2016

I don't think the HSC expects that specific language for the radioisotope however.

For fun

 

[chanandlerbong]

Re: HSC Chemistry Marathon 2016

Actually, you're right Leehuan. The questions too hard, and not required for the HSC.
_____________________________________
NEXT QUESTION:
1.
2L of concentrated (10M) hydrochloric acid was spilled in a laboratory accident. Three substances were considered for use to minimize the damage, solid sodium hydrogen carbonate, powdered limestone (calcium carbonate), and 2M sodium hydroxide solution.
(a) CALCULATE the minimum mass of calcium carbonate needed to neutralize the acid. [3]
(b) ASSESS each of the three substances, sodium hydrogen carbonate, powdered limestone and 2M sodium hydroxide for use in the neutralization of the spilt acid. [4]

 

[Shuuya]

Re: HSC Chemistry Marathon 2016

Actually, you're right Leehuan. The questions too hard, and not required for the HSC.
_____________________________________
NEXT QUESTION:
1.
2L of concentrated (10M) hydrochloric acid was spilled in a laboratory accident. Three substances were considered for use to minimize the damage, solid sodium hydrogen carbonate, powdered limestone (calcium carbonate), and 2M sodium hydroxide solution.
(a) CALCULATE the minimum mass of calcium carbonate needed to neutralize the acid. [3]
(b) ASSESS each of the three substances, sodium hydrogen carbonate, powdered limestone and 2M sodium hydroxide for use in the neutralization of the spilt acid. [4]

 

[Shuuya]

Re: HSC Chemistry Marathon 2016

Actually, you're right Leehuan. The questions too hard, and not required for the HSC.
_____________________________________
NEXT QUESTION:
1.
2L of concentrated (10M) hydrochloric acid was spilled in a laboratory accident. Three substances were considered for use to minimize the damage, solid sodium hydrogen carbonate, powdered limestone (calcium carbonate), and 2M sodium hydroxide solution.
(a) CALCULATE the minimum mass of calcium carbonate needed to neutralize the acid. [3]
(b) ASSESS each of the three substances, sodium hydrogen carbonate, powdered limestone and 2M sodium hydroxide for use in the neutralization of the spilt acid. [4]

Ok so I tried (b) and I'm not sure how to assess the substances... I wrote out the equations and found that NaHCO3 and CaCO3 produce a salt, water as well as CO2 whilst NaOH only produces a salt and water. Does this have anything to do with the suitability?

 

[leehuan]

Re: HSC Chemistry Marathon 2016

(That sodium hydrogen carbonate is giving me implications of an actual cube of it. Because otherwise why would the limestone be powdered. Note: Yes, the question was in this regard, ambiguous)

H(+) + OH(-) -> H2O; ΔH≈-57kJ mol^-1
The reaction between a strong acid and a strong base is highly exothermic. This is true for all strong bases and acids interacting as the enthalpy change is determined as a result of the formation of the water molecule in neutralisation. Therefore, the enthalpy change for a strong acid-base neutralisation is always approximately -57kJ mol^-1. However, when used with a weak base, this can be reduced to -47kJ mol^-1.

This makes the 2M NaOH solution immediately inviable for use, as the heat released from the reaction will be unnecessarily large. Additionally, if by accident we use an excess amount of NaOH to neutralise the HCl, we may only cause another bad situation where we need to neutralise a base instead. This is equally problematic as whilst 10M HCl poses acid burn hazards, 2M NaOH will create risks of base irritation.

Both NaHCO3 and CaCO3 are weak bases, and thus have both the advantages of causing limited irritation if using excess, as well as reducing the heat of reaction.

The NaHCO3, being a weaker base than powdered limestone (and of course, NaOH) is arguably a better choice than spreading the limestone around due to the heat released from the reaction being potentially minimised further. However, it is unsafe to essentially roll a cube of a substance around simply for the process of neutralisation. Whilst the process of neutralisation should be slow to minimise the rate of release of heat, it is quite inconvenient to have to carry the cube around due to it's small surface area (A 2L spill will be massive) and spread it out to allow the neutralisation process to take place. The user is also essentially exposed to prolonged heat exposure, which may cause burns in itself.

These problems do not outweigh the situation with heat released, thus the most viable option here is therefore the powdered CaCO3 as we will be able to spread it out evenly, then leave the laboratory as soon as possible to minimise contact with the substances. This neutralisation process will also allow for a more safer alternative to clean up later.

(Note: A better solution is to soak up the acid in sand first, then neutralise the acid elsewhere)

NEXT QUESTION:

A student sets up an enclosed environment to simulate acid rain. The system initially consists of only sulfur dioxide gas and litmus paper. When he sprayed water into this environment, the litmus turned red rapidly.

a) Use relevant equations to interpret the results of the experiment and assess its validity. (4)
b) Evaluate the impacts of increasing levels of sulfur dioxide in the environment today. (4)

 

[ragingcurry]

Re: HSC Chemistry Marathon 2016

Petroleum is a non-renewable fossil fuel, and supplies are being rapidly depleted due to heavy demands for petrochemicals for a variety of uses such as fuels, plastics, lubricants, solvents etc. The demand is increasing as world's populations increases. Consequently, crude oil is being rapidly depleted due to HIGH demand and limited crude oil reserves. Furthermore, the need for alternative sources of petrochemical products derivatives come down to two significant points: environmental impact and scarcity.

- Majority of crude oil is used up as a fuel, the consumption of fuel products is detrimental upon the environment which releases pollutants in the atmosphere (octane - major component of petrol, burns incompletely). In comparison to other potential fuels such as ethanol, the current petrol products burns relatively uncleanly, leading to environmental problems.

- Most polymers are currently derived from petroleum are currently non-biodegradable which places a considerable strain on our landfills. Alternative sources esp. biopolymers, are biodegrable and would alleviate such problems

- Another reason why alternative sources are needed because petrochemical products are derived from non-renewable sources of crude oil, with chemists placing the lifespan on current petroleum supplies well under 50 years, alternative sources are required simply due unsustainable trends

Alternatives sources such as biomass may be used, however they are more expensive than crude oil. In addition, using ethanol as an example for alternative sources; new infrastructure needs to be constructed (such as fermentation plants for ethanol), a process which takes time and money and just isn't economically viable currently.

84% of crude oil is used to produce energy. This includes petrol and diesel for cars. Potential alternative sources too alleviate these problems would be ethanol where it will meet with not only future energy needs, but with material needs as well. Ethanol can be produced by fermenting sugars from sugar cane crops:
C6H12O6 (aq) ----(yeast enzymes) ---> 2C2H5OH (aq) + 2CO2(g)

Benefits:
- Ethanol is able to be used as a petrol supplement, because it undergoes combustion: C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (l) + heat
It can be used as a substitute because it is a renewable resource as it is manufactured from carbohydrates such as glucose and starch that are produced by photosynthesis by plants. The products of its combustion (CO2 and H2O) are the reacts needed by plants for photosynthesis: 6CO2 (g) +
6H2O (l) ----light----> C6H12O6 (aq) + 6O2 (g)

- Burns completely in oxygen (cleaner burning fuel) in contrast to other fuels that undergoes incomplete combustion thus producing toxic CO and carcinogenic soot

Ethanol combined with its high energy-per-mole output, cleaner burning nature and ease of transport, ethanol has great potential as an alternative fuel.

Please correct my response!

NEXT QUESTION:
Outline two methods of producing ethanol. In your answer, construct chemical equations to demonstrate ONE renewable and ONE non-renewable method of ethanol production. (4 marks)

Hey RachelGreen did you write all these answers of the top of your head?
How do you remember all this?
Makes me feel dumb!!!

 

[leehuan]

Re: HSC Chemistry Marathon 2016

Hey RachelGreen did you write all these answers of the top of your head?
How do you remember all this?
Makes me feel dumb!!!

It's always been revising over notes (printed + HIGHLIGHTING for truly difficult stuff for me to remember), and practice questions.

Because there were certain dot points that I just need reaffirming what I knew every now and then (e.g. industry and medicine radioisotope), then those that I had a fair grasp on but I'd always lose info on x and y (potentials of polyethylene), then stuff I really struggled on (for me one of them was actually just the reflux practical).

If you just note take in class without trying to think about what's what, that doesn't help either. If you do computer notes, that's also a bit damaging because writing is one of the best forms of study. That's why if you want to properly know your stuff, copy out your notes over and over again, and do past papers.
___________________________________________________

Bump

A student sets up an enclosed environment to simulate acid rain. The system initially consists of only sulfur dioxide gas and litmus paper. When he sprayed water into this environment, the litmus turned red rapidly.

a) Use relevant equations to interpret the results of the experiment and assess its validity. (4)
b) Evaluate the impacts of increasing levels of sulfur dioxide in the environment today. (4)

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

Hey RachelGreen did you write all these answers of the top of your head?
How do you remember all this?
Makes me feel dumb!!!

Tbh, I just study a lot. If you're having problems answering questions here, then you should use UR notes to help you answer

 

[Flop21]

Re: HSC Chemistry Marathon 2016

Determine the number of moles of chromium atoms in 140.0g chromium(III) oxide.




I thought I knew how to do this but I can't get the answer given in the solutions.