HSC 2016 Chemistry Marathon (3 Viewers)

[leehuan]

Re: HSC Chemistry Marathon 2016

Next question:
Tincture of iodine is an antiseptic often found in medical kits. It is a solution of iodine in ethanol. Ethanol is used as the solvent as iodine is relatively insoluble in water. When an aqueous solution of iodine is required, iodine ions are added to iodine to form the triiodide ion which is more soluble in water.

(a) Write an equation for the reaction above for the solution of iodine
(b) Draw a labelled diagram to explain the solubility of iodine in ethanol.
(c) Explain why the triiodide ion is more soluble in water than is iodine.

Note, you wouldn't expect this type of question in the HSC though.

(a) I2(aq) + I-(aq) <--> I3-(aq)
(b) I-I
Molecular iodine is a linear molecule and non-polar, similar to the hydrocarbon chain in ethanol. The similar molecular structure of these molecules allows for dispersion forces to form between the molecules, which accounts for the solubility of iodine in ethanol.
(c) Whereas iodine is non-polar, the triiodide ion is a molecule with a -1 charge. Noting how water is a polar molecule; the hydrogen atoms being slightly positive and the oxygen atom slightly negative, ion-dipole interactions will form between the molecules, allowing for a sound solubility of the ion. By contrast, iodine is also a linear molecule, opposed to the bent structure of water, hence no significant intermolecular forces can form between molecules to promote solubility.

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

Oh wow that's perfect!

 

[leehuan]

Re: HSC Chemistry Marathon 2016

Assuming this is for 3 marks:
Next question: Discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry. [6 marks]

Let's reword this.

Using ethanol as an example, discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry. Explain the benefits that are carried along with the use of ethanol as said substitute. (6)

 

[RachelGreen]

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)

 

[Ambility]

Re: HSC Chemistry Marathon 2016

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)

Ethanol can be produced non-renewably from crude oil. Petroleum is separated into its constituent hydrocarbon molecules through a process of fractional distillation using boiling point as a difference in properties. One of the distillate fractions will contain ethylene but if more ethylene is required then higher molecular weight fractions can be catalytically or thermally cracked to ethylene which is the process of breaking the larger molecules down into smaller ones. Ethylene is then converted to ethanol through a hydration reaction which is an addition of a water molecule over ethylene's double bond. This requires 300 degrees Celsius and a sulfuric acid catalyst.



Ethanol can be produced renewably from fermentation. Biomass from plants containing cellulose is broken down both into glucose monomers and then into ethanol through a process of fermentation. This process produces ethanol along with carbon dioxide, requires an enzyme (yeast) and about 37 degrees Celsius. This process will only produce ethanol up to a concentration of 15%, further concentrations of ethanol can be derived through distillation.

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

Correct! 4/4
NEXT QUESTION:
"The carbon dioxide released from combustion of bio-ethanol is balanced by carbon dioxide captured through photosynthesis. Therefore combustion of bio-ethanol does not increase the total amount of carbon dioxide in the atmosphere. Thus, bio-ethanol has attracted attention as a carbon-neutral fuel - an energy source effective as a countermeasure to global warming."

Critically evaluate the statement above with reference to ethanol being a 'carbon-netrual' fuel. Support your answer with relevant chemical equations (5 marks)

 

[leehuan]

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)

Although I'm tired and can't absorb things in fully, I'd still say solid 6/6!

NEXT QUESTION:
A galvanic cell was set up as so: Cu(s)|CuSO4(aq)||AgNO3(aq)|Ag(s)
(a) Explain the purpose of a salt bridge in this cell. (2)
(b) Write down the net ionic equation of this cell and calculate the voltage it should produce. (2)
(c) When this experiment was performed in the laboratory, the values were inconsistent with that as calculated. Give three reasons as to why this may have been the case. (3)

 

[leehuan]

Re: HSC Chemistry Marathon 2016

Correct! 4/4
NEXT QUESTION:
"The carbon dioxide released from combustion of bio-ethanol is balanced by carbon dioxide captured through photosynthesis. Therefore combustion of bio-ethanol does not increase the total amount of carbon dioxide in the atmosphere. Thus, bio-ethanol has attracted attention as a carbon-neutral fuel - an energy source effective as a countermeasure to global warming."

Critically evaluate the statement above with reference to ethanol being a 'carbon-netrual' fuel. Support your answer with relevant chemical equations (5 marks)

I missed this question at the time. Admittedly, I also struggled when I was studying trying to answer this question.
------------------------------------------------------
Combustion of ethanol: C2H5OH + 3 O2 -> 2 CO2 + 3 H2O
Photosynthesis: 6 CO2 + 6 H2O -(UV)-> C6H12O6 + 6 O2
Fermentation of glucose to produce bioethanol: C6H12O6 -(yeast)-> 2 C2H5OH + 2 CO2

Primarily, bio-ethanol attracts attention as it can be considered a renewable energy source. It serves as a potential alternative to current fuel sources in petrol.

The equations given do imply that through the process of producing/using ethanol, the ability for plants to photosynthesise does indeed suggest its limited impact on contribution to CO2, hence seemingly providing supporting evidence for the justification. However, there are various other factors to consider in this process. Our current sources of biomass are mostly from dead plant matter, and to ensure we will always have sufficient amount of plants to undergo photosynthesis, there requires extensive arable land required to allow these plants to grow. The process of fermentation of glucose is also highly energy demanding, and sufficient energy can only be provided through alternate means. The easiest alternative to seek out becomes fossil fuels, which is what we seek to avoid in this process to reduce CO2 emissions. When considering these extrinsic factors, the statement is rather inaccurate as it limits itself to only a very narrow range of considerations.

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

I think you should talk more about how the whole processes absorbs and returns the same amount of CO2(g) in the atmosphere. Anyways, I'll just write what theyve here:
Bioethanol is produced by the fermentation of plant glucose, initially formed from CO2(g) and H2O(l) by the action of sunlight and the catalyst chlorophyll:
6CO2(g) + 6H2O(l) --(light,chlorophyll)--> C6H12O6 (glucose)

Six molecules of CO2(g) are ABSORBED in this process. The plant glucose is fermented to ethanol using the catalyst zymase (present in yeast):
C6H12O6---(zymase)--->2C2H5OH + 2CO2(g)
In this process, 2 molecules of CO2(g) are returned into the atmosphere. A further 2 molecules are RETURNED for every molecule of ethanol burned (Notice the ratios between ethanol and CO2 - 1:2).

Combustion: 2C2H5OH + 7O2(g) -----> 4CO2(g) + 6H2O (l) + E
If all of the ethanol in the above equation was combusted/burned, four molecules of CO2(g) would be returned to the atmosphere.

This whole process absorbs and returns the same amount of CO2(g) to the atmosphere. In this respect it is 'CARBON NEUTRAL'. However, this does not take into account the energy consumed in the manufacturing process such as:
- energy required to maintain the fermentation vats at the correct temperature and conditions
- energy required to distill the bioethanol from a concentration of 15% to 95-100%
- energy required to store and transport the fuel to the consumer

Because of this, the entire process cannot be considered completely 'carbon neutral' even though it is still more carbon neutral than the combustion fo fossil fuels is only discharged into the atmosphere.


I will have a go at your question later, I just came home from the city

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

NEXT QUESTION:
A galvanic cell was set up as so: Cu(s)|CuSO4(aq)||AgNO3(aq)|Ag(s)
(a) Explain the purpose of a salt bridge in this cell. (2)
(b) Write down the net ionic equation of this cell and calculate the voltage it should produce. (2)
(c) When this experiment was performed in the laboratory, the values were inconsistent with that as calculated. Give three reasons as to why this may have been the case. (3)[/QUOTE]


(a) The salt bridge maintains electrical neutrality and completes the circuit by allowing the migration of ions to occur between the two half-cells, hence allowing the cell to continue functioning.
[Additional information]. Since electrons are being removed from the anode, the anode compartment becomes more electron deficient (as a result, becomes more positively charged). Oppositely, the cathode compartment becomes more negatively charged due to excess electrons. Therefore, if electrons were the only moving charged particles, there would be an electrical imbalance and the cell wouldn't function. That is why salt bridges are required in galvanic cells, to maintain electrical neutrality

(b) Having problems?? Does the left side (Cu(s)|CuSO4(aq)) correspond to the anode compartment and the right side corresponds to the cathode compartment? Confused, need to work on this

(c) Cell voltages obtained in the laboratory was inconsistent due to a few reasons:
- Reliability is absolute crap by the looks of it. The experiment should be repeated many times, until consistent, similar results are obtained.
- While the concentrations were kept at approx. to 1M (1 significant figure), we could not ensure standard conditions were maintained i.e. solute concentration of 1.000mol/L (4 sign. figures), temperature of 25 degrees celsius, and pressure of 100.0kPA. This may have voltages slightly
- There may have been impurities in the electrolyte solution
- The metal may have not been completely pure. To solve this problem, use emery paper to clean the electrodes to remove oxide layers and other impurities
- The voltmeter had markings only at every 0.1V increment so there may have been errors reading the values of the analogue voltmeter. This could be improved by having a digital voltmeter.


Done and dusted

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

NEXT QUESTION:
A student decided to set up a galvanic cell containing the Zn(s)|Zn2+(aq) redox couple to power his gaming device.

Which other redox couple should be used so that the student's galvanic cell will have the largest possible voltage? In your answer, explain your answer with calculations.
Here are your options:
(a) K(s)|K+ (aq)
(b) Cu(s)|Cu2+ (aq)
(c) H+(aq) | H2(g)
(d) Ag(s)|Ag+(aq)

Telling it now, the answer is not A.

 

[leehuan]

Re: HSC Chemistry Marathon 2016

With your question that I answered, yes I didn't fully relate the equations because I didn't exactly state how many moles was involved (idk why). Other than that it was fine.

NEXT QUESTION:
A galvanic cell was set up as so: Cu(s)|CuSO4(aq)||AgNO3(aq)|Ag(s)
(a) Explain the purpose of a salt bridge in this cell. (2)
(b) Write down the net ionic equation of this cell and calculate the voltage it should produce. (2)
(c) When this experiment was performed in the laboratory, the values were inconsistent with that as calculated. Give three reasons as to why this may have been the case. (3)


(a) The salt bridge maintains electrical neutrality and completes the circuit by allowing the migration of ions to occur between the two half-cells, hence allowing the cell to continue functioning.
[Additional information]. Since electrons are being removed from the anode, the anode compartment becomes more electron deficient (as a result, becomes more positively charged). Oppositely, the cathode compartment becomes more negatively charged due to excess electrons. Therefore, if electrons were the only moving charged particles, there would be an electrical imbalance and the cell wouldn't function. That is why salt bridges are required in galvanic cells, to maintain electrical neutrality

(b) Having problems?? Does the left side (Cu(s)|CuSO4(aq)) correspond to the anode compartment and the right side corresponds to the cathode compartment? Confused, need to work on this

(c) Cell voltages obtained in the laboratory was inconsistent due to a few reasons:
- Reliability is absolute crap by the looks of it. The experiment should be repeated many times, until consistent, similar results are obtained.
- While the concentrations were kept at approx. to 1M (1 significant figure), we could not ensure standard conditions were maintained i.e. solute concentration of 1.000mol/L (4 sign. figures), temperature of 25 degrees celsius, and pressure of 100.0kPA. This may have voltages slightly
- There may have been impurities in the electrolyte solution
- The metal may have not been completely pure. To solve this problem, use emery paper to clean the electrodes to remove oxide layers and other impurities
- The voltmeter had markings only at every 0.1V increment so there may have been errors reading the values of the analogue voltmeter. This could be improved by having a digital voltmeter.


Done and dusted

b) Use your table of standard reduction potentials to find out which is the cathode and the anode.
Cu(s) + 2 Ag+ -> Cu2+ + 2 Ag(s)
Bottom one is always more readily reduced than oxidised
(The notation only means that we have a galvanic cell; it does not matter which side is the cathode or the anode)
c) 1M, 100kPa pressure and 25deg C was enough
Reliability - Although I have no problems with this, it's not the first thing the examiners have in mind when giving this sort of question
Accuracy - This one is irrelevant
Impurities - Doesn't matter, as Cu(s) can only correspond to CuSO4, as can Ag(s) only correspond to AgNO3

 

[leehuan]

Re: HSC Chemistry Marathon 2016

NEXT QUESTION:
A student decided to set up a galvanic cell containing the Zn(s)|Zn2+(aq) redox couple to power his gaming device.

Which other redox couple should be used so that the student's galvanic cell will have the largest possible voltage? In your answer, explain your answer with calculations.
Here are your options:
(a) K(s)|K+ (aq)
(b) Cu(s)|Cu2+ (aq)
(c) H+(aq) | H2(g)
(d) Ag(s)|Ag+(aq)

Telling it now, the answer is not A.

a) 2 K(s) + Zn 2+ -> 2 K+ + Zn(s) EMF = 2.94-0.76 = 2.18V - why not? You always flip the sign on the one that gets oxidised.
b) Zn(s) + Cu+ -> Cu(s) + Zn 2+ EMF = 0.76+0.34 = 1.10V
c) Zn(s) + 2 H+ -> H2(g) + Zn 2+ EMF = 0.76V
d) Zn(s) + 2 Ag+ -> 2 Ag(s) + Zn 2+ EMF = 0.76+0.80 = 1.56V

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

Yeah I know you always flip the sign on the one that gets oxidised but I don't understand why it's not A and not D. Confusing me tbh

 

[leehuan]

Re: HSC Chemistry Marathon 2016

I don't see any reason for it not to be A. Mind showing the source of your question?

 

[RachelGreen]

Re: HSC Chemistry Marathon 2016

Nevermind, I'm convinced it's incorrect now. Yeah it's A dw.

NEXT QUESTION:
To determine the concentration of Fe2+ ions in a sample of tank water, a student conducted a titration of a sample against a standard solution of potassium permanganate.

The reaction that occurs during the titration can be represented:

5Fe2+ (aq) + MnO4- (aq) + 8H+ (aq) -> 5Fe3+ (aq) + Mn2+ (aq) + 4H2O (l)

The unknown Fe2+ solution was titrated against 20.00mL of standard 0.0250M potassium permanganate in three trials.
An average of 32.50mL of tank water sample was required to change the colour of the solution in the conical flask from purple to colourless, which is the end-point in this titration.

(a) Calculate the CHANGE in oxidation number of manganese (Mn) that occurs as the titration proceeds and hence determine if the Mn is oxidized or reduced during the titration. (1M)

(b) Calculate the moles of potassium permanganate present in the conical flask during each trial. (1M)

(c) Calculate the mass of iron present as Fe2+ in 100mL of the tank water. (3M)

 

[leehuan]

Re: HSC Chemistry Marathon 2016

Nevermind, I'm convinced it's incorrect now. Yeah it's A dw.

NEXT QUESTION:
To determine the concentration of Fe2+ ions in a sample of tank water, a student conducted a titration of a sample against a standard solution of potassium permanganate.

The reaction that occurs during the titration can be represented:

5Fe2+ (aq) + MnO4- (aq) + 8H+ (aq) -> 5Fe3+ (aq) + Mn2+ (aq) + 4H2O (l)

The unknown Fe2+ solution was titrated against 20.00mL of standard 0.0250M potassium permanganate in three trials.
An average of 32.50mL of tank water sample was required to change the colour of the solution in the conical flask from purple to colourless, which is the end-point in this titration.

(a) Calculate the CHANGE in oxidation number of manganese (Mn) that occurs as the titration proceeds and hence determine if the Mn is oxidized or reduced during the titration. (1M)

(b) Calculate the moles of potassium permanganate present in the conical flask during each trial. (1M)

(c) Calculate the mass of iron present as Fe2+ in 100mL of the tank water. (3M)

a) In MnO4(-) manganese has an oxidation state of +7. The manganese ion has an oxidation state of +2. Hence, change in oxidation state = -5
b) n=CV -> n=0.0250*20.00*10^-3=5.00*10^-4mol
c) From the given equation, we have n(Fe2+) = 5*n(MnO4-) = 2.5*10^-3mol
Concentration of MnO4- in titration: C=n/V -> C=(2.5*10^-3)/(32.50*10^-3)=1/13 mol L^-1
Our tank has a volume of 0.100L
So in our tank, n=1/13*0.100 = 1/130 mol
So m = 1/130 * (4*16+54.94) = 0.9149230769...g = approx 9.15*10^-1 g (1s.f.)

I just rushed this so I could have made a silly mistake somewhere
---------------------------------------------------------------------
NEXT QUESTION:
Account for a radioisotope used in medicine and relate properties to it's use. (4)

 

[Ambility]

Re: HSC Chemistry Marathon 2016

NEXT QUESTION:
Account for a radioisotope used in medicine and relate properties to it's use. (4)

Cobalt-60 is used in medicine to irradiate cancerous cells in radiotherapy. Cobalt-60 has a half life of about 5 years which means it can last for a substantial period of time before needing replacement delivering the dosages required. Furthermore, cobalt-60 is useful for radiotherapy as the gamma rays emitted in beta emission which it undergoes have sufficient penetrating and ionising power to attack cancerous cells. It is this power that kills cancerous cells.

Honestly probably not the best answer, though I haven't learnt in school yet.

NEXT QUESTION: What is the oxidising agent in the following reaction?

 

[leehuan]

Re: HSC Chemistry Marathon 2016

Cobalt-60 is used in medicine to irradiate cancerous cells in radiotherapy. Cobalt-60 has a half life of about 5 years which means it can last for a substantial period of time before needing replacement delivering the dosages required. Furthermore, cobalt-60 is useful for radiotherapy as the gamma rays emitted in beta emission which it undergoes have sufficient penetrating and ionising power to attack cancerous cells. It is this power that kills cancerous cells.

Honestly probably not the best answer, though I haven't learnt in school yet.

NEXT QUESTION: What is the oxidising agent in the following reaction?

Wow. I had to pull out Google to realise Co-60 was also used in medicine. 3/4 though mainly cause information is lacking, which is probably due to you haven't learnt it yet.
------------------
The dichromate ion, as chromium is reduced from +6 to +3.
------------------
NEXT QUESTION:
Describe two processes used to produce transuranic elements and briefly contrast the products. Give relevant equations to support your answer. (5)

 

[leehuan]

Re: HSC Chemistry Marathon 2016

NEXT QUESTION:
Describe two processes used to produce transuranic elements and briefly contrast the products. Give relevant equations to support your answer. (5)

Thread revival.

There should be enough of you that have done this now.