HSC 2012-2015 Chemistry Marathon (archive) (5 Viewers)

[someth1ng]

AB940, some minor problems:
1. It's best to call them dispersion forces.
2. Say that they're non-polar.
3. Hydrogen bonding isn't necessarily the difference in electronegativity (otherwise, chlorine would make hydrogen bonds). It's just that N/O/F have smaller electron clouds so protons can get closer.
4. I would say that there's higher amounts of intermolecular forces. Saying that they're stronger is ambiguous (is each hydrogen bond stronger?)
5. You should also that that boiling point is dictated by the strength of intermolecular forces and how strong molecules are attracted to each other.

 

[jaycoo50]

re: HSC Chemistry Marathon Archive

alkanes < alkanols < alkanoic acids

The boiling point depends on intermolecular forces. Alkanes are non-polar molecules and weak dispersion forces are the only intermolecular forces. In alkanols, a polar OH group is present, allowing strong hydrogen bonds to be formed between molecules. Hydrogen bonds are stronger than dispersion forces and more energy is required to break the stronger hydrogen bonds than the weak dispersion forces, resulting in alkanols having higher boiling points than alkanes. Alkanoic acids can form two hydrogen bonds between each molecule, resulting in a higher boiling point than alkanols.


I don't think we need to know about boiling points of esters. Do we?


Question:

Describe the standard conditions in a galvanic cell and explain why they are necessary.

for you question question, it is not that hard, just memorize you textbook, I use spotlight chemistry,

standard condations:

Two electrode ( ie. cu and zn)
external wire,
electrolyte ( ion solution for movement of ions)
filter paper for salt bridge
voltmeter

now explain why each are necessary, ie. filter paper is necessary to allow the movement of ions from one beaker to another.

 

[JohnMaximus]

re: HSC Chemistry Marathon Archive

Why is a voltmeter necessary?

 

[someth1ng]

standard condations:

Two electrode ( ie. cu and zn)
external wire,
electrolyte ( ion solution for movement of ions)
filter paper for salt bridge
voltmeter

I'm going to be blatantly honest - this is a very bad response - I'd give it 0/3. We'll start with the fact those are NOT the standard conditions.

A galvanic cell doesn't need to be Cu and Zn - they can be any two combinations. You used "ie", which means "that is" - if you used "eg", it would be fine.

All the others are conditions are parts of a galvanic cell, not standard conditions.

Also, saying filter paper for a salt bridge is wrong. It's not always filter paper - it can be a gel and you need to say that it's soaked with an electrolyte such as KNO3.

 

[someth1ng]

By the way, standard conditions are:
1. 273.15K (IUPAC standard temperature) or 298.15K (standard ambient temperature).
2. Solutions at 1M concentration.
3. Pressure at 100kPa.

The reason why we need to keep them constant is because they can each affect the emf produced if they are changed. This makes it impossible to compare them. Hence, we use these standards to make all measurements consistent.

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Question: EA 0.010 M solution of nitric acid and a 0.14M solution of citric acid both have a pH of 2.0.

Briefly explain how it is possible that both of these solutions have the same pH.

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Citric acid is a weak acid so it only partially ionizes.

Nitric acid is a strong acid which completely ionizes.

So having the same concentration won't mean the pH will be the same. Therefore a much more concentrated solution of citric acid will equal the pH of a less concentrated nitric acid solution due to the nature of their ionization.

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Citric acid is a weak acid so it only partially ionizes.

Nitric acid is a strong acid which completely ionizes.

So having the same concentration won't mean the pH will be the same. Therefore a much more concentrated solution of citric acid will equal the pH of a less concentrated nitric acid solution due to the nature of their ionization.

yes remember to talk about [H+] before talking about pH

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Question 1: In a nuclear fission reactor iron-58 is bombarded with neutrons. Iron-58 forms a new isotope of iron. This new isotope then undergoes beta decay. Identify the new product after this beta decay (1 mark)

Question 2:
a) Identify the catalyst used in esterfication (1 mark)
b) Explain the purpose of this catalyst (2 marks)

Question 3: Explain the change in pH of the contents of a bottle of soda water after it has been opened (2 marks)

Question 4: Tony performed an experiment to ferment 12g of glucose with 1 g of yeast along with 50ml of water at 37 degrees Celsius. He left this for 1 days and recorded a mass change of 1.44 grams.
a) Calculate the mass of ethanol produced and compare this to the theoretical mass of ethanol (2 marks)
b) Explain the difference in masses ( 2 marks

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Q1) Idk...
Q2)
a)Concentrated sulfuric acid
b)It's a dehydrating agent. When it is added it will absorb water thus shifting equilibrium to right to make more ester. (Can't remember equation)
Q3) Soda water bottles contain dissolved carbon dioxide. The equilibrium is H₂O+CO_2aq<-> H₂CO₃aq. When you open the bottle you reduce the pressure of the equilibrium so it shifts to the left to produce more of the CO₂ which escapes(Le bubbles). When CO₂ gets removed the pH drops.
Q4) So late at night I'll do this question later.

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Q2)
a)Concentrated sulfuric acid [1 mark]
b)It's a dehydrating agent. When it is added it will absorb water thus shifting equilibrium to right to make more ester.[1 mark] - good, what about the actual effect of a all catalysts? (Can't remember equation)
Q3) Soda water bottles contain dissolved carbon dioxide. The equilibrium is H₂O+CO_2aq<-> H₂CO₃aq. When you open the bottle you reduce the pressure of the equilibrium so it shifts to the left to produce more of the CO₂ which escapes(Le bubbles). When CO₂ gets removed the pH drops. [0 marks] - equilibrium shift is correct, but when u reduce pressure, the disturbed system will shift to minimize the change, by shifting to the equation which produces MORE moles of GAS, so i think u should write the equation CO2 (g) <--> CO2 (aq) first to explain the shift in equilibruim and then explain the change in acidity using ur equation. Also, pH increase because of common sense. If u take a long time to drink a coke bottle, do u notice that the acidity deteriorates?

Check the bolded

 

[Menomaths]

re: HSC Chemistry Marathon Archive

When CO2 escapes of course the pH increases. I was sleepy my bad...Don't know what I was thinking

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

When CO2 escapes of course the pH increases. I was sleepy my bad...Don't know what I was thinking

yes, and also for Q2 (b), the catalyst used lowers the activation energy for the reaction to proceed, thereby increasing the rate of reaction

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Question: Compare the reactivity and solubility of ozone and dioxygen

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Question: Compare the reactivity and solubility of ozone and dioxygen

Dioxygen haha

Solubility: Oxygen is less soluble compared to Ozone. Reason for this is because oxygen has a O2 which is non-polar so it does not form strong inter-molecular bonds with polar water. Whereas ozone's polarity allows it to have inter-molecular interactions with water.
Reactivity: Oxygen is much more stable than Ozone. Oxygen is very stable since it has a double bond which requires high energy levels to break. Whereas Ozone has a single co-ordinate covalent bond which requires less energy to break so Ozone readily decomposes to a stable O2 molecule.

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Dioxygen haha

Solubility: Oxygen is less soluble compared to Ozone. Reason for this is because oxygen has a O2 which is non-polar so it does not form strong inter-molecular bonds with polar water. Whereas ozone's polarity allows it to have inter-molecular interactions with water.
Reactivity: Oxygen is much more stable than Ozone. Oxygen is very stable since it has a double bond which requires high energy levels to break. Whereas Ozone has a single co-ordinate covalent bond which requires less energy to break so Ozone readily decomposes to a stable O2 molecule.

yep

Question:
a) Identify the difference between CFCs and HCFCs (1 mark)
b) Explain why HCFCs have a lower ozone depletion potential compared to CFCs (1 mark)

 

[Menomaths]

re: HSC Chemistry Marathon Archive

yep

Question:
a) Identify the difference between CFCs and HCFCs (1 mark)
b) Explain why HCFCs have a lower ozone depletion potential compared to CFCs (1 mark)

a) HCFC = Hydrogenchlorofluorocarbon and CFC= Chlorofluorocarbon. The difference is HCFC contain Hydrogen while CFCs don't.
b) HCFCs are broken down in the troposphere due to the high reactivity of their bonds. Only a small percentage of HCFCs reach the stratosphere, but once they do, ozone destruction rapidly occurs. So they have much less ozone depletion potential than CFCs (CFCs are inert, so they stay can stay in the troposphere). Shouldn't this be like 2 marks, I could've written much more but 1 mark...

 

[AnimeX]

re: HSC Chemistry Marathon Archive

yep

Question:
a) Identify the difference between CFCs and HCFCs (1 mark)
b) Explain why HCFCs have a lower ozone depletion potential compared to CFCs (1 mark)

(unsure can someone mark this please )

a & b) CFC's are chloro fluro carbons and are harmful to the ozone layer due to their ability to break their c-cl bonds in the stratosphere (due to UV a and UV b radiation), they make it up to the stratosphere due to their low reactivity, potentially depleting the ozone layer. HCFCs are hydro chloro fluoro carbons and are more reactive than CFCs thus are broken down earlier and don't reach the stratosphere, thus help get rid of bad ozone, ie trophospheric ozone.

b) lol whoops answered a and b at once.

Discuss how a biopolymer you have studied is synthesised and compare it's uses to polyethylene (4 marks)

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

a) HCFC = Hydro(gen?)chlorofluorocarbon and CFC= Chlorofluorocarbon. The difference is HCFC contain Hydrogen while CFCs don't.
b) HCFCs are broken down in the troposphere due to the high reactivity of their bonds. Only a small percentage of HCFCs reach the stratosphere, but once they do, ozone destruction rapidly occurs. So they have much less ozone depletion potential than CFCs (CFCs are inert, so they stay can stay in the troposphere). Shouldn't this be like 2 marks, I could've written much more but 1 mark...

sorry it should be 2 marks

(unsure can someone mark this please )

a & b) CFC's are chloro fluro carbons and are harmful to the ozone layer due to their ability to break their c-cl bonds in the stratosphere (due to UV a and UV b radiation), they make it up to the stratosphere due to their low reactivity, potentially depleting the ozone layer. HCFCs are hydro chloro fluoro carbons and are more reactive than CFCs thus are broken down earlier and don't reach the stratosphere, thus help get rid of bad ozone, ie trophospheric ozone.

b) lol whoops answered a and b at once.

Discuss how a biopolymer you have studied is synthesised and compare it's uses to polyethylene (4 marks)

yes but u didnt identify the difference between CFCs and HCFCs
bolded - i think that is wrong

 

[AnimeX]

re: HSC Chemistry Marathon Archive

sorry it should be 2 marks


yes but u didnt identify the difference between CFCs and HCFCs

ahh my bad, i discussed both, must I explicitly say the difference?