VCE Chemistry Question Thread

[klippo]

Hi everyone!

Just wondering whether we need to know the different types of polymers and their properties for 3/4 chem?
e.g. thermoplastic polymers, thermosets or elastomers...
Or, do we just need to know the process of addition polymerization?

Thanks in advance!

[Sine]

Hi everyone!

Just wondering whether we need to know the different types of polymers and their properties for 3/4 chem?
e.g. thermoplastic polymers, thermosets or elastomers...
Or, do we just need to know the process of addition polymerization?

Thanks in advance!

just the process of addition polymerisation and condensation polymerisation

[Sine]

whats a good explanation for the purpose of boiling chips?

Why do we cool the ester and add cold water once an ester has been formed (methyl salicyclate)

[JellyBeanz]

Are we required to know how to draw the bonding pairs between Adenine-thymine and Guanine-cytosine?

[Individu]

Can someone please explain why (c) is the answer...

0.132 g of a pure carboxylic acid (R–COOH) was dissolved in 25.00 mL of water and titrated with
0.120 M NaOH solution. A volume of 14.80 mL was required to reach the endpoint of the titration.

Question 9
Which of the following best represents the pH of the solution at the equivalence point and the name of a suitable
indicator for this titration?
 pH at the equivalence point suitable indicator
A. less than 7 phenolphthalein
B. less than 7 bromophenol blue
C. greater than 7 phenolphthalein
D. greater than 7 bromophenol blue

From VCAA 2009 report:

At the equivalence point all of the RCOOH will have reacted with NaOH(aq) according to
RCOOH(aq) + OH-(aq) → RCOO-(aq) + H2O(l).
The pH at the equivalence point is due to RCOO-(aq) which as the conjugate base of the carboxylic acid is a weak base, hence the solution has pH > 7.
During the titration the pH of the solution increases and to accurately identify the endpoint, as close as possible to the equivalence point, the indicator must change colour once the pH becomes greater than 7.
Phenolphthalein fits this criteria.
Perhaps students who chose option B did not recognise that even though carboxylic acids are weak acids they react to completion in the presence of the stoichiometric amount of a strong base.

[HasibA]

Are we required to know how to draw the bonding pairs between Adenine-thymine and Guanine-cytosine?

pretty sure you do , idk if you need to draw the nitrogenous base pairs in detail and show where the bonds occur, but you have to know the fact that the A-T bond has two hydrogen bonds and the C-G has 3 hydrogen bonds (heinemann has a nice diagram for this in their textbook!) hope this help

tl;dr- yes, not in detail tho

From VCAA 2009 report:

At the equivalence point all of the RCOOH will have reacted with NaOH(aq) according to
RCOOH(aq) + OH-(aq) → RCOO-(aq) + H2O(l).
The pH at the equivalence point is due to RCOO-(aq) which as the conjugate base of the carboxylic acid is a weak base, hence the solution has pH > 7.
During the titration the pH of the solution increases and to accurately identify the endpoint, as close as possible to the equivalence point, the indicator must change colour once the pH becomes greater than 7.
Phenolphthalein fits this criteria.
Perhaps students who chose option B did not recognise that even though carboxylic acids are weak acids they react to completion in the presence of the stoichiometric amount of a strong base.

https://www.youtube.com/watch?v=LxsVqu5w7pA explains it

[JellyBeanz]

Like literally all you need to know is that there are 2 hydrogen bonds between Adenine-thymine and 3 hydrogen bonds between Guanine-cytosine. You get the structural formulas for these in the data book, so as long as you know where the H-bonds are, you should be fine.

pretty sure you do , idk if you need to draw the nitrogenous base pairs in detail and show where the bonds occur, but you have to know the fact that the A-T bond has two hydrogen bonds and the C-G has 3 hydrogen bonds (heinemann has a nice diagram for this in their textbook!) hope this help

tl;dr- yes, not in detail tho


https://www.youtube.com/watch?v=LxsVqu5w7pA explains it

Thanks heaps guys

[Swagadaktal]

Can someone please explain why (c) is the answer...

0.132 g of a pure carboxylic acid (R–COOH) was dissolved in 25.00 mL of water and titrated with
0.120 M NaOH solution. A volume of 14.80 mL was required to reach the endpoint of the titration.

Question 9
Which of the following best represents the pH of the solution at the equivalence point and the name of a suitable
indicator for this titration?
 pH at the equivalence point suitable indicator
A. less than 7 phenolphthalein
B. less than 7 bromophenol blue
C. greater than 7 phenolphthalein
D. greater than 7 bromophenol blue

This q has been answered like 1324 times so ima be the 1325th one.

When you titrate something,you just get a stoichiometric equivalence, in this q u need a 1:1 ratio. This doesn't dictate the pH at that point. Because the base is strong and any carboxylic acid is weak, the pH will be greaterthan 7 as the equivalent mol amount of acid is not enough to overpower the strong base (to make it below 7)

No idea if that made sense

[Swagadaktal]

whats a good explanation for the purpose of boiling chips?

Why do we cool the ester and add cold water once an ester has been formed (methyl salicyclate)

Add cold water to get rid of any excess hoes. Esters are insoluble coz any polar fams be gone (equivalent pull from c on both ends of the O and the double bond O has no free electrons.) Thus, u get rid of the excess hoes to ensure purity of ur banana smelling compound

[JellyBeanz]

Guys could someone explain how restriction enzymes work? Like why it cleaves at certain places and why it is used?

[Sine]

Guys could someone explain how restriction enzymes work? Like why it cleaves at certain places and why it is used?

a specific restriction enzyme will recognize a particular sequence of nucleotide bases,( the recognition sequence )and cut at a particular point. It is used for DNA profiling.*VCE Biology*-->which it would then cut producing blunt ends when cuts are directly opposite each other or sticky ends when cuts are not directly opposite each other (has overhanging bases) sticky ends can easily join up with other sticky ends but blunts ends require enzymes to do so.

[Swagadaktal]

Guys could someone explain how restriction enzymes work? Like why it cleaves at certain places and why it is used?

Pre sure this isnt in VCE chemistry - am i correct?

[Sine]

Pre sure this isnt in VCE chemistry - am i correct?

yeah not VCE chem it's in VCE biology. However i think in VCE chemistry we just need to recognise that they are a thing and that they are used for some of the DNA forensic applications.

[Individu]

The thermite process can be used to weld lengths of railway
track together. A mould placed over the ends of the two rails
to be joined is filled with a charge of aluminium powder and
iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron, which joins the rails together.

a Write a half equation for the conversion of iron(III) oxide
to metallic iron.
b Is the half equation you wrote for part a an oxidation or a
reduction process?
c Write the overall equation for the thermite process.


My initial answer for a was Fe3+ (s) + 3e- --> Fe(l) but this is wrong apparently. It's meant to be Fe2O3(s) + 6e– —> 2Fe(l) + 3O 2–(s) according to the textbook. My main question is why do we have to include the oxygens in this half-equation? (oxygens don't seem to change oxidation number)

[JellyBeanz]

The thermite process can be used to weld lengths of railway
track together. A mould placed over the ends of the two rails
to be joined is filled with a charge of aluminium powder and
iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron, which joins the rails together.

a Write a half equation for the conversion of iron(III) oxide
to metallic iron.
b Is the half equation you wrote for part a an oxidation or a
reduction process?
c Write the overall equation for the thermite process.


My initial answer for a was Fe3+ (s) + 3e- --> Fe(l) but this is wrong apparently. It's meant to be Fe2O3(s) + 6e– —> 2Fe(l) + 3O2–(s) according to the textbook. However, I got c correct. How does one approach question a - would you guys work out the overall equation first and then the half equation(s)?

I just work out the half equations and add the half equations together to get the overall equations and cancel out the electrons.