HSC Chemistry Question Thread

[Dragomistress]

The anode is always the site of oxidation and the cathode is always the site of reduction

How do I know which is the site of oxidation and the site of reduction?

[jazcstuart]

How do I know which is the site of oxidation and the site of reduction?

For a galvanic cell you figure out which undergoes oxidation and reduction using the list of standard potentials, which tells you the reduction potential. The metal with the higher reduction potential will undergo reduction (of course  ). In an electrolytic cell this is reversed; the negative terminal supplies electrons, therefore forcing reduction, so the metal with the lower reduction potential will undergo reduction (I think you only need to know about electrolytic cells if you are doing Shipwrecks).
Hope this makes sense

[Fergus6748]

How do I know which is the site of oxidation and the site of reduction?

For a galvanic cell you figure out which undergoes oxidation and reduction using the list of standard potentials, which tells you the reduction potential. The metal with the higher reduction potential will undergo reduction (of course  ). In an electrolytic cell this is reversed; the negative terminal supplies electrons, therefore forcing reduction, so the metal with the lower reduction potential will undergo reduction (I think you only need to know about electrolytic cells if you are doing Shipwrecks).
Hope this makes sense

I've always been thinking of the Standard Potential as the higher metal on the chart oxidises and thus the anode, Aha. Thus, the lower one reduces (cathode). I've never hheard about it being told the other way. Anox Redcat is a popular mnemonic to remember that the anode oxidises and the Cathode reduces. Electrolytic cells are in industrial chemistry as well. Dot point 5, for the production of sodium hydroxide.

[horse9996]

For saponification, you know how you add oil, sodium hydroxide and a water/ethanol mixture into a beaker which is heated to make the soap? Why is the ethanol added, what's its purpose?

[Fergus6748]

For saponification, you know how you add oil, sodium hydroxide and a water/ethanol mixture into a beaker which is heated to make the soap? Why is the ethanol added, what's its purpose?

Hey, from what I've been told the ethanol is just there to help the reaction between the oil and the sodium hydroxide. It doesn't have any other purpose from that. Hope this helps!!

[horse9996]

Hey, from what I've been told the ethanol is just there to help the reaction between the oil and the sodium hydroxide. It doesn't have any other purpose from that. Hope this helps!!

Does it act as a catalyst?

[Fergus6748]

Does it act as a catalyst?

Not too sure to be honest, aha. All I was told is that it assists with the reaction. I think it essentially is, as it doesn't take part in the reaction but helps the reaction.

[horse9996]

Not too sure to be honest, aha. All I was told is that it assists with the reaction. I think it essentially is, as it doesn't take part in the reaction but helps the reaction.

Ok, so if they ask what the purpose of the ethanol is I just say it assists in the saponification process while the mixture is being heated. Thanks for the help

[jazcstuart]

Ok, so if they ask what the purpose of the ethanol is I just say it assists in the saponification process while the mixture is being heated. Thanks for the help

I don't do industrial chem, but remeber that ethanol is a solvent which can dissolve both polar and non-polar substances. So if the oil is non-polar, it won't dissolve in water but it will dissolve in ethanol, then the ethanol dissolves in water. I would say this is why ethanol is used, hope this helps.
(Ps you should look at the shoutouts and appreciation thread if you haven't already  )

[horse9996]

I don't do industrial chem, but remeber that ethanol is a solvent which can dissolve both polar and non-polar substances. So if the oil is non-polar, it won't dissolve in water but it will dissolve in ethanol, then the ethanol dissolves in water. I would say this is why ethanol is used, hope this helps.
(Ps you should look at the shoutouts and appreciation thread if you haven't already  )

Ohh that makes heaps of sense, also with why the soap has to be seperated at the end (soap is similar with a polar head and non polar hydrocarbon tail, so it allows water so dissolve oils/grease, I think of it like a tadpole)

[Dragomistress]

Hey, how am I meant to find the isomer for this question?

[Fergus6748]

Hey, how am I meant to find the isomer for this question?

Hey, so isomers are just the same molecule with a different structure, it's a bit hard to describe without diagrams but for example, both 1-bromo-1-chloropentane and 1-bromo-2-chloropentane are both isomers of that molecule. So, the isomers of C3H6BrCl are:

1-bromo-1-chloropentane
1-bromo-2-chloropentane
2-bromo-2-chloropentane
2-bromo-1-chloropentane

Those are the only isomers of C3H6BrCl so the answer is 4.

With the other q, the first thing you need to do is make an equation:

HCl + NaX --> NaCl + HX

With that you need to find the moles of hydrochloric acid that was needed to neutralise the solution so:

Moles = Concentration x Volume
= 0.100 x 0.0244
= 0.00244 moles

From above, we know that the moles of HCl equals the moles of NaX so:

nHCl=nNaX
=0.00244 moles

We know that Moles = Mass/Molar Mass so Molar Mass = Mass/Moles

MNaX= (1)/(0.00244)
= 409.8360656....g/mol

Therefore, the molar mass of NaX is 410 g/mol, I think, pretty sure what was the answer? Hope this helps!!

[horse9996]

Hey, how am I meant to find the isomer for this question?

Draw out the carbon chain and count the possible arrangements (remember naming conventions eg 1-chloropropane is the same as 3-chloropropane). I don't think there's any other way really

Also see working for the titration question attached.

[Dragomistress]

Hey, so isomers are just the same molecule with a different structure, it's a bit hard to describe without diagrams but for example, both 1-bromo-1-chloropentane and 1-bromo-2-chloropentane are both isomers of that molecule. So, the isomers of C3H6BrCl are:

1-bromo-1-chloropentane
1-bromo-2-chloropentane
2-bromo-2-chloropentane
2-bromo-1-chloropentane

Those are the only isomers of C3H6BrCl so the answer is 4.

With the other q, the first thing you need to do is make an equation:

HCl + NaX --> NaCl + HX

With that you need to find the moles of hydrochloric acid that was needed to neutralise the solution so:

Moles = Concentration x Volume
= 0.100 x 0.0244
= 0.00244 moles

From above, we know that the moles of HCl equals the moles of NaX so:

nHCl=nNaX
=0.00244 moles

We know that Moles = Mass/Molar Mass so Molar Mass = Mass/Moles

MNaX= (1)/(0.00244)
= 409.8360656....g/mol

Therefore, the molar mass of NaX is 410 g/mol, I think, pretty sure what was the answer? Hope this helps!!

Sadly, they are both wrong
I should have posted the answer but for 14 it is C and for 18 it is A.

[horse9996]

Sadly, they are both wrong
I should have posted the answer but for 14 it is C and for 18 it is A.

My one is the solution to a short answer in the 2001 paper, I assume that multiple choice is a similar method

What paper is q14 from?