HSC Chemistry Question Thread

[cnimm2000]

Hey!! Pressure is caused by gas molecules hitting the side of the container - so the more molecules in the container, the higher the pressure will be. So, when there is an increase in pressure, equilibrium will shift to favour the side that reduces the pressure (by Le Chatelier). This will be the side that produces fewer moles, as this will reduce the number of molecules in the reaction vessel, thus reducing pressure.

When there is a decrease in temperature, equilibrium will shift to favour the side which increases the temperature (by Le Chatelier) - this will be the exothermic reaction. If the equation has an exothermic forward reaction, it will shift right; if it has an endothermic forward reaction, it will shift left.

Let me know if any of that doesn't make sense!!

This is a really great explanation.
Thank you so much. This really helped me to understand the concept better.

[riya.bhavsar]

Hi,

I am was writing my practical report. Can someone please explain to me what is reliability, accuracy and validity in an experiment, how can we improve that reliability, accuracy and validity.

Thanks

[RuiAce]

Hi,

I am was writing my practical report. Can someone please explain to me what is reliability, accuracy and validity in an experiment, how can we improve that reliability, accuracy and validity.

Thanks

This is explained quite well here.

[riya.bhavsar]

This is explained quite well here.

Thank you that helps

[Calley123]

Hey,
In the 2013 Exam it said: Construct separate flow diagram to show the steps used in the production of polyethylene and those used in the production of a recently developed biopolymer (5 marks)
If tho the example answer and the marking criteria didn't include structural equations, is it a good idea to include them anyway or it is just a waste of time?

Thanks  

[RuiAce]

Hey,
In the 2013 Exam it said: Construct separate flow diagram to show the steps used in the production of polyethylene and those used in the production of a recently developed biopolymer (5 marks)
If tho the example answer and the marking criteria didn't include structural equations, is it a good idea to include them anyway or it is just a waste of time?

Thanks  

First note that the answers BOSTES/NESA provide are not necessarily of band 6 level. They will just grab any random answer for their sample response.

I would say it is a great idea, because it makes the whole process a lot clearer. At the same time, where possible you could just replace it with some chemical equations.

[cnimm2000]

Hey guys:
I was doing this dot point:
P. 3.6 Identify data sources, choose resources and perform a first-hand investigation to determine and compare heats of combustion of at least three liquid alkanols per gram and per mole
And i came across a question asking calculate the heat of combustion per gram and per mole of each fuel.
To calculate the heat of combustion per gram I've heard that we divide the molar heat of combustion by the molar mass of the alkanol.
i just wanted to confirm if thats correct or if we divide by the mass of the alkanol burnt
Thanks in advance.

[Natasha.97]

Hey guys:
I was doing this dot point:
P. 3.6 Identify data sources, choose resources and perform a first-hand investigation to determine and compare heats of combustion of at least three liquid alkanols per gram and per mole
And i came across a question asking calculate the heat of combustion per gram and per mole of each fuel.
To calculate the heat of combustion per gram I've heard that we divide the molar heat of combustion by the molar mass of the alkanol.
i just wanted to confirm if thats correct or if we divide by the mass of the alkanol burnt
Thanks in advance.

Yes! Dividing kj/mol by g/mol will result in kJ/g, which is the HoC per gram

[kaustubh.patel]

Hey chem livers I have a question about titration.
Why do we use specific indicators like phenolphthalein (only indicates from pH 8.6 to 10) and why not a universal indicator.

[kauac]

Hey chem livers I have a question about titration.
Why do we use specific indicators like phenolphthalein (only indicates from pH 8.6 to 10) and why not a universal indicator.

Phenolphthalein has 1 obvious colour change as [H+] changes, making it easy to find the end point of the titration. The colour/shade of universal indicator constantly changes as [H+] is altered, and is therefore hard to observe a distinct end point when the mole ratio is balanced. Hope this helps!

[cnimm2000]

Yes! Dividing kj/mol by g/mol will result in kJ/g, which is the HoC per gram

Sorry i don't quite understand what you mean dividing it by grams per mole - are you referring to the molar mass?

[RuiAce]

Sorry i don't quite understand what you mean dividing it by grams per mole - are you referring to the molar mass?

Yeah, because g/mol is basically the SI units that correspond to the molar mass.

She's taking advantage of the fact that in formulas in science, the units actually cancel out in the numerator and denominator. In your case, you initially had the molar heat of combustion, measured in kJ/mol. If you divide the molar heat of combustion by the molar mass, then units wise you divide kJ/mol by g/mol, which lands you to kJ/g. Incidentally, kJ/g is the units used for heat of combustion per gram

[cnimm2000]

Yeah, because g/mol is basically the SI units that correspond to the molar mass.

She's taking advantage of the fact that in formulas in science, the units actually cancel out in the numerator and denominator. In your case, you initially had the molar heat of combustion, measured in kJ/mol. If you divide the molar heat of combustion by the molar mass, then units wise you divide kJ/mol by g/mol, which lands you to kJ/g. Incidentally, kJ/g is the units used for heat of combustion per gram

Ok thank you

[Calley123]

Hey,
When I was writing half-equations for galvanic cells, I noticed that in the answers they stated that 1/2 Cl(g) {on data sheet} is the same thing as Cl2 (G). Please explain me how or why that is true.

Thanks

[RuiAce]

Hey,
When I was writing half-equations for galvanic cells, I noticed that in the answers they stated that 1/2 Cl(g) {on data sheet} is the same thing as Cl2 (G). Please explain me how or why that is true.

Thanks

It's not.

You don't have 1/2 Cl_(g). You have 1/2 Cl_2(g), which is half a mole of the chlorine element in the gaseous phase. Note that in the chlorine element molecule, you always have two chlorine atoms, not one.

And then when it gets reduced (i.e. gains electrons), you end up with Cl^-_(aq), which is one mole of the chloride ion. You only have one chloride ion appearing at a time.