VCE Chemistry Question Thread

[HopefulLawStudent]

So quick (and probably dumb) question but can distilled water become undistilled? Like if we were using distilled water for like the dissolution of some chemical, and we had distilled the water a loooooong time ago and it hadn't been stored properly or whatever, is it possible that it was no longer "distilled" and therefore, could be a potential source of error? Is that possible? And does this question even make any sense?

[conic curve]

So quick (and probably dumb) question but can distilled water become undistilled? Like if we were using distilled water for like the dissolution of some chemical, and we had distilled the water a loooooong time ago and it hadn't been stored properly or whatever, is it possible that it was no longer "distilled" and therefore, could be a potential source of error? Is that possible? And does this question even make any sense?

Distilled water is water that has had many of its impurities removed through fractional distillation. Fractional distillation involves boiling the water and then condensing the steam into a clean container.

[blacksanta62]

Hey guys, I have my SAC tomorrow and I would like to know what are some experimental errors which occur when burning food to see the heat released by the burnt food? I cannot for the life of me think of any errors
Is the above an example of heat of combustion?
Also, there was some pre-lab questions:
- How do I convert 2000 calories into KJ?
- And why do food labels use KJ/g instead of KJ/mol? I was thinking it's because most of the general public would not be able to identify what a mol is, in chemistry terms anyway
Thank you

[sweetiepi]

Hey guys, I have my SAC tomorrow and I would like to know what are some experimental errors which occur when burning food to see the heat released by the burnt food? I cannot for the life of me think of any errors
Is the above an example of heat of combustion?
Also, there was some pre-lab questions:
- How do I convert 2000 calories into KJ?
- And why do food labels use KJ/g instead of KJ/mol? I was thinking it's because most of the general public would not be able to identify what a mol is, in chemistry terms anyway
Thank you

Hey!
A couple of errors that I found in my calorimetry pracs were that the food didn't burn completely and that the flame kept getting extinguished, which affects the temperature of water, as we recorded a lower temperature of the water, which affected the calculated q(or E)=cmΔt.

Heat of combustion is defined as the energy released when a substance burns in oxygen completely, so I'm pretty sure this is calorimetry prac is an example of this.

To convert from calories into kJ, you multiply the number of calories by 4.18

However I'm not 100% sure why kJ/g is used, however, I believe it has to do with why we use specific heat energy, as that is measured in J/g ^oC.

[sweetcheeks]

kJ/mole is only valid if you are testing a pure substance. Foods contain a mixture of substances and hence does not have a specific molecular formula.

It is similar to the biodiesel question on the 2014 VCAA paper.

[Sarinamfgh]

A standard 0.103M sodium Carbonate solution was prepared in a volumetric flask. 25.00mL aliquots were measured into a conical flask and titrated against a dilute solution of approximately 0.2M hydrochloric acid. Which of the following indicators in most suitable for use in this situation?

• Methyl red
• Phenol red
• phenolphthalein
• bromothymol blue

Hey guys the answer is methyl red. It will be amazing if someone can explain why? I thought that it would be bromothymol blue becuase its Ph range is where we are expecting the equivalence to be.

[Gromekk]

im pretty noob so maybe wait until someone can confirm, sodium carbonate isnt that strong of a base, so its conjugate is a weak acid. HCl is a strong acid and its conjugate is a very weak base, so you would expect the equivalence point to be <7. looking at the data book methyl red is the only option  that changes colour at a ph<7 (out of the options)

edit, just realised bromothymol blue is 6-7.6 as well, not sure how i'd decide then.

[blacksanta62]

Another question guys, with the attached question, why do they find delta T of the reaction by taking away the temperature after calibrating?
And if asked to find the "energy change" of a reaction, what do I have to do?
Thank you
Edit:

kJ/mole is only valid if you are testing a pure substance. Foods contain a mixture of substances and hence does not have a specific molecular formula.
It is similar to the biodiesel question on the 2014 VCAA paper.

Thank you

[gsinghmann]

Hey guys do you know where I can find the 2015 vcaa chemistry 3&4 worked solutions. I know that if I go to the vcaa website I can look at the examiner's report but they don't show much working out.

[gsinghmann]

Hey guys do you know where I can find the 2015 vcaa chemistry 3&4 worked solutions. I know that if I go to the vcaa website I can look at the examiner's report but they don't show much working out.

[blacksanta62]

Hey guys do you know where I can find the 2015 vcaa chemistry 3&4 worked solutions. I know that if I go to the vcaa website I can look at the examiner's report but they don't show much working out.

Don't think there are any unfortunately
Someone can confirm though

[zsteve]

Another question guys, with the attached question, why do they find delta T of the reaction by taking away the temperature after calibrating?
And if asked to find the "energy change" of a reaction, what do I have to do?
Thank you
Edit:Thank you

They calibrate the calorimeter by burning benzoic acid or something. After calibration, we run our reaction. Hence, the change in temperature is given by [temp after reaction]-[temp before reaction], but [temp before reaction]=[temp after calibration].

Hence, Delta T = [temp after reaction]-[temp after calibration].

To find the energy change of reaction, you need to use \(q = mc\Delta T\) or \(q = CT\) where C is the heat capacity (i.e. calibration factor) of your calorimeter. This gives the amount of heat (energy) that was released by the reaction/went into the water to raise the temp.

[sweetcheeks]

Hey guys do you know where I can find the 2015 vcaa chemistry 3&4 worked solutions. I know that if I go to the vcaa website I can look at the examiner's report but they don't show much working out.

I am not sure if that exists. If there is anything you would like explained or shown feel free to ask here.

[HopefulLawStudent]

Explain why gas chrom generally cannot be used to test for the presence of compounds with relative molecular mass greater than 300.

In my answer, would it matter to VCAA whether or not I wrote "It can't be volatised" vs. "it can't be vaporised"? Is there like a preferred answer? Can the two words be used interchangeably?

[jyce]

In my answer, would it matter to VCAA whether or not I wrote "It can't be volatised" vs. "it can't be vaporised"? Is there like a preferred answer? Can the two words be used interchangeably?

No, they're not technically the same; I'd stick with "vaporised". Regardless of which word you use, however, your answer doesn't actually explain WHY molecules with a molar mass of greater than 300 g mol^-1 often cannot be analysed by GC. I would write something like "Because often such molecules cannot be vaporised in a gas chromatograph without decomposing".