Chemistry 3/4 2013 Thread

[thushan]

Pencil 4 lyf.
What happens if you make a mistake?

Cross it out, start on new line.

[clıppy]

Cross it out, start on new line.

That's ridiculous. There's only so many lines.

[lolipopper]

That's ridiculous. There's only so many lines.

im sure they can read pencil. the scanners arent so bad that a simple HB pencil wont get read. i sure hope they can read pencil because i make so many mistakes, i would have to use two exam booklets with a pen

[thushan]

That's ridiculous. There's only so many lines.

Eh, worked for me. If I ran out of lines I would put an asterisk next to a qn, then draw an arrow to bottom of page, and complete answer there

[clıppy]

Eh, worked for me. If I ran out of lines I would put an asterisk next to a qn, then draw an arrow to bottom of page, and complete answer there

So it's seriously recommended to use pen because they scan the exam? That's good to know, I would've just kept using my pacer the whole time

[thushan]

So it's seriously recommended to use pen because they scan the exam? That's good to know, I would've just kept using my pacer the whole time

Yeah, and pencil is more likely to not scan through properly and there would be issues. It's just a disclaimer, so that if a student writes in pencil and scanner doesnt scan it properly, VCAA can be like "well we told you to use a pen so suck it up"

[lolipopper]

question: the STAV 2009 exam 2
asked "determine the heat of reaction for the fermentation of glucose in kj mol-1"

so i worked it out and got 68.7kj mol-1. however the answer has (delta)H= -68.7kj mol-1.
i was under the impression that heat of reaction is not the same as enthalpy. am i right?

[brightsky]

nope heat of reaction means exactly the same thing as enthalpy of reaction. there are also terms such as 'heat of vaporisation', 'heat of condensation', etc. all mean the same thing as enthalpy.

[lolipopper]

then whats the term for just the magnitude of the energy produced or consumed in a reaction?

[brightsky]

if you were asked to calculate the amount of heat/energy evolved as per the amount ratios specified in the balanced chemical equation, then you would put 68.7 kJ. rarely will you be asked to do this however.

[lzxnl]

Honestly, heat of reaction can be a slightly misleading term. You know that the heat is positive, but the enthalpy change is negative. Problem?

Actually, there's no problem if you think about what heat actually is. Heat is a transfer of energy, so it actually can have a sign. When the enthalpy is negative, that means the enthalpy of the system (slightly more complex than just the chemical energy of the system but don't worry about that) drops. Hence, what you can think of heat of reaction being is the heat transferred by the surroundings to the system. An endothermic reaction, for instance, has a positive heat of reaction because the heat transferred to the system is positive.

It's just that when we think of heat, we only think of the heat transferred to the surroundings as that is what we feel.

[Alwin]

Eh, [pen] worked for me.

Maybe that's because you don't make any mistakes thushan?

I always use pen too (and for maths as well) but it gets petty messy when I realise I've made some silly error in part (a) then have to change my working and answers for the rest of the question =w=

[lzxnl]

Cue graph error in maths exam

[thushan]

Maybe that's because you don't make any mistakes thushan?

Nah, I've made a number of mistakes in the exam and had to cross out and rewrite. I hate pencil coz it takes forever to rub out whereas with pen I just cross out the line and move on.

[Holmes]

I found this to be interesting, quote from page 169 of Hienemann Chem book, biochemical fuels.

It's about making ethanol by fermentation, and then evaporating and then dehydrating the fermentation mixture to extract "99.7% pure" ethanol. Then:

In Australia, the final ethanol is then ‘denatured’ (poisoned) by adding up to 5% petrol to make it unsuitable for consumption as drinking alcohol. The process falls under the control of Australian excise laws.

Why do you suppose they denature it?

Also, interestingly, in Australia, it is "Legal for individuals to manufacture their own alcohol without paying excise with two provisions, that they do not employ the use of a still, and that they do not sell the product".

Aaaaaaand, you need a license to own a still (home distillery kinda thing I'm guessing) larger than 5 litres, and the rate to pay "is in excess of $71 (AUD) per liter of alcohol produced" at home. http://en.wikipedia.org/wiki/Homebrewing#Legality That is so expensive, and it has to be paid even if the alcohol one makes is not to sell but for personal use.

Thought it was mildly and temporarily interesting.