VCE Chemistry Question Thread

[Vaike]

Would it be necessary or useful to memorise the specific order of different bio/fossil fuels in order of highest to lowest energy content?

Probably not. It's a good idea to have some understanding of general trends in energy content, but I don't believe you specifically need to know the energy value for each fuel, or exactly how they compare. It won't hurt to do so, but just a having some idea of where abouts they lie in relation to each other should suffice.

[TheAspiringDoc]

^^ thanks again Vaike!

Could someone please explain transesterification. I get that it is supposed to convert triglycerides to biodiesel esters, but I am confused as to whether to view it as two reactions or one, as well as what the roles of KOH, NaOH and H2SO4 are. Also, there is something to do with needing a basic or acidic environment for the reaction and to reverse it?

Thanks

[Vaike]

^^ thanks again Vaike!

Could someone please explain transesterification.

No problem As you mentioned, a transesterfication reaction involves converting one type of ester into another type of ester, hence the name! In the case of producing biodiesel, triglycerides derived from plant oils or animal fats undergo transesterification to produce Fatty Acid Methyl Esters (FAME), a.k.a biodiesel. Often in VCE I found the reaction to generally be considered one step, in which a triglyceride is reacted with 3 methanol molecules in the presence of a hydroxide catalyst in order to produce 3 FAME and a glycerol molecule. However, it can be described as two steps in that first the triglyceride undergoes hydrolysis, before the fatty acid esters undergo esterification (H₂SO₄ is a catalyst for esterificaiton).

NaOH/KOH are commonly used catalysts for this reaction, and if I recall correctly (a bit rusty!) the reaction can occur in both acidic or basic conditions, however I remember only really seeing basic conditions during VCE. The reaction is also reversible as you mentioned, but again I don't really remember having to know much about this throughout VCE chem.

[Bri MT]

^^ thanks again Vaike!

Could someone please explain transesterification. I get that it is supposed to convert triglycerides to biodiesel esters, but I am confused as to whether to view it as two reactions or one, as well as what the roles of KOH, NaOH and H2SO4 are. Also, there is something to do with needing a basic or acidic environment for the reaction and to reverse it?

Thanks

+1 to everything vaike said!

Just so you know, you are likely to find this easier to understand after beginning unit 4 as you'll spend time on the mechanics of esterification then. I wouldn't stress too much if you have trouble right now - just make sure you set a reminder to revisit it.

[Ajakara]

Hey people!1!1 So several (and I mean the entire year level above me and my tutor and several others) people have told me that some of the figures in the Data Booklet have changed (e.g. mainly for gases, such as 1atm is no longer 101.3kPa and is now 100kPa). However, I just printed the data booklet from the VCAA Website and the figures have not changed (e.g. 1 atm still equals 101.3kPa, not 100kPa as I have been told). Can someone please clarify which figures I should use, if this is a mistake from VCAA or am I the only one with this problem :/// ? Cheers in advance

[Vaike]

*snip*

Heya! As long as you've downloaded the 2017 data booklet, the information in there is definitely correct. What you may instead be referring to is that the conditions defined for SLC and STP changed from 2016 to 2017. Both used to be defined at a pressure of 101.3 kPa, which is equivalent to 1 atm in the old study design, but now are considered to be at 100 kPa by VCAA. So basically if you see a question referring to SLC conditions, you should now used 100 kPa, not 101.3 kPa for the pressure.

[VanillaRice]

Hey people!1!1 So several (and I mean the entire year level above me and my tutor and several others) people have told me that some of the figures in the Data Booklet have changed (e.g. mainly for gases, such as 1atm is no longer 101.3kPa and is now 100kPa). However, I just printed the data booklet from the VCAA Website and the figures have not changed (e.g. 1 atm still equals 101.3kPa, not 100kPa as I have been told). Can someone please clarify which figures I should use, if this is a mistake from VCAA or am I the only one with this problem :/// ? Cheers in advance

In addition to what Vaike has said above, I suspect there may have been confusion between that and another change - to Table 5. Unit Conversions on page 6. 100kPa = 0.987atm is the same as 101.3kPa = 1atm, it's just they've changed the 101.3kPa to 100kPa.

[Ajakara]

Ahh Alright, what I thought was happened was VCAA rounded 1atm to 100kPa and 750 mmHg to 100kPa. Alright that's fair enough, so nothing has changed except the STP and STC right?

[TheAspiringDoc]

Hello
For the equilibrium formula

(LaTeX fail  )
Doesn't this not account for temperature? Why is this not an issue since increased temperature can change the value of K by favouring the endothermic half reaction?

Edit: I have another question; with KOHES, if you actually set your working out in that order in an assessment, will marks potentially be deducted for not adding states until the final step of working?

[chooby]

Hello
For the equilibrium formula

(LaTeX fail  )
Doesn't this not account for temperature? Why is this not an issue since temperature can change the value of K?

I think when using this formula, you'd have to keep it at the same temperature. Regardless of their starting concentrations, K is the same at a given temperature.

[Vaike]

Hello
For the equilibrium formula

(LaTeX fail  )
Doesn't this not account for temperature? Why is this not an issue since increased temperature can change the value of K by favouring the endothermic half reaction?

Correct, the value of K is dependent on and only on the temperature of the equilibrium system. In VCE, it is always stated that the equilibrium system is kept at a constant temperature when dealing with equilibrium constants.

[TheAspiringDoc]

So with a given set of reactant and product concentrations, the value of K will be the same if the calculation is performed at a constant temperature of 280K or 1000K? (Provided they are the same state..)

Edit: I have another question; with KOHES, if you actually set your working out in that order in an assessment, will marks potentially be deducted for not adding states until the final step of working?

Bumping

Thanks 

[Vaike]

So with a given set of reactant and product concentrations, the value of K will be the same if the calculation is performed at a constant temperature of 280K or 1000K? (Provided they are the same state..)

Yup, you got it In regards to your bump, you won't be penalised for not including states in your working, but of course you must include them in your answer to receive full marks. So yes, adding states in the last step is fine.

[VanillaRice]

Edit: I have another question; with KOHES, if you actually set your working out in that order in an assessment, will marks potentially be deducted for not adding states until the final step of working?

I would say no. As long as you make your final answer clear, you shouldn't have a problem. In fact, by that logic, you could lose marks at any step, since your answer isn't complete. So probably not

EDIT: Beaten by Vaike

[TheAspiringDoc]

Thank you all

Sorry - I'm still uncertain about the whole equilibrium K value stuff with regards to changes in temperature and pressure.

So how (and under what circumstances) is it that ΔTemperature or ΔPressure affects K? Is it only whilst the ΔT or ΔP is occurring, or is it after having changed the temp/pressure already?
Example:
For aA + bB <=> cC + dD with given values of the concentrations and coefficients, if K is measured at 8*C, and then the temperature is changed to 50*C, will a different value of K be measured (not that I'm NOT meaning measuring it whilst K is changing; only measure it after the temperature change has been performed)

Note sure if I'm explaining my confusion correctly, but I'd greatly appreciate if anyone can try to decipher and answer my babble haha