VCE Chem Questions Thread
hey guys! just a question...
when drawing a sequence of amino acids, can you draw each amino acid similar to how it is presented in the data book, or does vcaa expect you to show all bonds? like instead of NH2 doing H-N-H, etc?
tia!!
hey guys I'm a bit confused for titrations with the whole endpoint and equivalence point thing - i know that they're not the same - but if they're not the same what's the actual difference between them?
- Edited
bluebettafish I’ve made a foolproof guide to titrations. It also answers your question:
X titrated against Y = Y (standard in burette/titrant) added to X (analyte/titrand in conical flask)
Titre = volume of titrant/standard solution required to reach endpoint
You wanna be using an indicator with a similar endpoint (pH at which it changes colour) to the predicted equivalence point (pH where reactants are present in stoichiometric ratio). You predict the equivalence point based on the strengths of the acid and the base. Note that you don’t necessarily need an indicator for redox titrations since some redox reactions produce visible colour changes
The burette is essentially a scientific beer bong and you wanna be rinsing it with the standard solution to avoid diluting it and hence overestimating the analyte concentration. Ditto for rinsing a bulb pipette. The conical flask should be rinsed with deionised water only since you don’t want residual analyte in there to overestimate your analyte concentration. You can wash volumetric flasks with deionised water too since you’re adding water when making up the standard solution anyways
Most of titrations is about stoich, they might give you a dilution factor to take into account at the end
Billzene omggg thank you so much!!! 
So I'm not too sure if my understanding is correct - but if we use a really dodgey indicator (which we know is not the same as the predicted equivalence point) - it would give us an endpoint that is completely different to the equivalence point? And so, we want to use an indicator that has a pH matching the predicted equivalence point so that we can visualise the equivalence point? (I hope that makes sense? )
Basically, I guess I'm wondering - can an endpoint happen anywhere and is the endpoint solely dependent on the indicator?
bluebettafish The equivalence point depends on the strength of the acid/base. Eg for HCl + KOH, the equivalence point would be around 7 since it’s a strong acid and a strong base. However for ethanoic acid + KOH, the equivalence would have a pH above 7 as ethanoic acid doesn’t fully ionise to produce H+ which neutralises KOH. And yes you should use an indicator with a similar endpoint range to the predicted equivalence point
Billzene awesome thanks so much!
also for a weak acid and weak base - would the equivalence point be at around a ph of 7?
bluebettafish
It should be around 7 but since you won't see a sharp increase/decrease in pH on the titration curve, you usually don't titrate weak acids against weak bases and vice versa
Hey guys, how does one go about trying to memorise organic reaction pathways and the chemicals that cause each step?
SnekiSnek I've been just doing a heap of exam practice questions on organic pathways and using active recall. I usually do this through blurting, meaning I'll initially read through the content and reagents, and then I'll try and recall them on paper. The second time I do it is without reading about it prior. You'll then get to the point where you'll just know them off by heart. I also use blurting for other memory recall (that said I probably used it more for bio).
Anki/quizlet can be good too…
hey guys - what's the difference between retention time and retardation factor n chromatography? are they the same?
also for all the organic analysis stuff like volumetric analysis, NMR, etc, etc if we were asked to explain the 'principles' of it or its 'applications' what would you say for each?
Like for IR would you just say "identifies functional groups through changing vibrational energy states" and applications: "determines purity of substance, identifies molecules" - Mass Spec "fragments molecules into positively charged ions", (lol not sure on applications here?) NMR "changes nuclear spin stage", chromatography "separates molecules based on adsorption to stationary phase and desorption in mobile phase", volumetric "reacting known concentration with unknown concentration"? (What extra information would I need to include for each of these if this were a question to come up?) Is applications just that it helps us identify molecules?
also lol what's the solvent front in chromatography? (is it just some molecule we put on the sheet and see how far it travels?)
Hi!
So the solvent used depends on the molecule… and is usually stated in the question (as either polar/nonpolar).
We don’t need to know about retardation factor for VCE ‘22, just the retention time relationship.
For that long SA question, it is probably best to give them everything, and hope you say what they want. VCAA is really bad like that….
But use the no. of marks as an indication.
The solvent is used to carry the substance through the chromatography medium (like the tube thing in HPLC, or the paper).
If the substance is polar, and the solution is polar, it will desorb to the mobile phase. (Being attracted to the solvent, it sticks to it more than the paper/medium)
So the solvent will affect the retention time for a molecule.
God thanks so much !!! also lol in my textbook - there's only retardation factor for like TLC?
also - in column chromatography - is the solvent continuously added since the eluent is leaving the column?
sorry for another question - but do we have to know about gas chromatography?
- Edited
bluebettafish
My chem notes:
Chromatography:
Chromatography is used to separate and analyse complex mixtures. All methods involve:
- A mobile phase - which is the solvent containing the sample (often a liquid)
- A stationary phase - which is the surface the mobile phase flows over/through.
The retention time (time taken for a component to pass through) is dependent on the ability for different substances to interact with each phase. This includes how well the stationary phase adsorbs to the sample - and how well the mobile phase dissolved the sample (desorption). It is often influenced by the polarity of the molecules.
High performance liquid chromatography (HPLC) uses a solid stationary phase consisting of tiny particles. The mobile phase is pumped in at high pressure and the eluent stream (mixture of sample + mobile phase) is monitored by a UV detector. This is presented on a chromatogram.
Each peak on a chromatogram represents a component of the sample. If a set of standards is used, with known concentrations, a regression line can be calculated providing a relationship between the area under a peak and the concentration.
Factors affecting retention time include:
- The polarity of the stationary/mobile phase
- The length of the column
- The temperature of the column
- The mobile phase flow rate
- The surface of the stationary phase
Explain how HPLC works:
- HPLC relies on the repeated transfer of components from the stationary and mobile phase
- Components that are more strongly adsorbed to the stationary phase will have higher retention times as they travel slower than components that more strongly desorb to the mobile phase.
- The different retention times lead to separation.
An increase in temperature will mean that:
- the components in the HPLC column are less adsorbed to the stationary phase and more strongly desorbed in the mobile phase
- as the components are more soluble with an increased temperature.
- This leads to a decreased retention time.