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?)

  • God replied to this.

    bluebettafish

    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?

          • God replied to this.

            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:

            1. The polarity of the stationary/mobile phase
            2. The length of the column
            3. The temperature of the column
            4. The mobile phase flow rate
            5. 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.

              For this year's study design, we only need to know about HPLC (and perhaps a little bit about the paper one).

              Hope this helps 🙂

                hey guys sorry to bother you again but is the molar volume of gas 22.7 or 24.8?

                  I should add, if ur in y11 - u may have to know about paper chromatography and all that

                  But u shouldnt if ur in y12

                  bluebettafish

                  The constant we use in VCE Chemistry for the molar volume of gases is 24.8 V/n because we are dealing with gases that are SLC (Standard Lab Conditions). This is irrelevant to our study design but 22.7V/n is the molar volume at STP (standard temperature and pressure).

                  SO 22.8 V/n is the magic number 😉

                    Hey guys, can someone please explain this to me:

                    Consider the following statements about reactions involving enzymes:
                    I Coenzymes can be metallic ions.
                    II Coenzymes can accept electrons during a reaction.
                    III Coenzymes can donate groups of atoms during a reaction.
                    IV Coenzymes can increase the rate of reaction.
                    Which of the statements above are correct?
                    A. I and IV only
                    B. II and III only
                    C. I, III and IV only
                    D. II, III and IV only
                    How is point IV correct- they dont directly contribute to increasing rate of reactions do they?

                    Consider an alkaline hydrogen fuel cell.
                    Which one of the following will reduce the amount of usable energy produced per kilogram of fuel entering
                    the fuel cell?
                    A. supplying oxygen in excess
                    B. increasing the porosity of the cathode
                    C. reducing the operating temperature from 40 °C to 20 °C
                    D. reducing the rate at which oxide ions move through the electrolyte

                    Also How is D wrong for this qs? If oxide ions move more slowly through the electrolyte, wouldn't the production of electricity decrease?

                      N1ck3lGall1um Coenzymes can speed reactions up as some enzymes can't bind the substrate without a coenzyme.

                      2nd question isn't well written (it examines a specific cell type whereas VCAA says no specific cell required in the study design) although I think the correct answer is C since electrochem is based on equilibrium and if you remove heat, the system will want to generate heat to compensate according to Le Chat's. D can't be right since OH- moves through the electrolyte not O2-

                        N1ck3lGall1um

                        Yeah that question was weird af.

                        You just have to know that, when temperature increases, fuel cells are more efficient.

                        Or you can think of it in an energy profile diagram. However, when the temp is higher, it starts higher. (So the ∆H value is higher). Apparently this is wrong (scientifically).... because temperature doesn't effect enthalpy or something... I can't remember.

                        Hope this helps 🙂

                          Hi! Can anyone here explain why branched alkanes have a lower melting and boiling point? I've been told they have a greater surface area compared to straight chain alkanes but weaker dispersion forces, but I'm sort of confused how that works.