jyce how can I improve in chem?
These are my errors:
- pH questions - always get them wrong, dont even know what pH means. how can i fix this?
- I cant seem to get those fuel cells and like commercial cells. i am good/alright with galvanic and electrolysis but not industrial cells
- spectroscopy etc.
Hi cosine,
First of all pH is a measure of the
power, or concentration, of
hydrogen ions. As pH =
-log
10[H
+], the greater the pH the
lower the concentration of hydrogen ions, and vice versa. For strong acids, we assume that they have ionised in aqueous solution completely and therefore if, for example, we had a 0.1 M hydrochloric acid solution it will have ionised to produce 0.1 M of hydrogen ions and its pH will be -log
10(0.1) = 1. Calculating the pH of a weak acid, on the other hand, is more complicated as there is not a complete ionisation; in fact, we often safely assume that a weak acid has not ionised at all. In this case, we need to write an expression for the equilibrium constant of the reaction, excluding the concentration of water, and go from there. If you had 0.1 M solution of a weak acid, it would ionise to produce less than 0.1 M of hydrogen ions and therefore have a pH
greater than 1. This is by no means everything you need to know on pH, but it's some background information that might help you out.
On to your second point, industrial galvanic cells (i.e. primary, secondary and fuel cells) will most definitely be assessed. Here's some background information again:
- these three types of cells are all galvanic cells, and therefore: the cell reaction is spontaneous, with the oxidant being higher up on the electrochemical series than the reductant, with a positive E
0 value for the cell reaction, and with the anode being negative and the cathode being positive
- primary cells cannot be recharged, meaning that the cell reaction cannot be reversed once it has reached equilibrium
- secondary cells on the other hand are able to be recharged,
because the products of the discharge reaction remain in contact with the electrodes (i.e. the sites of reaction) and therefore remain available to become reactants in a reverse reaction, but an external power source is required as the reverse reaction is non-spontaneous
- when a secondary cell is recharging, it acts like an electrolytic cell: the recharge reaction is literally the reverse, so the anode becomes the site of reduction (i.e. the cathode) and the cathode becomes the anode
but the polarities of the electrodes remain the same so the cathode is now the negative electrode and the anode is now positive- fuel cells are not rechargeable but they also don't go flat like primary cells, and this is because they have a
continuous supply of reactants.
Some tips regarding these commercial cells include to remember that they are galvanic cells and not electrolytic, to identify off the bat the anode and cathode and their polarities, the direction of electron flow and the direction of ion flow, to acknowledge that the electrolyte tends to participate in the cell reaction but with no net consumption or production (e.g. H
+ ions might be produced at the anode, but then the same amount will be consumed at the cathode), and lastly to acknowledge that 'KOHES' does not always work to balance cell reactions as, for example, the environment may be alkaline rather than acidic.
Lastly, with spectroscopy note that you do NOT need to know how the instrumentation (e.g. the monochromator) works, but rather for each spectroscopic technique you need to know which region of the electromagnetic spectrum it utilises, how this region of the spectrum interacts with the analyte, and how to interpret the data obtained:
- atomic absorption spectroscopy utilises visible light which promotes the valence electrons of atoms to higher energy levels; the data obtained is a series of absorbance measurements, one for the analyte and the rest for a series of standards, and the absorbances of the standards are graphed against the standard concentrations in what is known as a calibration graph, in order to relate absorbance and concentration in a mathematical relationship, AND then the concentration of the analyte is simply read off; be sure to account for dilution factors
- UV-visible spectroscopy utilises, in addition to visible light, ultraviolet light which has enough energy to promote electrons in
atoms, ions and molecules; the data interpretation is basically the same as with AAS
- IR spectroscopy utilises infrared radiation which promotes molecules to higher vibrational energy levels (i.e. makes the covalent bonds in molecules more active); what comes out of IR spectroscopy is an IR spectrum and all you need to do is estimate the wavenumber ranges of major peaks and then compare these to the wavenumber ranges in the data book in order to identify the bonds responsible; particularly noteworthy peaks are those at about 1700 cm
-1, indicative of a C=O bond, and a very broad band at around 2500-3300 cm
-1, indicative of the O-H bond within a carboxyl functional group; IR spectroscopy is useful for identifying functional groups
- lastly, NMR spectroscopy utilises radio waves which change nuclei to a higher spin state; NMR spectra provide a number of useful pieces of information: the number of peaks/peak sets indicates the number of
different hydrogen/carbon environments, the relative areas the peaks/peak sets indicates the relative numbers of hydrogen/carbon atoms in each different environment, the peak-splitting of a high-resolution
1H NMR spectrum indicates the number of hydrogens in
adjacent environments according to the 'n + 1 rule' (e.g. in propane, the CH
2 environment has six hydrogens in the adjacent environments and therefore the peak representing the CH
2 group will be split into a 6 + 1 = septet), and lastly the chemical shifts, measured in ppm, could be useful (e.g. a peak set at 9-13 ppm of a
1H NMR spectrum is indicative of a carboxyl group); NMR spectroscopy is particularly useful for distinguishing between isomers.
Again, this is by no means an extensive explanation.
As for improving in these areas, I would recommend the same thing I said to anat0my: practice questions, preferably VCAA ones! It's really the only way to get better in Chemistry. You'll hopefully start to pick up on the main things VCAA draws from each of the above topics. It's hard for me to give you more specific help on these topics without more specific questions from yourself. So, if when doing practice questions on these topics you're having trouble, shoot me some more questions! Keep in mind there are many things in the course aside from pH, commercial galvanic cells and spectroscopy, and therefore obviously these three topics will only make up a fraction of the exam.
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