40daysofCHEM 9.4.4 = Part 18

Some Videos - Titrations and Haloalkanes

I decided to make some spur-of-the-moment videos in Period 6 today whole I had some free time in L2.

The first was going to be an in-class PowerPoint presentation in response to the BOS and the HSC 2012 Haloalkane question.

The other two I thought might be useful to watch before we started titrations. I'm rethinking the second titration one - while making the video I figured out a better way to explain one idea. Perhaps I'll use that idea when explaining it in class.

The video belowgoes with the notes in our 9.3.4 booklet - the colour scheme was inspired by an awesome badge that I saw recently.

 

9.4.4 HSC Question Feedback

This set of questions was a perfect example of why it is so important to be thorough with your Chemistry. Most people for most extended response questions lost a mark for a small error or because thye didn’t use an important Chemistry term in their answer. As a one off it does not really matter but across the whole paper it counts for approximately 25% of the available marks.

But generally the answers were very good and you should feel proud that you have completed so many HSC questions this early in the course. There are many students who wont even start doing this sort of work until stuvac. We have a whole 6 months to clarify any misconceptions, reinforce the depth of out understanding and fine-tune our exam technique.

 Specific feeback

2001

Q27) This was a tricky question to navigate because the source was unusual. It was a diagamatic graph. But because it was a graph it contained data and that measn you need to specifically refer to it. Eg “Ozone is mostly found in the stratosphere (up to 4.7ppm), while it is a maximum of 0.9ppm in the troposphere”. You then needed to describe its effects ‘nearby’ and ‘ip high’, which we did well BUT you need to sue the correct terms. Don’t call it the lower atmosphere  - call it the troposphere. Don’t say the ozone is in the upper atmosphere – say the stratosphere.

2002

Q25) b) This should have been an easy two marks but you need to be able to draw Lewis dot diagrams correctly (dots or lines – NOT both lines and dots for the bonds). You should always label the coordinate covalent bond.

Q25) c) Remember the dot next to the free radical. Also a phrase you need to work into ALL of your answers about CFC’s impacting ozone is “Cl free radicals catalyse ozone decomposition”.

2003

Q12) Because the graph showed the breakdown of one mole, you could just read off the graph C:F:Cl = 2:4:2 and remember wee don’t simplify covalent compounds (only ionic).

2004

Q12) We will cover this in the Acidic Environment topic

Q27) Answered really well but you need to refer to the data in the table. You need to describe CFC’s impacts (another opportunity to write: “Cl free radicals catalyse ozone decomposition”), refer to HCFC and HFC’s. You need to say the + and – of each and assess. which is most suitable. Also – 7 marks is a LOT – you MUST use chemical equations in all answers >3 marks

2006)

Q5) These isomer Q can be tricky – but I just draw myself some diagrams and eliminate the impossible multiple choice answers as I go.

Q28) The image shows a global analysis of ozone – thus they must have used a satellite system. BUT  - you MUST know that TOMS is not longer being used. They are using the OMPS (Ozone mapper profiler suite) on the AURA NPP Satellite. Finally –when it says discuss the ozone hole over Antarctica you must mention general depletion by CFC’s and the ozone hole that forms in Spring. But also NB that there have been some recent signs that the annual hole-size is not getting bigger.

2007

Q15) – I’ve said it enough times above ‘“Cl free radicals catalyse ozone decomposition”…

Q16) b) Once again an extended response question greater than three marks – You must use at least one balanced chemical equation (Eg ozone absorbing UV)

2008)

Q18) b) Apart from drawing a correct diagram of both, the other key things are to:

i)                    Note that ozone is bent and has a dipole, thus it is more soluble in water than oxygen (like dissolves like)

ii)                   Ozone greater size means it takes greater energy to vibrate – thus higher mpt and bpt

iii)                 Note that the single bond in ozone is easier to break then the double bond in oxygen – thus it is more reactive

It is very important to note that it is NOT the polarity or the coordinate covalent bond that makes ozone reactive. Once formed the coordinate covalent bond acts as a regular covalent bond.

Q21) A lot of students skipped this but really – it would have ten 30 seconds to look up the BOS and download a copy. I also wrote a description of the graph in the ‘graph space’ to help you out.

2009) – 2010)

Q12 & Q4 on the topic of naming – please don’t forget all the tricky rules (include hyphens, 2 – di, 4 = buta etc)

9.4.4 Feedback

PART 1  - THE ATMOSPHERE

1)   9.4.4.1.1 Describe the composition and layered structure of the atmosphere

 (A) Of all the ozone in the atmosphere, 10% is found in the troposphere. This is not the same as saying ‘10% of the troposphere is ozone’ (it would be hard to squeeze in that 10% considering that ~80% of the troposphere is N₂ and ~20% is O₂)

(P) Be careful with your diagrams, the layers of the atmosphere get thicker (in km) as you head toward space – your diagrams don’t have to be to scale but they should reflect this. Also, make sure the ozone layer is IN the stratosphere, it does not take up the whole stratosphere.

2)    9.4.4.1.2  Identify the main pollutants found in the lower atmosphere and their sources

3)     9.4.4.1.3   Describe ozone as a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant

(Q) Always include the ^● for free radicals (even O^●)

(R) When discussing the dangers of UV, it is important to mention both its effect on plant photosynthesis and on animal cells (e.g. sunburn, DNA damage, skin cancer).

PART 2 – THE OZONE MOLECULE

4)   9.4.4.1.4  Describe the formation of a coordinate covalent bond

(B) Remember from yr 11 – All atoms ‘want’ to get a ‘full outer shell’ of electrons (ie a ‘noble gas configuration’). Non metals can obtain this stable full outer shell by gaining electrons or by sharing them with other non-metals (ie forming a covalent bond).

(C) Always draw ozone with its bent shape. It is an important structural feature. It is also important to draw all the electrons in their correct positions around each atom.

5)     9.4.4.1.5   Demonstrate the formation of coordinate covalent bonds using Lewis electron dot structures

(S) Be careful, a single covalent bond has a PAIR of electrons being shared (ie one from each atom). This means there are four shared electrons in an…

(X) Lewis dot diagrams need the square brackets [ ] around ions with the charge specified e.g. a sodium ion is [Na]⁺. For clarity it can help to draw the electrons for each atom with different symbols (eg x, o, ●) and it is advisable to label the coordinate covalent bond. See here for the correct way to illustrate the fomation of a polyatomic ion with a coordinate covalent bond).

6) 9.4.4.1.6     Compare the properties of the oxygen allotropes O₂ and O₃ and account for them on the basis of molecular structure and bonding

(D) It is vital to define all Chemistry terms properly. You need to know the correct (thorough) definition of allotrope.

(E) There is nothing wrong with putting the actual values for O₂ and O₃ desnity, solubility, mpt and bpt in your table but you DO NOT need to know these values – just that O₃ has higher values than O₂.

(F) O₂ and O₃ have a low mpt BECAUSE they are covalent molecular ie they HAVE have weak intermolecular bonds which require LITTLE energy to break allowing the molecules to separate. ALSO - because thye have low bpt they are gases - because they are gases they have low density.

(G) You need to explain the differences in mpt, bpt and water solubility for O₂ and O₃. Firstly, O₃ is slightly polar (ie has dipole-dipole intermolecular bonding as well as weak dispersion) due to the uneven distribution of electrons in the molecule – thus it forms slightly stronger bonds with water molecules (a polar solvent, like dissolves like) and thus has higher solubility). This stronger intermolecular bonding, AND because it is a larger molecule requiring more energy to vibrate, means it takes more energy to separate the molecules giving it a higher mpt and bpt.

(H) The single covalent bond in ozone is easier (take less energy) to break than the O=O double bond. Thus O₂ which only has the O=O double bond is less reactive then O₃.

(T) It is vital to know the difference between intermolecular and intramolecular bonds. Clearly, these terms only apply to covalent molecular substances (the only substances that are ‘molecules’, not ‘lattices’) – so the strong intramolecular bonds are the covalent bonds between atoms and the intermolecular bonds are the weaker forces (bond) between molecules (eg hydrogen bonding… & …).

(Y) Oxygen free radicals don't have a measured bpt and mpt etc because to measure these values you need to have a number of them together - but as soon as you out these reactive molecules together they react to form O₂ preventing the properties of O^● being measured.

(Z) To compare the strucutre of O^●, O₂ and O₃ you need to detail their # atoms, intermolecular bonds, types of intramolecular bonds (v. imp!) and draw a Lewis dot diagram. NB - sometimes you will see ozone drawn with two single bonds and a 'dotted arch'. The dotted arch is called a ‘resonance bond’ – we don’t worry about that in yr 12 HSC Chem – just draw it with the bent shape and with ONE double bond and ONE single coordinate covalent bond.



7)     9.4.4.1.7   Compare the properties of the gaseous forms of oxygen and the oxygen free radical

PART 3 – CHLOROFLUOROCARBONS & IMPACT

9)     9.4.4.1.8      Identify the origins of chlorofluorocarbons (CFCs) and halons in the atmosphere

10)      9.4.4.1.10   Discuss the problems associated with the use of CFCs and assess the effectiveness of steps taken to alleviate these problems

(I) The ozone hole is a seasonal (Spring) event in the Antarctic where there is rapid depletion of O₃, forming the ozone ‘hole'.** Ozone depletion refers to the general decrease is global O₃ concentrations that have occurred over the last 40 years.
** NB when the ozone hole 'breaks up' in late Spring, severly ozone depleted air can spread north to Southern Australia.

(J) The cold dark Antarctic winter results in ‘ice’ crystals forming in the stratosphere. These ice crystals catalyse the reaction between chlorine nitrate and hydrogen chloride to form hydrogen nitrate and chlorine gas. It is this chlorine gas that decomposes to produce the chlorine free radicals when the sunlight returns in spring (the large amount of Cl^●  results in rapid ozone depletion and the ozone hole)

(U) You must mention (along with your series of equations) that the Cl^● CATALYSES ozone destruction (ie it takes part in the reaction, providing a reaction path with low activation energy and thus increasing the rate of reaction, but it is reformed and not consumed in the reaction)

11)    9.4.4.2.1     Present information from secondary sources to write the equations to show the reactions involving CFCs and ozone to demonstrate the removal of ozone from the atmosphere

12)      9.4.4.1.11    Analyse the information available that indicates changes in atmospheric ozone concentrations, describe the changes observed and explain how this information was obtained

(K) Dobson spectrometers monitor a wavelength that O₃ does and a wavelength that O3 does not absorb – by comparing their relative intensities the concentration of O₃ can be calculated. Similarly satellite systems monitor wavelength of light that ozone does and does not scatter to determine O₃ concentrations. There is a nifty little video about this here: http://www.youtube.com/watch?v=BCAQR-4ue0U

(L) You need to know at least one advantages and one disadvantage of balloons, Dobson spectrometers and satellite-based spectrometers. Also: Nimbus, Aura and Suomi-NPP are satellites, TOMS, AURA and OMPS are the ozone monitoring instruments they carried (respectively). The one being used now is OMPS. If you talk about TOMS you sound outdated and will be penalised for not using reliable (ie current) resources). There is a nifty little video about this here: http://www.youtube.com/watch?v=BCAQR-4ue0U

13)    9.4.4.2.3     Present information from secondary sources to identify alternative chemicals used to replace CFCs and evaluate the effectiveness of their use as a replacement for CFCs

(M) When comparing CFC’s, HCFC’s and HFC’s:

a) Say what atoms they contain

b) Note that Cl is the halogen that damages ozone

c) NB that HCFC’s decompose in the troposphere (so little reaches the stratosphere) but NB they have some ODP (and define this the first time you use it) because thye still contain Cl

d) NB that HFC’s have no ODP because…

(N) When writing a criteria, be smart and be specific. Rather then saying that “One criteria to be examined is the ODP”. Say “To be a successful replacement, the chemical must have low to no ODP to overcome the problems produced by CFC’s” – after that, if you say that HFC’s have no ODP you have addressed the criteria and it is clear that HFC’s are a good option.

(O) When assessing the Montreal protocol, you should consider the fact that it has been followed worldwide, and that CFC’s use and productions rates have dropped and that Cl concentration in the atmosphere has started decreasing and that O₃ concentrations are currently relatively steady.

(V) When you state that CFC’s continue to be released into the atmosphere  despite the Montreal Protocol (and thus the recovery will be slow), you need to specify that this is from old disposed CFC’s containing equipment in landfill

14) EXTRA – you DO need to know ppm and ppb in the HSC course

(W) – Oops, the answer to (a) should be 0.2 GRAMS, and (d) should be 0.013 ppb (read the Q Dr Blurg!)

Interesting superconductor article

Hi Physicists,

We haven't talked much about the 'real' reason why Cooper pairs don't interact with the lattice but this article does a pretty good job. See the four paragraphs after the BCS diagram for a good explanation.

But as, as with all simplifications of quantum physics, NEIGH!