Major help required (1 Viewer)

[*Pooja*]

Hello ppl. Summarising season has begun in the study sector of my life yet again. I couldn’t do some of the dot points in this topic b/c my teacher ended up finishing the topic with exercise sheets. i need help! So pls any hints or suggestions or real help will be appreciated. Ok here goes:

(1) flame test colours for the syllabus cations (ie. barium, calcium, lead, copper and iron)
can someone confirm this for me:
 barium – yellow or green?
 calcium – brick red
 copper – green?
 Lead – I don’t know
 Iron (Fe2+ and Fe3+) – I don’t know

(2) AAS – do we have to be able to answer questions on explaining how the AAS apparatus works like with the bulbs and everything?

(3) 3.3.2. Gather process and present information to describe and explain evidence for the need to monitor levels of one of the above ions in substances used in society. I chose lead. Where can I find more up-to-date info on it?

(3) 3.3.4. Analyse information to evaluate the reliability of the results of the above investigation and to propose solutions to problems encountered in the procedure.
Our class didn’t do the filtration method experiment. We did this decanting one where we left the precipitate to set overnight several times and then we poured off the top aqueous layer. In that case, what kinds of things should I mention for this dot point?

(4) 3.3.5. Gather, process and present information to interpret secondary data from AAS measurements and evaluate the effectiveness of this in pollution control.
Is this just being able to draw and interpret the calibration graphs (concentration vs absorbance)? What is it exactly that we have to mention in the second part – ‘evaluate the effectiveness of this in pollution control’?

(4) 4.2.4. Describe the formation of a coordinate covalent bond.
I just defined a coordinate covalent bond and mentioned that it is identical to a normal covalent bond. Is that all we have to do here?


(5) 4.2.6. Compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding.
For this dot pt and also 4.2.7. (compare the properties of the gaseous forms of oxygen and the oxygen free radical) – so far the properties I mentioned are: shape of molecule, bonding, colour, odour, mp and bp, solubility in water, density, reactivity and uses. I included an electron dot diagram as well. Any others?
And for the second part of dot pt 4.2.6.…I don’t know.

(6) 4.2.10. Discuss the problems associated with the use of CFCs and assess the effectiveness of steps taken to alleviate these problems.

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

These look like two major dot points and probably is worth something like 6-8 marks in the HSC and I don’t have anything on it! Where can I find info for this stuff?

(7) 4.3.3. 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.
Where can I find the latest info for this? My notes for this so far only go to about three small 2/3-sentence paragraphs.

Ok. This was probably annoying but I seriously need help especially for section four of this topic. Once again, any help will be appreciated. Thanks ppl !

 

[kimmeh]

1) barium –apple green
calcium – brick red
copper – blue-gren
Lead/Iron - umm our teacher told us that we're not supposed to be putting these elements in flames to start off with but she said that they have no colour

2) what bulbs? i'm sure all you have to do is just describe how it works (the specific light used (this bulb?), the flames, the receiver etc..)

3)talk about the causes, effects and sources of lead. http://www.dec.nsw.gov.au/home.htm will help you-also, trusy ol' google

4)Evaluate the validity and reliability of the process you did and then talk about what other procedures that can be done to minimise errors-this dot point is asking you to identify your possible places of error and to propose ideas into ways of fixing it for future reference.

5)Yes understanding and reading calibration graphs. Evalutate its effectiveness - is AAS advanteous for pollution control? Why/why not?

6)Yes, you have described it already

7)propeties of 02 and 03 are fine. Though a diagram i dont think is a property. the second part is asking you to explain why there are differences in their propeties.
-The boiling point of diatomic oxygen is lower than that of the ozone as diatomic oxygen has a lower molar mass requiring less energy in the boiling process.
-solubility: Non-polar O2 does not form strong intermolecular forces in the polar water. Ozone has a bent structure, which provides for some polarity of the molecule in its interaction with water
-chemical stability: Ozone is easily decomposed into oxygen molecules: The two identical oxygen to oxygen bonds in ozone consist of a single bond and a partial bond. This results in lower stability of the ozone molecule, compared with the diatomic oxygen molecule

8)A good idea is a chemistry textbook-thats where i got all my information from. Do a google search as well for CFCS. NASA has some good pictures and graphs from satellites monitoring the ozone concentrations. Treaties signed by different countries and the efforts to stop the production of CFCs

9)Search up HCFs and HFCs. i typed into google "Alternatives to CFCs" and got oads of sites.

my strong suggestion is that you consult the textbook before asking uneccessary questions. i pulled these answers from Conquering chemistry and Google. I also think you need to understand your verbs more clearly, as all it is, is the verbs. Here's the glossary of key words: http://www.boardofstudies.nsw.edu.au/syllabus_hsc/glossary_keywords.html which will help you in answering your dot points. btw, you numbered 3 and 4 twice

 

[CM_Tutor]

I could answer these, but it'd take ages and so I agree with Kimmeh. Try a more focussed question / questions, after some more work.

Kimmeh, the BP difference of oxygen and ozone has more to do with polarity than it does molar mass (because of dipole-dipole interactions). Also, the chemical stability question is much more satisfactorily answered in terms of molecular structure and the origins of the polarity. I know that the commonly used answer is to talk of bond strength because O₂ is supposed to have a double bond, but frankly that argument is chemically highly dubious.

 

[kimmeh]

ahaha thanks CM, i just copied that section out of someone's notes. So dont blame me i was just about to make a thread about it just to make sure because it didnt sound too right. So thanks CM

 

[kimmeh]

Our teacher told us its the greater MW though, come to think of it. Is then okay to say it is due to the stronger dispersion forces between the molecules? Ohh and btw, that guy got the notes from hsc.csu

 

[xiao1985]

I could answer these, but it'd take ages and so I agree with Kimmeh. Try a more focussed question / questions, after some more work.

Kimmeh, the BP difference of oxygen and ozone has more to do with polarity than it does molar mass (because of dipole-dipole interactions). Also, the chemical stability question is much more satisfactorily answered in terms of molecular structure and the origins of the polarity. I know that the commonly used answer is to talk of bond strength because O₂ is supposed to have a double bond, but frankly that argument is chemically highly dubious.

cm tutor, but doens't oxygen have the same electronegativity??? so wouldnt' there be no dipole dipole interaction?? but rather dispersion forces???

 

[CM_Tutor]

Kimmeh, I wasn't trying to blame you. Sorry if it sounded that way.

I see these answer in relation to MP / BP and stability of ozone quite often.

To clarify, with BP of ozone and oxygen:

1. It is true that, due to it having a higher molar mass (and hence more electrons), the number and overall strength of the dispersion forces between molecules is higher for ozone than it is for oxygen. As a result, you would expect the BP of ozone to be higher, as more / stronger intermolecular interactions require more energy to overcome, and hence boiling occurs at a higher temperature.

2. However, the difference in BP is too large to be due just to differences in dispersion forces - look at the difference in BP between ethane and propane. The difference in molar mass is about the same, as are the molar masses themselves about the same. However, the difference in BP of ozone / oxygen is nearly twice as large.

3. The reason for the difference is that ozone is polar whereas oxygen is not. As a result, ozoone has dipole-dipole interactions between its molecules in addition to dispersion forces, and these interactions must also be overcome for ozone substance to boil, explaining the size of the difference.

4. There is other evidence for the polarity of ozone (other than its measured dipole moment). Ozone is markedly more soluble in water than is oxygen, this is easily explained if ozone is polar, but much harder to explain if it is not.

 

[CM_Tutor]

cm tutor, but doens't oxygen have the same electronegativity??? so wouldnt' there be no dipole dipole interaction?? but rather dispersion forces???

Xiao1985, as you know, polarity is explained in terms of electronegativity differences at school. Also, as you have noted, electronegativity differences would predict that O₃ couldn't be polar.

Unfortunately, electronegativity differences are not the only possible source of polarity. If you consider the usual structure for O₃, O=O-O, the central O is electron deficient, and the final O is electron rich. This can be indicated by calculating formal charges (they'd be in your textbook, if you don't remember them / haven't yet met them in lectures). The first O is formally uncharged, the central one is +1, and the final one is -1. Thus, polar.

These formal charges are also useful for explaining the instability of O₃. With a formally positively charged oxygen atom, it is not surprising that ozone is unstable, especially given it is the second most electronegative element. If an atom that is extremely good at attracting electrons is positive, then it is very likely to react in some way to gain extra electrons. As a result, ozone is highly reactive, and hence unstable.

The argument about partial bonding that Kimmeh mentioned above (which is really an argument about bond orders) is widely used in the HSC, but is fundamentally flawed, for two reasons:

1. Resonance is a stabilising, not a destabilising, phenomenon (look at benzene, for example).

2. Oxygen, O₂ doesn't have a double bond. (HSC People - do not try this argument in an exam. It is highly likely that a teacher will object to the statement that I just made, correct though it is. Talk about formal charges, or even use the argument that Kimmeh mentioned - just be aware that the situation is no where near as simple or clear cut as the HSC likes to portray. )

 

[Xayma]

2. Oxygen, O₂ doesn't have a double bond. (HSC People - do not try this argument in an exam. It is highly likely that a teacher will object to the statement that I just made, correct though it is. Talk about formal charges, or even use the argument that Kimmeh mentioned - just be aware that the situation is no where near as simple or clear cut as the HSC likes to portray. )

Although Im not stupid enough to try the argument (because I couldnt back it up) what would the bonding best be described as?

 

[CM_Tutor]

Although Im not stupid enough to try the argument (because I couldnt back it up) what would the bonding best be described as?

O₂ has a diradical ground state. This means that the two O atoms are joined by a single bond, with 5 non-bonding electrons on each O atom (ie two pairs and an unpaired electron). (Yes, I do realise this violates the octet rule.)