Recently Asked Questions - Chemteach - University of Canterbury - New Zealand

Recently Asked Questions

  1. 3.5 Unit standards to assess Level 3 Organic
  2. 2.4 standard: Intermolecular forces
  3. General: Chemiluminescence demos
  4. General: Code of Practice
  5. 2.3 Significant figures sufficiency

Q1. I am keen to find exemplars assessments for the 6344 Level three Unit Standard in Organic Chemistry. I am hoping to offer this as an alternative to Achievement Standard 3.5. But have no experience with Unit Standards. Any way you can help?

Go to our new website for Chemistry teachers ( www.chemteach.ac.nz ) and to the page on internal assessment for Year 13 where you will find a large number of unit standards exemplars. The page is pass word protected but you can obtain the password by following the instructions on the page. You need to check that these exemplars match the standard as they were written some time ago and they may require a little tweaking. They are a very good start point, however.

Q2. There appears to be considerable variation (in different texts etc) as to which intermolecular forces of attractions, the "van der Waals forces" generic term actually encompasses.  We avoid the debate by using the phrase temporary (or instantaneous) dipole attractions in an analogous manner to the permanent dipole attractions used in this resource. Apart from avoiding the debate about what van der Waals forces are it gives the students a much clearer picture of what type of forces are involved in each case.

A. If you visit the Activities folder on the Chemteach website ( www.chemteach.ac.nz ) there are two activities that you can download that give students practice with inter and intramolecular forces.

Q3. Do you have any ideas about some chemical reactions that can be done in the classroom that could produce light, much like a glowstick etc. ?

A. A number of common substances (such as the quinine in tonic water) will fluoresce when exposed to a "black light" (UV source). To get luminescence in the classroom the easiest way involves the oxidation of luminol - mixing luminol and an appropriate oxidant will produce a lovely blue chemiluminescent glow for some minutes in a darkened room. Fancy mixtures are given in texts such as Chemical demonstrations by Shakhashiri but strong solutions of hydrogen peroxide with a metal catalyst will work as below.

The reaction can either be done in a beaker or tubing can be arranged in a spiral or other form with a funnel at the top. Two solutions are required:  Anhydrous sodium carbonate (4 g) is dissolved in 500 mL of water and then 0.2 g luminol (3-aminophthalhydrazide) is added and stirred until dissolved. Sodium bicarbonate (24 g) is then added, followed by 1 mL of concentrated aquesou ammonia. Finally, 0.4 g of copper sulfate pentahydrate is added and the solution is stirred until most of the copper salt has dissolved. Dilute to 1 L. Take 5 mL of 30% hydrogen peroxide and dilute to 1 L.

Presentation: Darken the room and then mix the two solutions either in a beaker or by pouring them simultaneously through some tubing. If desired a small amount of fluorescein can be added to give a yellow-green light.

Q4. I have a question about chemicals we have at school that are not stated on the school exempt labs list, none of them are expressly forbidden. What is your interpretation of this? Do we need to get rid of them, there is a mixture of these from titanium to heptane. if you need more details of some of the others please get back to me and I will give you a better idea of what they are - there are probably around 60ish items with many of these being organics.

A. P18 of the Code of Practice has a flow diagram that explains how to make a decision on this. Essentially you check Is it on the allowable list in table 1 Appendix 2? If so, OK to use. Is it on the forbidden list Appendix 3? Dispose of appropriately and safely. 

Is it on the ERMA list of transferred substances - go to http://www.ermanz.govt.nz/search/registers.html  Search for name and choose "Controls Word" or "Controls PDF" to determine its classification. If it belongs to a class that is banned from schools (see Table 4.1 in Code of Practice)then it must be disposed of safely. e.g pentane is classified as 3.1 ( a flammable liquid and is not exempt so needs to be disposed of).

If the chemical is not listed on Ermanz site you need to search on the internet (e.g. http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/index.htm for the appropriate SDS sheet to determine its safety classification and then again compare it with the list of banned classes from Table 4.1. Whew!

Q5. Last year the moderator told us that two sig fig errors out of eight would limit the student to merit even though they got every question correct in paper (just not to right number of sig figs). The moderator said all questions had to be correct. I read the comments about the sig figs on this site which suggests otherwise. Now i'm just confused to whether i ignore the moderator or be more realistic to what I think should be allowed. Can you help please clear this up for me?

A. I would not expect any errors with significant figures in answers to calculations. Errors suggest the student doesn't really understand correct use of significant figures. 

However, a volume that is stated as from a pipette has this accuracy built into it and I would be happy to read it as such. If a volume, say 20 mL is recorded as a pipetted volume, I would be happy to interpret this as 20.00 mL even if this is not stated.  

Often an assessor only looks at the answers leading to excellence opportunities for correct use of significant figures but the requirement for correct usage applies to the whole activity.

There will soon be new activities on TKI for version 2 of AS 90763. You will notice that correct significant figure usage is required for all answers in these activities. I will publicise this when it comes on line.
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