General Questions and Answers - Chemteach - University of Canterbury - New Zealand

General Questions  and Answers

  1. Significant figures
  2. Limiting reagents demonstration
  3. NCEA assessment design
  4. Complex ions
  5. When to use kJ and when to use kJmol-1
  6. MindSpring
  7. Safety
  8. Thinking Skills Resource
  9. SmartBoard resources
  10. Inserting Flash animations into a PowerPoint presentation
  11. Field trips in the Auckland area
  12. Is the term NaCl(aq) correct?
  13. General: Chemiluminescence demos
  14. General: Code of Practice

Q1. It crossed my mind that if any of the data given to the student to use was only to 2 significant figures, then 3 significant figures is not valid. Physics are very keen on students noting this as an indication to the accuracy of their data. Chem and Biol use 3 significant figures routinely even if eg: a conc is given as 0.5 mol dm-3.

A. Essentially this is correct and you may have noticed that for this reason almost all data provided in NZQA exemplars (whether external or internal) is now provided to 3 significant figures - even if it means having to write 0.100 mol etc.

Note that data such as titre readings are often known to 4 significant figures (eg: 20.45 mL) and molar masses may also be sometimes given to 4 significant figures but, as you point out, the accuracy of the overall result is usually limited to the least accurate bit of data. This why a little flexibility is allowed in the requirement regarding consistent and appropriate use of significant figures for excellence.

However, I'm sure we don't need to get too hung up about this (we can leave that to the Physicists) but it is a good reminder to us all to get in the habit of always giving data to 3 significant figures where possible. Back to top.

Q2. An attendee at last years Why Chemistry is Difficult Course requested details on the limiting reagents demo so here it is.

  • 6 small flasks each containing 20 mL of 1.0 M ethanoic acid.
  • Solid sodium bicarbonate is pre-weighed into a balloon and attached to the rim of each flask.
  • The successive amounts for each flask are: 0.4 g, 0.8 g, 1.2 g, 1.6 g, 2.4 g and 4.0 g
  • Successively lift the balloon up on each flask to release the solid contents into the ethanoic acid (shake well).
  • The limiting conditions occur at the 4th flask so no further increase in volume occurs for final flask ballons.

Students are required to give their predictions for each balloon before they are activated. Back to top.

Q3. I have a query regarding the new system of marking where the highest grade is awarded for a question. Suppose the test had overall 27 Achieve / 15 Merit / 5 Excellence. What would be the sufficiency statement based on the new system?

Would I be correct if I set it as 14 Achieve / 10 Achieve + 8 Merit/8 Achieve + 5 Merit + 3 Excellence? If a student got 10 Achieve / 9 Merit / 1 Excellence what would their grade be? Alternatively, if she got 10 Achieve / 9 Merit / 4 Excellence what would her grade be? Can the Merits and Excellences be brought down and if so how many?

The test has 2 large questions, 26 question parts in all.

Overall there are 27 opportunities for Achieve, 23 opportunities for Merit and 5 opportunities for Excellence.

  • 7 parts are straight Acheive.
  • 15 parts are Achieve/Merit.
  • 3 parts are straight Merit.
  • 5 parts are Achieve/Merit/Excellence.

Students completed it in 50 minutes.

A. Ok – a few points and principles here. Firstly the “system” you refer to is hardly new as recording of only the highest grade attained for a particular question has been used for quite a few years.

Secondly, it is not a question of bringing Merit's and Excellence's down.  A question that is more complex and requires a student to make links between concepts will result in some students demonstrating some recall or understanding about chemical principles, but not enough to achieve the higher grade.  In this case they can get some credit for the level of literacy that they are demonstrating .  

Decisions are essentially based not only on the student attaining the majority of the available opportunities at each level, but also the overall sufficiency requirements increase as the grade moves fro Achieve to Merit to Excellence.

So, in your example, the student will need to correctly meet the requirements of at least 14 of the 27 total possible answers (assuming they are all valid questions) to be awarded an Achieve. (The 14 answers can include any of the Achieve, Merit or Excellence type questions.)

To attain Merit, because there are a total of 23 Merit opportunities, the student would need to meet the requirements of at least 12 of the Merit (or Excellence) opportunities plus perhaps a further 6 Achieve opportunities to keep up a reasonable sufficiency requirement at Merit level.

Finally, to meet Excellence, students would need to meet the requirements of 3 of the 5 Excellence opportunities, perhaps 13 or 14 Merit opportunities and a further 6 or 7 Achieve opportunities so that the overall sufficiency level would be around about 80% -85% of total opportunities.

So in this situation a student getting 10 A/9M/1E would only be an Achieve and if she got 10A/9M/4E it would only be a Merit as she has a total of 13 at Merit or higher plus the required extra Achieves to meet sufficiency. She would not be an Excellence as although she has met 3 of the 5 Excellence opportunities, she has only got a total of 10 extra Merit opportunities (counting the one “unused” Excellence as a Merit) - not the required extra 13 (or 14) separate Merit opportunities required for overall sufficiency.

The other issues relates to the design of the task itself which is always fraught with difficulty when one hasn't actually seen the task!

Having a total of 27 Achieves, 23 Merits and 5 Excellence opportunities in 50 minutes sounds as if they are very small outcomes and quite possibly repeating the same learning outcome in some cases. Given that we are wanting to encourage holistic rather than atomistic learning it is recommended that individual grades be awarded for clumps of similar questions. eg: rather than awarding 3 different grades for balancing 3 different redox questions, a single grade could be awarded for correctly writing 2 of the 3 available equations. Apart from reducing the total number of grades to “count” at the end, this also helps ensure that the overall grade is a fair reflection of the student ability over all aspects of the Achievement Standard – not just a limited no of the outcomes. eg: if there were 5 questions on oxidation number, 5 on identifying redox and 5 on balancing equations, a student could successfully meet the requirements for Achievement without even having to answer any questions on the appearance of common oxidants or any other aspect covered by the standard.

Therefore when designing a task you need to consider a balance of questions that fairly reflect all of the outcomes of the standard and also want to clump outcomes wherever possible to ensure the final grade judgement is a fair reflection of their performance in the overall standard.

Finally, the proportion of Achieve, Merit and Excellence questions needs to be considered. In his case there seems to be too heavy an emphasis on the low end type questions and probably clumping at this level would definitely help this imbalance. Back to top.

(Warning – the following communication may prove hazardous to your sanity)

Q4. Aqueous Copper ions. Are they [Cu(H2O)4]2+ or [Cu(H2O)6]2+ from an exam point of view? I know about the equatorial H2O's being closer to the Cu ion than the H2O's at the top and bottom of the Cu ion. The shape in some books is given as square planar, in others as octahedral.

If it is 6 H2O's, then surely we should all be writing [Cu(H2O)2(NH3)4]2+for the ammine complex. And similarly for the chloro complex too. I actually don't care which it is. I just want the official exam answer.

A . Firstly there is no such beast as “an official exam answer” as a marking panel always has to have some flexibility in what they will and will not accept as an acceptable answer. So as always with these responses, they need to be seen simply as the best advice available based on my prior experience and current contacts within the system.

So let's put the question in perspective first before attempting to answer.

Since the revision of the Level 3 Chem standards, much of what we used to cover in relation to transition metal complex ions has effectively been removed from the assessment (as opposed to the curriculum) requirements.

As well as the knowledge of transition metal species involved in redox reactions required for AS 90696, the new “combined' C3.4 Ach Std (# 90780) now includes only the following relevant areas:

  • special characteristics of transition metals (variable oxidation state, colour) related to electron configuration.  Transition metals will be limited to iron, vanadium, chromium, manganese, copper and zinc
  • Lewis structures and shapes (up to six electron pairs about the central atom for molecules and polyatomic ions, including those with multiple bonds)

Given that the first requirement is limited to relating variable oxidation state and colour to electron configuration I can't see why a student would ever be expected to know the “correct” formula of the Cu(II) aquo complex. As the complex ion remains in the +2 oxidation state regardless of attached ligands it can't be related to “variable oxidation state”. Presumably the intent of “colour” is to emphasise the requirement of partially filled d orbitals to account for the colour with the concept of energy gaps/jumps relating to different frequencies of light, although I guess at Scholarship level some sensible discussion about the ligand environment affecting colour could possibly be expected. e.g. when the H2O ligands in the Cu(II) ion are replaced by Cl- ligands the colour changes from blue to yellow  and if only some of the waters are replaced the colour of the solution will be green.

With respect to shape, I guess the examples you raise could be used as examples of square planar or octahedral structures (does polyatomic ions include complexions???) but for this purpose the formula would surely be provided as I cannot believe that an examiner at level 3 would have an expectation of a student knowing this detail. Incidentally, from my own experience with marking I would be delighted if students were able to provide the formula for either of the possible structures).

So my gut feeling is that knowledge of such information would never be required at Level 3 but could conceivably crop up as an example in Scholarship, particularly as Scholarship requires knowledge from all level 2 and 3 standards (both internal and external) and of course knowledge of the existence of certain complex ions is a requirement for the internally assessed Qualitative analysis standard.

So for the sake of inquisitive (curious?)  Scholarship students let's try and address the specific question so you can enjoy the rewards of Easter in a stress free environment.

The generally accepted formula for the aquo Cu(II) complex is [Cu(H2O)6]2+ despite the 5th and 6th ligand not being bound very strongly and as you say being further than the other four from the transition metal. Successive replacement of up to four water ligands by ammonia occurs readily and while its formula is strictly [Cu(H2O)2(NH3)4]2+   to answer your specific query I cannot imagine any examiner or marker at this level ever rejecting the commonly used form of [Cu(NH3)4]2+.

Given the amount of distortion present in either of these “octahedral” complexes, where it is difficult to distinguish between a tetragonally distorted “octahedral” coordination and square coordination it would be a “brave” examiner that didn't accept either answer and if they were sensible would not be requesting an answer in the first place because of the complexity involved.

So to summarise, from the perspective of NCEA requirements I don't think it matters at all and I wouldn't lose any sleep over it. If a Scholarship student asks about it - great! Let them go away and find out about from advanced text books, the web, tertiary sources but don't see it as a piece of critical information that they must know to meet NCEA requirements.  Back to top.

Q5. We are a little confused with when to use kJ and when to use kJmol-1. One text book quotes:

Water made from 2 mol of hydrogen burnt in 1 mole of oxygen releases 570kJ of energy under standard conditions.  

2H2(g) + O2(g) > 2H2O(l) ΔrH = -570kJ mol-1

Water made from 1 mole of hydrogen burnt in ½ mole of oxygen releases 285.5 kJ of energy under standard conditions

H2(g) + ½ O2(g) > H2O(l) ΔrH = -285.5kJ mol-1

Does the first equation mean kJ per mole of oxygen?  Does the second equation mean kJ per mole of hydrogen?  

A. For enthalpy changes the units of kjmol-1 always refer to one mole of the equation as written rather than with respect to any particular component of the equation, so both of your examples are correctly expressed. The value of the first equation is double that of the 2nd because it describes twice the amount of reactants (and products). For energy changes which are defined wrt 1 mole of either reactant or product (e.g heat of combustion or heat of formation) the equation does always include one mole of the identified chemical but even in these situations there will be other species present in amounts other than 1 mole which is why the definition has to be like this.

When a stated energy change relates to a specified quantity of a substance (either moles or grams) the unit for the energy change is simply given as kJ. eg: 2H2(g) + O2(g) > 2H2O(l) ΔrH = -570kJ mol-1 but the energy change when 10g of hydrogen gas is burnt is 1425 kJ. Back to top.

Q6. Can you explain to me what the "mindspring" site is, what it can be used for.  It sounds interesting.

A. It's a site set up by a Christchurch group with some Ministry funding to encourage sharing of teacher resources on line so that teachers throughout the country don't have to continuously try and reinvent the wheel.

Some subject areas have been more successful than others, but unfortunately Chemistry has only involved a couple of teachers who have been prepared to add resources to the site. The site url is http://www.mindspring.school.nz but you need to get a login. Back to top.

Q7. While sorting out material in our Science Practicals we took a closer look at the regulations covering the use and storing of certain dangerous substances.

We found the NZASE web site and were surprised to find that substances such as metallic sodium and mercury, and mercury oxide and chlorides were allowable, according to the data sheets posted there, even if there were restrictions on their use, such as heating the substances etc. See http://www.nzase.org.nz/codepractice/Appendicesv4.pdf

It seemed strange to me that, at a time when dentists (and their patients) have become very aware of the hazards of handing mercury, that there seems to be no guidelines on the use of mercury metal on the NZASE site other than a restriction on heating that substance (or any of its compounds).

Are these guidelines adopted by MOE for secondary schools? I'd appreciate your opinion on this.

A. As far as I know it's a fairly new development to clarify some confusion that has been around in secondary schools for a while as to legal obligations re acceptable substances etc. Personally, having just returned from a ChemEd conference in North America I would hate to see us follow their path where lots of chemicals are banned and lots of practical work is only done by demo or by watching a video. With respect to Hg I would imagine that it is only used by teachers as an illustrative example of a liquid metal and also for its density (so used in a sealed container as demo) or very rarely in a demo such as the "beating heart" example of electrochemical cells. As far as mercury compounds are concerned the only one I know of in use in schools would be Hg(II) chloride which is used to demonstrate the extreme reactivity of aluminium with oxygen once the protective aluminium oxide coating is removed. Again done as a controlled demo with no student contact. Similarly the controlled reaction of sodium would only be done as a demo and if carried out by an experienced chemistry poses negligible risk to students and certainly is well worth while doing. Watching a video of this reaction does not convey the same level of reactivity to students. Hope this helps allay your concerns?

Follow up

Yes, I too feel that too stringent measures on the use of particular substances in schools removes the fun of what could be inspiring lessons for budding scientists. On the other hand, awareness (by the teacher) of just how dangerous these substances can be can also enhance this excitement for the students if it is handled well. Back to top.

Q8. Can you please tell me where I can get the resource you mentioned in your newsletter - Thinking Skills Resource marketed by McTogi on behalf of Bob Bucat from Univ of Western Australia.

A. All McTogi Resources are available on CD. Back to top.

Q9. For those of you lucky enough to have access to a SmartBoard (or similar interactive whiteboard) you may be interested in checking out the teaching resources available from www.prometheanplanet.com (free registration required).

A flipchart viewer that allows all resources to be displayed on a PC (without a SmartBoard) is freely downloadable from the same site.Chemistry interactive resources include flipcharts on Redox, Organic, Isotopes, Ionic formula, Proteins, a chemical formula Jigsaw and Sudoku. Another source is http://www.echalk.co.uk but this is a paid subscription although a sample of resources are avalable free. Back to top.

Q10. Inserting Flash animations into a PowerPoint presentation

Microsoft Office PowerPoint® 2003
Microsoft PowerPoint 2002

If you have an animated graphic that was created with Macromedia Flash® and saved as a Shockwave® file (.swf file extension), you should be able to play it in a PowerPoint presentation using a specific ActiveX control and the Macromedia Flash Player. To run the Flash file, you add an ActiveX control to the PowerPoint slide and create a link from it to the Flash file. You also have the option of embedding the file in the presentation.

Important   

  • The ActiveX control, called Shockwave Flash Object, must be "registered" on your computer for you to play the Flash file within PowerPoint. If it is registered, it will appear in the list of controls opened from the Control Toolbox (detailed in the steps below). If it is not registered, download the latest version of the Macromedia Flash Player from the Macromedia Web site; this will register the control on your computer.
  • Older versions of the Shockwave Flash Object may be registered on your computer. To guarantee that complex animations run properly, we recommend that you install the latest version of the Macromedia Flash Player.

To play a Flash file in your presentation, follow these steps:

  1. Install the Macromedia Flash Player on your computer.
  2. In normal view in PowerPoint, display the slide on which you want to play the animation.
  3. On the View menu, point to Toolbars , and then click Control Toolbox .
  4. In the Control Toolbox , click More Controls (the button with the hammer and wrench icon).
  5. In the list, scroll down and click Shockwave Flash Object , then drag on the slide to draw the control. You can resize the control at any point by dragging the sizing handles if you need to adjust it to the size of the animation.
  6. Right-click the Shockwave Flash Object, and then click Properties .
  7. On the Alphabetic tab, click the Movie property.
  8. In the value column (the blank cell next to Movie ), type the full drive path including the file name (for example, C\:My Documents\MyFile.swf) or Uniform Resource Locator (URL) to the Flash file that you want to play.
  9. To set specific options for how the animation plays, do the following, and when you're done, close the Properties dialog box:
    • Make sure the Playing property is set to True . This plays the file automatically when the slide is displayed. If the Flash file has a Start/Rewind control built into it, the Playing setting can be set to False .
    • If you don't want the animation to play repeatedly, in the Loop property, select False (click the cell to get a down arrow, click the arrow, and select False ).
    • To embed the Flash file so you can pass this presentation on to others, in the EmbedMovie property, click True . (In order for the Flash file to run, however, the Shockwave Flash Object control must be registered on any computer that runs this presentation.)
  10. To run the animation: With the slide displayed in normal view, click the Slide Show button in the lower left of the PowerPoint window (or press F5, or, on the Slide Show menu, click View Show ).
  11. To exit the slide show and return to normal view, press ESC Back to top.

11. Do you have a list of places to visit in Auckland for EOTC for level 2 chemistry?

A. There are possible tours of the Glenbrook steel and Lion brewery that can be done. My concern with these is the lack of chemistry that the students actually see. Another option might be to look at local businesses E.g.

Electroplaters, water quality or fonterra (although not for Auckland ). It would very much depend on the local companies and whether they can handle the large numbers of students and their safety rules. Back to top.

Q12. Is the term NaCl(aq) correct? It seems to imply that there is a species , NaCl, in solution.

A. There is nothing wrong with NaCl(aq). It describes the state (dilute aqueous solution) that the NaCl is in. It is just as valid as NaCl(s) or NaCl(l). I understand that you may be concerned that the ions will be separated when dissolved up, but formulae, particularly those from compositional nomenclature such as this, do not necessarily imply proximity, or even give a completely accurate description of the chemical species present. That bottle of PCl5 sitting on the shelf (well actually it should be stored in a cabinet) is actually an ionic solid, [PCl4][PCl6], when in the solid state. Indeed, your NaCl(s) does not involve little pairs of ions being stacked up in pairs like a room full of married couples! Back to top.

Q13. 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. Back to top.

Q14. 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! Back to top.

  • Department of Chemistry
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    Phone +64 3 364 2413
  • Chemteach
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    New Zealand
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