Friday, March 25, 2016

Acid-Base Equilibria and Beginner Teachers

Cathy is a Year 12 student in a Brisbane school (capital city of Queensland, Australia). Last week she had a chemistry exam, the topic was equilibrium which included acids, bases and acid-base titrations. I met her on her way to school on the morning of this exam. When I asked her if she felt confident about her exam, I was horrified by her response,
"Sort of. I've got a new teacher this year and she's not any better than the one I had last year. We did an experiment, she said it took too long so we didn't do any more."
Seasoned teachers are used to:

  • student claims that their teacher is "no good" (especially if the student is performing poorly)
  • student exaggeration (only 1 experiment in a whole term of equilibrium, surely not!)
"Oh come on", I said in my best 'you're kidding me' voice, "you studied equilibrium for a whole term and only did one experiment?"
"Yeah", she confirmed, "we watched a video on titration though".
"Didn't you do a titration experiment?"
"Nah. She said we didn't have time."

Even allowing for the possibility of student exaggeration, the thought that you would play a video showing someone else performing a titration rather than giving your own students the opportunity to carry out even a simple titration, is, quite frankly, appalling. 

From Cathy's description of the teacher I assume this is the teacher's first year out teaching (beginner teacher). Reflecting on my own first year of teaching (a long, long time ago), I remember struggling to meet the requirements of the chemistry syllabus in the time-frame allowed, and, I also remember that while teaching the techniques of titration was time consuming, the students learned more in a few practical sessions than they learned during the whole of the preceding theoretical lessons, and once they have mastered the techniques they can be put to use in real-world problems.

A typical sequence that is often taught for (monoprotic) acid-base equilibria assumes prior knowledge of solutions, concentration (molarity) and equilibrium concepts and calculations (including self-dissociation of water, Kw ):
  1. What are the properties of acids and bases?
  2. How do we define an acid and a base?
  3. What is meant by the terms "strong acid"  and "strong base"?
  4. How do we measure the strength of an acid or a base (pH scale)?
  5. What happens when you add an (Arrhenius) acid to a (Arrhenius) base (neutralisation)?
  6. How much (Arrhenius) acid do we need to add to a known amount of (Arrhenius) base in order to neutralise it (acidic, basic, neutral solutions)?
  7. Discussion of titration techniques, including preparation of a standard solution.
  8. Performing a strong acid - strong base titration.
  9. Using the results of the experiment to calculate the concentration of the unknown acid or base.
  10. Perform calculations for each 1 mL addition of strong acid to strong base in the titration experiment and graph the results (strong acid - strong base titration curve)
  11. Discussion of weak acids (Ka).
  12. Discussion of other acid-base reactions (proton transfer reactions) and other titration curves
  13. What indicator should you use for a particular acid-base reaction? (optional, how does an indicator work)
  14. Titration of weak acid - strong base (such as determination of acetic acid in vinegar)
If you see your students 4 or 5 times a week, this teaching program for acid-base equilibria will take about 4 weeks using a traditional, structured approach. If you have the luxury of being able to time your practical work so that it occurs in the correct sequence, and take time to link the practical work to the theoretical concepts, your students have a good chance of understanding and being able apply the concepts to unfamiliar problems.
If you don't do any experimental work, you could probably bowl it over in 2 to 3 of weeks, and be faced with a lot of bored students wondering why they ever took a course in chemistry.
If you take a student-centered constructivist approach (for example, start with the questions  like "what gives vinegar its tangy taste?",  "if acids are corrosive and burn skin, how come you can drink vinegar?",  "how can you measure the strength of an acid?", "how can we determine which brand of vinegar has the greatest concentration of acetic acid?"), be prepared to add another week (unless you give the students a lot of reading/research for homework). The benefits, however, are enormous. Your students are more likely to be engaged with the content and "on task", they will have to be able to justify decisions they make in order to design and perform experiments thereby linking concepts and practical work, and because they "invest" in the whole learning process they are more likely to be apply the understanding and knowledge gained to other problems.

So, if you are new to teaching acid-base equilibria, here a few suggestions:
  1. Even if you firmly believe that constuctivist approaches to teaching are the most effective way to teach chemistry, be prepared to spend your first year of teaching chemistry taking a more traditional approach, using guided questioning to lead students towards the experiment(s) you need them to do (syllabus requirements) while still giving them "ownership" of the experiment and its results. Keep a list of the misconceptions you come across when you teach, this will help you be better prepared for next year. As you feel more confident in your ability to meet the syllabus requirements within the time you have, and you have a better feel for the misconceptions you will meet, you can start "loosening your hold" and give more time to truly constructivist approaches.
  2. Let the students do as much practical work as possible (students not only need to be exposed the practical techniques of chemistry, they need to do the experiments in order to fully appreciate the significance of what you are trying to teach them). You also need to devote time to discussing the results of their experiments with them, and reinforcing the concepts, calculations, techniques etc involved. 
  3. Spend time discussing the self-dissociation of water (that is, it is a lesson in its own right, not just a passing reference before you discuss acid-dissociation). Students will have been exposed to an "acids and bases" topic sometime between Years 7 and 10, but even so, many of them may still think that an acid has a pH less than 7, a base has a pH greater than 7, and that a neutral substance has a pH of 7. Believe me, it can be an uphill struggle to separate the two concepts of "acid, base, neutral" from the concept of "pH" in a student's mind (and if you don't believe me, think about the number of times you have seen/heard advertisements for products which talk about the "neutral pH" of skin/hair etc). If the students do not have a good grasp of the self-dissociation of water then they will not understand the pH of  aqueous solutions. (And a word of caution, just because a student can calculate the pH of an aqueous solution of base at 25oC, it doesn't mean they understand the relevance of pH + pOH = 14, or [H+][OH-] = 10-14, and if you want to test this statement, ask you students to calculate the pH of 0.001 M NaOH(aq) at 50oC, or ask them to find the pH of 0.01 moles of HCl(g) dissolved in 1 L of ethanol and see what happens, because the chances are they will simply do a pH + pOH = 14 calculation without even thinking about it!)
  4. Spend time making the distinction between "strong acids", "weak acids", "dilute aqueous solutions of acids" and "concentrated aqueous solutions of acids" (similarly for bases) because once again, you are likely to have an uphill struggle to separate the two concepts "strength of an acid/base" and "concentration of an acid/base". Remember, they have already been exposed to statements such as, "I need a cup of strong coffee", or, "this cordial is a bit strong" which, in chemical terms should be "I need a cup of concentrated aqueous solution of coffee (or cordial as the case may be)". On the other hand, they have also been exposed to ads which say things like "concentrated laundry detergent" which is a slightly more appropriate use of the technical term "concentrated" (although I do remember one example that used "concentrated laundry liquid" which introduces the other problem of the loose usage of the word "liquid" instead of "solution"). One way to do this is to give each pair of students a bottle of acetic acid labelled with its concentration, and have them measure its pH with a pH meter. Also provide them with volumetric flask of HCl(aq) of known concentration (say 0.1 M) and have them measure its pH, then have them perform sequential 1:10 dilutions and measure the pH at each stage say they can see that pH is dependent on the concentration of the strong acid and that you can reach a point at which the pH, and therefore the concentration, of a strong acid is the same, and even greater than, the concentration of an aqueous solution of weak acid. When you tabulate the class results and ask them for an explanation be prepared for many of them to believe you somehow "tricked them", it can take time for them to break the strength/concentration misconception and replace it with a more appropriate separation of the two concepts. If the students do this activity themselves, it will easily take a lesson, if you do it as a demonstration it will take about 10 minutes, BUT, it is better for the students to do it themselves partly because it reduces the instances of "there must be a trick in this" thinking, but mostly because they can see the pH change with the concentration and they are going to have to justify that all the way to the point at which the pH of the strong acid is  greater than the pH of a weak acid.
  5. Related to point 2 is the common misconception students may have that when you add an acid to a base you end up with a neutral solution that has a pH of 7. Personally, I think the best way to deal with this is to let the students work it out for themselves before you attempt to explain it to them. For example. give each pair of students a bottle of methyl orange indicator (you will need a fair degree of tolerance in establishing the end-point so don't use a pH meter) and a conical flask and have them add a 10.00 mL aliquot of standardised 0.1 M NaOH to the flask and record what happens to the indicator colour. Have them calculate the moles of hydroxide ions in the flask, as well as calculate the pH of the solution (so they are convinced the indicator is giving a true reading). Then give each pair a 100 mL volumetric flask containing 0.1 M monoprotic acid (some will get a strong acid such as HCl(aq), some will get a weak acid such as acetic acid). . Have them calculate the volume of acid they will need to add to the NaOH(aq) to neutralise it. Have them add this volume (straight from the pipette to the flask), give it a swirl, and record the colour of the indicator. Tabulate the results on the board (yellow vs red). Ask them why some changed colour and some didn't (be prepared to let the students with weak acids try adding more acid, many will believe they made a mistake in the calculations or in adding the solutions), if the students do not come to the realisation that only students with strong acids got a colour change at neutralisation, you can use questions to help guide them. I have found this is a far more effective method than just "telling them" and it need only take 15 minutes if all the solutions and equipment are prepared before hand AND you don't expect them to write it up as a prac (a demonstration takes even less time, but may not be quite as effective, that is, some students will believe you have somehow "tricked" them).
  6. Have the students perform the calculations that will enable them to draw a strong monoprotic acid - strong base titration curve. If you have a class of 20 students, they only need to do one of the calculations each, you can tabulate the results and then they can graph the class results. There are a number of reasons for this, it reinforces the nature of the neutralisation reaction, stoichiometry, and of "limiting reagents" and "reactants in excess". It is also enables them to come to a greater understanding of the shape of the curve than if you just present it to them and discuss key points. Finally, if the students do not have a good grasp of why titration curves are the shapes they are, they will have a much harder time coming to terms with the nature of different indicators and why some indicators are more appropriate than others for particular titrations.
  7. Do use "real-world" examples. The acetic acid concentration of brands of vinegar is not hard to do, and empowers them (if you have mothers whinging that their daughter will now only let them buy brand X because its better value because it has a higher concentration of acetic acid than other brands, then pat yourself on the back for a job well done!) If you are in a position to be able to safely determine the concentration of sulfuric acid in a lead-acid battery, then this is also not hard to do (but check whether it can be done at your school). Similarly, you will find concentrated HCl(aq) available at you local hardware store (for cleaning bricks) or pool suppliers (for addition to pools) and, if your safety guidelines allow, you can determine the concentrations of these.If you are prepared to take your students through back titrations (indirect titrations) then a wealth of new "real-world" opportunities is open to you.
  8. Finally, do not deceive yourself. It is NEVER about what you "teach", it is ALWAYS about what the students "learn". YOU can make up time by giving the students notes you have prepared for them, making them read stuff for homework, making them watch a 30 minute video instead of doing a 2 or 3 day prac, then you can happily tick this off on your list of things to teach, BUT, you must also find out what the students have learned, because you may very well find out that you have been a bit hasty in ticking something off your list! 

Thursday, March 24, 2016

Hidden Salt in Food

Researchers at VicHealth and Deakin University compared how much salt people thought they consumed with how much salt they really had consumed and found that Australians were not only eating too much salt, but were also eating more salt than they thought they were!

Australians were found to be consuming between 8 and 10 grams of salt per day, about twice the amount recommended by the World Health Organisation (WHO) that recommends adults should eat less than 5 g of salt (a bit less than a teaspoon) per day. The "salt" they are referring to is "table salt" which has the chemical name "sodium chloride" and the chemical formula NaCl. Sodium chloride is actually an ionic substance made up of sodium ions (Na+) and chloride ions (Cl-) in a ratio of 1:1 and it is the sodium ions (Na+) that are the cause for concern because elevated levels of sodium ions increase a person's risk of high blood pressure, heart disease and stroke. Unfortunately, non-Chemists often refer to this as elevated "sodium" levels rather than as elevated "sodium ion" levels.

If you want to reduce your sodium chloride intake, the first thing you can do is NOT add "table salt" to your food when you eat it. However, only about 20% w/w of our daily intake of sodium ions comes from adding sodium chloride to our food at the table before we eat it. The other 80%  w/w of the sodium ions we consume is already present in our food, either naturally or because it has been added during processing.

Natural sources of sodium ions in our food include:

  • milk and cream: 50 mg of sodium ions per 100 g
  • eggs: 80 mg of sodium ions per 100 g
  • carrot: 69 mg of sodium ions per 100 g
  • spinach: 79 mg of sodium ions per 100 g
  • green beans, potatoes: 6 mg of sodium ions per 100 g
  • pumpkin: 1 mg sodium ions per 100 g
  • apple, banana, pear: 1 mg sodium ions per 100 g
By far the greatest source of sodium ions in our diet comes from eating processed foods:
  • 1 slice of white bread (30 g) can contain 140 mg Na+
  • 1 slice of cheddar cheese (20 g) can contain 140 mg Na+
  • 1 foil pack of butter (7 g) can have 55 mg of Na+
  • 1 small bowl of breakfast cereal (30 g) can have 140 mg of Na+
  • 1 small packet of potato chips (45 g) can have about 300 mg of Na+
  • 1 can (12 fl oz, about 350 mL) diet coke has 40 mg of Na+
  • tap water contains about 20 mg Na+ per 1 L
Food you buy in packets from a supermarket will have a list of ingredients and you can read this to find the amount of sodium ions (Na+) present in the food.

However, there are many foods we buy that do not come in a packet which tells us how much sodium ion is present. These foods make up our "hidden salt intake". You may find this information on company websites, such as

  • 1 McDonalds Big Mac contains 859 mg of Na+
  • 6 KFC chicken nuggets with sauce has 1040 mg of Na+
  • 1 slice (1/8 th) of medium pan Pizza Hut Meat Lover's pizza has 740 mg of Na+
  • 1 Taco Bell black bean burrito contains 1030 mg of Na+
Many people forget that sodium ions are also present in many medicines. Effervescent medicines contain sodium hydrogen carbonate (or sodium bicarbonate) which helps them dissolve in water.
For example,:

  • 1 effervescent Berocca tablet contains about 280 mg of Na+ while the film-coated Berocca tablet contains only 1.85 mg of Na+
  • 1 Gaviscon Advance tablet contains 55 mg Na+ but 10 mL of liquid Gaviscon contains 141 mg of Na+
  • 1 Panadol Actifast caplet contains 173 mg Na+ but 1 Panadol soluble tablet contains 428 mg of Na+
So, if you want a diet that's low in sodium, eat lots of fresh fruit and vegetables, drink lots of water, and avoid packaged food and "fast food", and, remember to choose "low sodium" medicines.

Reference:
http://www.abc.net.au/news/2016-03-24/reducing-salt-intake-could-save-thousands-of-lives-each-year/7274140



Further Reading:
Mass Conversions
Percent by Mass

Suggested Study Questions:

  1. Convert the following masses in grams to masses in milligrams:
    • 5 g
    • 8 g
    • 10 g
    • 30 g
    • 100 g
  2. Convert the following masses in milligrams to masses in grams:
    • 1.85 mg
    • 55 mg
    • 69 mg
    • 173 mg
    • 859 mg
    • 1040 mg
  3. Calculate the percentage of sodium ions and the percentage of chloride ions in sodium chloride.
  4. Use the information in the article to calculate the mass of sodium ions each adult Australian currently consumes as a result of :
    • adding table salt to food before eating it
    • table salt that is naturally present or is added to food during preparation
  5. 100 g of milk contains 50 mg of Na+ . What is the percentage by mass of sodium in the milk?
  6. One 30 g slice  of white bread contains 140 mg Na+. What is the percentage by mass of sodium in white bread?
  7. For lunch, a student ate a sandwich made up of 2 slices of white bread, 2 foil packs of butter and a slice of cheddar cheese. She also ate a 200 g banana, and washed it all down with 250 mL of plain, unflavoured milk.
    • Calculate the mass of sodium ions the student consumed for lunch.
    • Calculate her consumption of sodium ions as a percentage of the WHO recommended daily intake of sodium ions.
  8. A different student consumed a Big Mac, 1 can of diet coke, and a 200 g packet of potato chips.
    • Calculate the mass of sodium ions the student consumed for lunch.
    • Calculate her consumption of sodium ions as a percentage of the WHO recommended daily intake of sodium ions.
  9. For her birthday, a Chemistry Teacher's class gave her a 500 g block of dairy milk chocolate. The label included the information that the block of chocolate contained 82 mg of sodium per 100 g.
    • What is the mass of sodium ions in the block of chocolate?
    • If each person in the class of 22 received an equal share of the block chocolate, what mass of sodium ions would each person consume?
  10. In Australia, the maximum recommended dose of paracetamol (the active ingredient in panadol tablets) is 4000 mg per day. 1 soluble Panadol tablet contains 500 mg of paracetamol.
    • What is the maximum number of soluble Panadol tablets per day that an adult Australian should consume?
    • If an adult Australian consumed the maximum recommended dose of soluble panadol tablets in 1 day, what mass of sodium ions would they have consumed?
    • What percentage of the WHO recommended maximum intake of sodium would this amount of panadol be?

Saturday, March 19, 2016

Lipstick Evidence

Lipstick is sticky stuff!
Kissing someone on the cheek with your lusciously lipsticked lips will invariably leave a colourful impression. And, after you've had your sip of coffee your "lips" are left behind in vivid colour on the cup. Lipstick can even end up on tissues after  a momentary touch as you blow nose, or wipe tears from your eyes. And we've all seen movies in which a wife discovers lipstick (not her own) on her husband's collar. Needless to say then that lipstick can be found at a crime scene and is considered to be an example of "trace evidence".
Researchers at  Western Illinois University have been investigating better ways to lift and analyse this lipstick evidence.
In general, lipstick is composed of

  •     65% castor oil
  •     15% beeswax
  •     10% other waxes
  •     5% lanolin (also known as wool wax or wool grease)
  •     5% dyes, pigments and perfume
In other words, most of the mass of a lipstick is made up of lipids (fats, oils and waxes).
To lift the lipstick from the material, the researchers developed a two part process:
  1.   Add an organic solvent to remove most of the oils and waxes.
  2.   Add a basic organic solvent to extract the remaining residue.
The components of the lipstick are now present in solution.
In order to determine the chemical composition of the solutions, they will need to undergo separation and analysis. Three common methods of doing this are:
Different brands of lipsticks have different chemical compositions so they produce different chromatographs.
Using known brands and colours of lipsticks, the researchers can produce a database of chromatographs. When lipstick evidence is found at the scene of a crime, forensic scientists can produce a chromatogram of it and compare this with the database of known brands and colours in order to find a match. In this way forensic scientists can determine the brand and colour of the lipstick. Law enforcement officials could then investigate whether a suspect uses that particular lipstick.
The researchers are still performing analyses of lipsticks, but at this stage they have reported that the best results are achieved with gas chromatography (GC).

Reference:
American Chemical Society. "Tying lipstick smears from crime scenes to specific brands." ScienceDaily. ScienceDaily, 14 March 2016.

Further Reading:
Percentage composition 
w/w % concentration
Parts per million (ppm) concentration
Lipids (oils, fats and waxes) 
Properties of Carboxylic Acids 
Preparation and Naming of Simple Esters


Suggested Study Questions:
  1. A tube of lipstick contains 4.0 grams of lipstick. Calculate the mass of each of the following components of the lipstick:
    • castor oil
    • beeswax
    • lanolin
  2.  The castor oil used to make the 4.0 grams of lipstick is itself made up of a number of fatty acids notably about 90% ricinoleic acid, 4% oleic acid and 3% linoleic acid. Calculate the mass of each of these fatty acids present in the lipstick.
  3. Why do you think the concentrations of chemical compounds found in lipstick are given as % w/w (percentage by weight or percentage by mass) rather than in units of mol L-1 or ppm?
  4.  What is meant by the term "fatty acid" in chemistry?
  5.  Draw and name the functional group that is present in both carboxylic acids and fatty acids.
  6.  Acetic acid (ethanoic acid) is miscible (soluble in all proportions) in water,  whereas the solubility of pentanoic acid is 3.4 g mL-1, and of hexanoic acid is 1.0 g mL-1. Would you expect oleic acid (C17H34O2) to be soluble in water? Explain your answer.
  7.  What is meant by the term "triglyceride" in chemistry?
  8.  Draw the functional group that is common to both triglycerides and esters.
  9.  Esters are immiscible in water so an organic solvent is used to extract the triglycerides from the lipstick marks. Imagine you have been given samples of cyclohexane, ethanol, and acetone. Which of these do you think would be the best solvent to use on the lipstick mark, and explain your answer.
  10.  Design an experiment that you could perform to test your hypothesis in question 9 above regarding which of the solvents would be best to use on the lipstick mark.

Wednesday, March 9, 2016

World Science Festival 2016

The World Science Festival which began in New York in 2008,  kicked off for the first time in Brisbane last night (9th March 2016) and will end this Sunday 13th March 2016).
You can see the program of events at http://www.worldsciencefestival.com.au/program-listing/
If you have an interest in physics or biology you will find quite a lot of activities, there is even a maths film, and a couple of coding activities.
I think one of the highlights of the festival will be 'Breakfast with the Brians", the Brians being Brian Greene (professor of physics and mathematics at Columbia University, who has written a number of books designed to make recent developments in physics accessible to everyone) and Brian Schmidt (Nobel Prize winning Australian National University astronomer). The discussion is to be moderated by arguably the best science journalist and broadcaster in Australia, Robyn Williams (Radio Nationals' "The Science Show" Saturdays at noon and repeated Thursday at 9pm, and "Ockham's Razor" Sundays at 7:45am).
If your interest is primarily in the extraordinarily fascinating and fundamentally important discipline of chemistry, then, unfortunately, it seems your activities will  be limited to the rather hackneyed making of "slime" (borax and PVA glue) and "snow" (water added to sodium polyacrylate which can absorb hundreds of times its own mass in water causing it to "fluff up" and look vaguely like snow if you are 5 years old and have never seen snow) in the street science program.
Every day we rely on chemistry to keep us alive and functioning. Oxygen is successively extracted from the air we breathe and transported around the body to be used in chemical reactions. The products of those chemical reactions can be used immediately, stored for later use, or excreted, all by using particular chemical reactions. The same can be said for the food (including fluids that we ingest). How many times have you found yourself scrutinizing the ingredients list on a food packet to see how much "sodium", "sugar", "fat", "protein", etc it contains? And all of that is determined by chemical methods! Even when you are sick, you rely on chemists to develop drugs to make you better! As I sit here writing this, I am thankful for the developments in chemistry that provide the materials to make the computer I use, and the mug I drink my coffee out of. Later on, when I get in my car I am pleased to know that chemists have been essential in developing the materials used to make and fuel my car, and that chemists continue to make progress in developing renewable fuels and materials.
Without chemists I would not have access to modern surfactants to keep my clothes, and myself, clean. Nor would I have portable energy sources (batteries) to power my laptop, phone, torch, ipod etc Without chemists I would not have clean drinking water available through a tap in my kitchen, and maybe this is the problem ...
Do we take chemistry, and chemists, for granted?

Sunday, March 6, 2016

Polylactic Acid



The London Organizing Committee of the Olympic and Paralympic Games (LOCOG) was committed to making 2012 the very first "zero-waste" Olympic and Paralympic Games.
11 million attendees produced 8,500 tons of solid waste, mostly in the form of plastic packaging.
8,000 tons of this plastic waste was transported to a site where it was composted within 9 weeks and subsequently used to fertilize local crops.
That's right!
There are plastics that can be composted, and the plastic used at the London Olympic Games was the most common compostable plastic which is known as polylactic acid or PLA.
And the chemistry of polylactic acid (PLA) is the subject of this edition of AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter.
You can subscribe to our newsletter by contacting us at http://www.ausetute.com.au/contact.html