Problem Solving in Chemistry

Chemistry students are expected to be able to solve chemistry problems.
On every exam paper, there will be an assortment of problems for students to solve.
Sometimes they do it well, sometimes they do it OK, and sometimes they do it badly.
Often, we give students time before exams to practice answering exam questions.
But how often do chemistry teachers actually explicitly teach problem solving skills?
Often we expect students to "follow our lead" when we demonstrate how to solve particular problems, but do we ever give them a good general framework that they could use to solve any problem they are likely to face in exams?
Most likely the answer is no.
"Surely, by the time students get to the senior years of high school they should be able to solve problems right?" I hear you ask.
While this is a reasonable expectation, the reality is that quite a few can't, just try reading the annual examiners reports and you will get a feel for the kinds of difficulties many students face when trying to solve problems.
So, I've spent some time doing some reading, quite a lot of thinking, and more typing than I'd like, in order to produce a framework for problem solving in chemistry.

You can see the results on the AUS-e-TUTE page on Problem Solving in Chemistry:
http://www.ausetute.com.au/stopgops.html

and a results-only demonstration of the problem solving process in action has been added to the bottom of the amended Dilution Calculations page:
http://www.ausetute.com.au/dilucalc.html

The problem solving page might seem like a lot of reading, but once your students become familiar with the process it is really very quick. It helps them identify potential difficulties BEFORE they actually start doing calculations, ensures they answer the question they were asked and that they check the answer to make sure it is reasonable.

If you happen to teach physics and/or maths as well as chemistry, the method can be applied to these subjects as well.

Because acronyms are useful, I've called this the StoPGoPS approach to problem solving, for reasons that will become self-evident when you read the problem solving page, and I've used a set of traffic lights as a visual aid to recall.

Please feel free to comment on the usefulness of this problem solving model.

Trans Fats

Some fats, such as polyunsaturated fats, are thought to be good for us.

They lower the "bad" type of cholesterol which has been linked to heart disease.

Other fats, such as saturated fats and trans fats, are considered to be bad for us because they increase this "bad" type of cholesterol.

Since the beginning of the 21st century, health authorities all over the world have been calling for the elimination of trans fats from commercially produced food products.

But what is a trans fat and where does it come from?

Go to the June 2014 issue of AUS-e-NEWS for the chemistry of Trans Fats.

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Keeping Glass Clear

Windows get "foggy" when water condenses on them. The glass of the window is colder than the surrounding air, and the air contains moisture which is gaseous water. When the gaseous water hits the surface of the cooler glass, the water turns into a liquid.
What happens next depends on the nature of the glass surface.

If the glass were perfectly flat, and perfectly clean, the strong adhesion between the water molecules and the hydrophilic glass would allow the water to spread out evenly over the surface of the glass so you would still be able to see through the glass.
If the glass is dirty it has an uneven cover of hydrophobic particles which reduces the adhesive forces between the water and the surface resulting in a multitude of small water droplets collecting on the surface which means it is harder to see through the glass.

Traditional ways of keeping the glass of car windows "fog free" include wiping the water off if it is on the outside, or turning on the heater or air-conditioner to increase the rate of evaporation if the "fog" is on the inside of the car.

From a chemistry point of view, we can see there are two possible approaches to making a coating for the glass surface so that it will stay "fog free":

• Make an extremely hydrophobic, transparent coating, then any water that comes into contact with the surface will not adhere at all and will just run off. This is the traditional approach used to create water repellent coatings and materials.
• Make an extremely hydrophilic, transparent coating so that any water than comes into contact with the surface will spread out evenly, making an extremely thin, transparent, layer over the surface. This is what scientists at A*STAR's Institute of Materials Research and Engineering (IMRE) have done. They have created a new technology, CleanClear, which is a durable and permanent ceramic coating that is transparent and superhydrophilic, which means it attracts water instead of repelling it.

The new transparent, superhydrophilic ceramic coating is anticipated to have many application; on car windows, glass cooking pot covers, hot food displays, it might even be used to keep your spectacles clear.

Reference:
The Agency for Science, Technology and Research (A*STAR). "Creating a water layer for a clearer view." ScienceDaily. ScienceDaily, 12 June 2014. www.sciencedaily.com/releases/2014/06/140612212430.htm.

Further Reading
http://www.ausetute.com.au/members/surfacetension.html
http://www.ausetute.com.au/members/wetting.html
http://www.ausetute.com.au/members/heatlatent.html

Suggested Study Questions

1. Write a balanced chemical equation to show the condensation of gaseous water on a glass surface.
2. Write a balanced chemical equation to show how liquid water evaporates off a glass surface.
3. Use (kinetic) particle theory of matter to explain what happens when water condenses and evaporates.
4. Define the terms hydrophilic and hydrophobic.
5. Define the terms adhesion and cohesion.
6. Explain why water forms spherical droplets.
7. Explain why glass is a hydrophilic surface.
8. Explain why dirty glass, glass covered with oily particles, is hydrophobic.
9. Explain how a water repellent coating on glass might work.
10. Discuss potential problems with using CleanClear on car windows, that is, what factors might reduce its effectiveness and why.

Packing Density

In the particle theory of matter, all matter is made up of tiny particles. You've probably drawn circles in boxes to represent solids, liquids and gases. Each of those circles is a 2-dimensional representation on the paper of a 3-dimensional sphere. So, your representation of the particle theory of matter also implies that all matter is made up of particles which are spherical in shape.

If you draw the circles so close to each other that they are touching, you label the drawing "solid".
If the circles are little bit further apart, you label the diagram "liquid".
If the circles are much further apart, you label the drawing "gas".
But, have you ever wondered just how close you can pack those spheres together?

The study of soot is becoming very important because soot in the air relates to the balance of climate: heating from light absorption versus cooling from light reflection.
Soot is made up of small round particles of carbon about 10 or 20 nanometres across. The particles stick together randomly in short chains and clumps of a half dozen or more spheres. These, in turn, clump loosely together to form larger, loose aggregates of 10 or more which over a few hours will compact into a somewhat tighter ball which is atmospheric soot.
The closer you pack the soot spheres together, then the more dense the soot becomes. That is, since all the atoms have the same mass, if you pack them more tightly together then they will occupy a smaller volume, and, since density is mass per unit volume, the density of the soot will increase.

Mathematicians have been looking at sphere packing problems for more than a hundred years. In 1831, Carl Friedrich Gauss (one of the greatest mathematicians the world has known), determined that a close-packed arrangement in which 1 sphere is surrounded by 12 other identical spheres has an average density of π/√18 ≈ 0.74
The assumed density of soot in models of atmospheric soot has, until now, been 0.74.

A research group at the National Institute of Standards and Technology (NIST), have made measurements of actual soot particles and found the density of soot to be 0.36 not 0.74
So, the researchers set out to model soot particles using 6 mm plastic spheres glued together in thousands of random combinations forming clumps of from 1 to 12 spheres which were then used to fill every available size of graduated cylinder. When they graphed their results as a function of clump size,  they got a curve which levelled off at 0.36, the same as the density of soot particles they measured, but not the same as that predicted by the close-packing of spheres according to Gauss.

It now appears that, under normal conditions (not extreme temperature or pressure), the density of close-packed particles of any size, from nanometres to tens of metres is 0.36

Reference:
C. D. Zangmeister, J. G. Radney, L. T. Dockery, J. T. Young, X. Ma, R. You, M. R. Zachariah. Packing density of rigid aggregates is independent of scale. Proceedings of the National Academy of Sciences, 2014; DOI: 10.1073/pnas.1403768111


Further Reading
http://ausetute.com.au/density.html
http://ausetute.com.au/massconv.html
http://www.ausetute.com.au/voluconv.html

Suggested Study Questions

1. Draw a diagram to represent each of the following states of matter using the particle theory of matter:
   • solid
   • liquid
   • gas
2. Calculate the volume of a spherical carbon particle that is 20 nanometres in diameter.
3. Calculate the volume of a cube in which the length of each side is 20 nanometres.
4. Calculate the ratio of the volume of the sphere to the volume of cube.
5. Why is this ratio NOT 0.74?
6. Draw a diagram showing the close-packing of just 2 carbon particles each with a diameter of 20 nanometres.
7. Draw a square around each sphere to represent the volume of space occupied by the close-packed spheres and label the total volume occupied and the volume of carbon particles.
8. For the close-packing of 2 carbon particles as drawn above, calculate the ratio of the volume occupied by the spheres to the total volume of the rectangular prism.
9. Consider placing 1 more sphere above the 2 spheres you have already drawn. Draw a diagram of an arrangement that will occupy the
   • maximum total space
   • minimum total space
10. Research the difference between hexagonal close packing and cubic close packing.

Wet, Wetting and Wettability

In Surface Chemistry, the concept of wetting or wettability is very important. Understanding wetting enables scientists to make water repellant fabrics, or, to make sponges that soak up oil spills, or to produce detergents that remove grease and oil in water.

Learn more about wetting and wettability with AUS-e-TUTE's news tutorial, game and test.
Members should log-in and go to the Surface Chemistry section for links to the new resources.

Not an AUS-e-TUTE Member?
There is a free tutorial available: http://www.ausetute.com.au/wetting.html

Essential Elements

Most of the mass of the human body is made up of just 6 elements:

• 65% oxygen
• 18.5% carbon
• 9.5% hydrogen
• 3.2% nitrogen
• 1.5% calcium
• 1% phosphorus

Carbon and hydrogen are the building blocks making up organic (carbon) molecules, and so, since the human body is just an incredibly complicated organic chemical factory, it is no surprise that carbon and hydrogen make up such a huge amount of the mass of a human body. Oxygen is present in most of the biochemicals making up your body, while nitrogen is an essential component of proteins. Why is there so much phosphorus? Phosphorus and oxygen combine to make up the glue that holds your DNA together, no phosphorus, no DNA, no you! And calcium is ofcourse present in bones and teeth.
Then there are minor amounts of other elements such as potassium, sulfur, sodium, chlorine and magnesium which make up less than 1% of the remaining mass of the human body.

Some elements, like iron, are absolutely essential in order for the human body to survive, but are present in extremely minute amounts, for iron this is about 0.006%

H																		He
Li	Be												B	C	N	O	F	Ne
Na	Mg												Al	Si	P	S	Cl	Ar
K	Ca		Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr		Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Cs	Ba	*	Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Fr	Ra	**	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Cn	Uut	Fl	Uup	Lv	Uus	Uuo
		*	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb
		**	Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No

Vanderbilt University Scientists have just found out that bromine is also essential.It appears that bromine is important to an enzyme that is used to make a particular type of sulfur-nitrogen bond within the collagen IV structure used to make the scaffold for cells.

Reference:
A. Scott McCall, Christopher F. Cummings, Gautam Bhave, Roberto Vanacore, Andrea Page-McCaw, Billy G. Hudson. Bromine Is an Essential Trace Element for Assembly of Collagen IV Scaffolds in Tissue Development and Architecture. Cell, 2014; 157 (6): 1380 DOI: 10.1016/j.cell.2014.05.009

Further Reading
Periodic Table
Metals and Non-metals

Percentage Composition
Mass-Mole Calculations 

Suggested Study Questions:

1.  Give the chemical symbol for each of the following elements:
   • oxygen
   • carbon
   • hydrogen
   • nitrogen
2. Give the name for each of the following elements:
   • Fe
   • Ca
   • P
   • K
   • Mg
3. Give the atomic number for each of the following elements
   • sulfur
   • sodium
   • iodine
   • Cl
   • F
   • Br
4. Make a table of the metals and non-metals named in the article above
5. Chris the Chemist has a mass of 90 kg. Calculate the mass, in kilograms, of each of the following elements in Chris' body
   • carbon
   • hydrogen
   • oxygen
   • nitrogen
6. Calculate the mass of iron in Chris' body in
   • kilograms
   • grams
   • milligrams
   • micrograms
7. Calculate the moles of each of the following elements found in Chris' body
   • carbon
   • hydrogen
   • oxygen
   • nitrogen
8. Do you think Scientists will one day claim that lead is an element essential for human life? Explain your answer.