PBDEs

Polybrominated diphenyl ethers, PBDEs, have been used as flame retardants in many common household products such as blankets, couches, food wrappers, as well as in motor vehicles and airplanes.
A diphenyl ether molecule has the following structure:

One of the polybrominated diphenyl ether compounds is 2,2',4,4',5-pentabromodiphenyl ether and it has following structure
There are 209 possible substances which are included in the family of polybrominated diphenyl ethers (PBDEs) and these are called congeners. Congeners include dibromodiphenyl ethers, tribromodiphenyl ethers, tetrabromodiphenyl ethers, pentabromodiphenyl ethers, hexabromodiphenyl ethers, heptabromodiphenyl ethers, octabromodiphenyl ethers, nonabromodiphenyl ethers and decabromodiphenyl ethers.

PBDEs with 1-5 bromine atoms per molecule are considered to be more dangerous than PBDEs with more bromine atoms per molecules because they are more efficient at accumulating in living things. In humans they effect hormone levels in the thyroid which can lead to alterations in behaviour. Studies have shown that children in developed countries have higher levels of blood PBDEs than adults, ranging from 24 to 114 parts per billion.

Most PBDEs have either been banned or are in the process of being phased out, eg,:

• Penta and octa PBDEs are no longer manufactured in the USA because of the health risks while the European Union banned their use in 2004.
• Deca PBDE has commonly been used in the electronics industry but has now been banned in Europe.

There is, however, growing concern that PBDEs may have a long environmental life.

Reference
Crystal Y. Usenko, Eleanor M. Robinson, Sascha Usenko, Bryan W. Brooks, Erica D. Bruce. PBDE developmental effects on embryonic zebrafish. Environmental Toxicology and Chemistry, 2011; DOI: 10.1002/etc.570

Further Reading
Nomenclature
Haloalkanes
Structural Isomers
Functional Groups
Percentage Composition
Parts per Million Concentration

Study Questions

1. For the diphenyl ether molecule:
   • identify the ether functional group
   • give the molecular formula
   • calculate the molecular mass (formula weight)
   • calculate the percentage composition
2. For the 2,2',4,4',5-pentabromodiphenyl ether molecule:
   • identify the ether functional group
   • give the molecular formula
   • calculate the molecular mass (formula weight)
   • calculate the percentage composition
3. How many bromine atoms would be found in a molecule of each of the following:
   
   • dibromodiphenyl ether
   • tribromodiphenyl ether
   • tetrabromodiphenyl ether
   • pentabromodiphenyl ether
   • hexabromodiphenyl ether
   • heptabromodiphenyl ether
   • octabromodiphenyl ether
   • nonabromodiphenyl ether
   • decabromodiphenyl ether
4. Draw one structural isomer of each of the molecules above.
5. Name each of the molecules you have drawn in question 4 above.
6. Give the molecular formula for the molecules you have drawn that have been considered to be the least dangerous.
7. The blood of a child is found to have 70ppb PBDE. Convert this to:
   • ppm
   • mg/g
   • g/L
8. Assuming the child above has 4.7L of blood in his/her body, calculate the mass of PBDE present.
9. In the USA, the EPA set a "safe" limit of 7 micrograms per kilogram of body weight for PBDE. If the child above has a mass of 18kg, is the amount of PBDE in his/her blood considered safe? Explain your answer.
   
   
   
   

NMR Without Magnets

Nuclear Magnetic Resonance (NMR) is an important tool in analyzing the structure of organic compounds, and, its relative, Magnetic Resonance Imaging (MRI) is used in medical diagnosis.

Nuclear Magnetic Resonance (NMR) depends on the fact that many atomic nucleii possess spin and their own dipolar magnetic fields. During conventional NMR spectroscopy these nuclei are lined by a strong external magnetic field, then knocked off axis by a burst of radio waves. The rate at which each kind of nucleus then wobbles, or precesses, is unique and identifies the element. For example, a hydrogen-1 nucleus (a lone proton) precesses four times faster than a carbon-13 nucleus (6 protons and 7 neutrons).

Being able to detect these signals depends first of all on being able to detect net spin. If the sample were to have as many spin-up nuclei as spin-down nuclei it would have zero polarization, and the signals would cancel out. But since the spin-up orientation requires slightly less energy, a population of atomic nuclei usually has a slight excess of spin ups, if only by a few in a million.

The lines in a typical NMR spectrum reveal more than just different elements. Electrons near precessing nuclei alter their precession frequencies and cause a "chemical shift", moving the signal or splitting it into separate lines in the NMR spectrum. This is the principal goal of conventional NMR, because chemical shifts point to particular chemical species; for example, even when two hydrocarbons contain the same number of hydrogen, carbon, or other atoms, their signatures differ markedly according to how the atoms are arranged. But without a strong magnetic field, chemical shifts are insignificant.

The down-side is that NMR relies on huge, very low-temperature, superconducting magnets so it is an expensive and non-portable tool. Scientists at Berkeley Lab and UC Berkeley have shown that chemical analysis with NMR is practical without using any magnets at all.

Firstly the scientists have increased the net spin orientation via hyperpolarization which increases the proportion of parahydrogen (in which the proton in each hydrogen nucleus spins in the opposite direction resulting in spin 0) in relation to orthohydrogen (in which the proton in each hydrogen nucleus spins in the same direction resulting in spin 1).

Second, the scientists use optical-atomic magnetometers instead of the huge superconducting magnets used in conventional NMR. Optical-atomic magnetometers measure whole atoms, not just nuclei. An external magnetic field is measured by measuring the spin of the atoms inside the magnetometer's own vapor cell, typically a thin gas of an alkali metal such as potassium or rubidium. Their spin is influenced by polarizing the atoms with laser light; if there's even a weak external field, they begin to precess. A second laser beam probes how much they're precessing and thus just how strong the external field is.

Third, the scientists use J-coupling, instead of chemical shift, for the chemical analysis because you cannot detect chemical shift in a zero field. Discovered in 1950 by the NMR pioneer Erwin Hahn and his graduate student, Donald Maxwell, J-coupling provides an interaction pathway between two protons (or other nuclei with spin), which is mediated by their associated electrons. The signature frequencies of these interactions, appearing in the NMR spectrum, can be used to determine the angle between chemical bonds and distances between the nuclei. You can even tell how many bonds separate the two spins.

Experiments to date have been performed on molecules that are easily hydrogenated and therefore easily hyperpolarized. Beginning with styrene, a simple hydrocarbon, J-coupling has been measured for a series of hydrocarbon derivatives including hexane and hexene, phenylpropene, and dimethyl maleate, important constituents of plastics, petroleum products, even perfumes.

Reference
T. Theis, P. Ganssle, G. Kervern, S. Knappe, J. Kitching, M. P. Ledbetter, D. Budker, A. Pines. Parahydrogen-enhanced zero-field nuclear magnetic resonance. Nature Physics, 2011; DOI: 10.1038/nphys1986

Further Reading
¹H NMR Spectroscopy
Quantum Numbers
Isotopes

Study Questions

1. For conventional ¹H NMR, define the following terms:
   • chemical shift
   • magnetic coupling or spin-spin coupling
   • J-coupling
   • internal standard
2. In conventional ¹H NMR, what is the purpose of an internal standard such as TMS?
   
   
3. In conventional ¹H NMR, what does the number of signals tell you about a sample molecule?
4. In conventional ¹H NMR, what does the relative area of each signal tell you about the sample molecule?
5. In conventional ¹H NMR, what does the relative position of the signals tell you about the sample molecule?
6. For an atom of hydrogen-1, given the possible value(s) for the spin quantum number, m_s.
7. Consider a diatomic molecule of hydrogen, H₂. For each possible spin quantum number for each hydrogen atom, draw a representative diagram using ↑ to represent up-spin and ↓ to represent down-spin.
   
8. For each diagram above, add the values of the spin quantum number for each nucleus in the molecule to find the net spin on the molecule.
9. How many parahydrogen and orthohydrogen molecules did you draw?
   

Hydrogen from Water Splitting

The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.

Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts.
The formula for birnessite is (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O
As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.

The water splitting reaction has two steps:

1. Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
   
2. The protons and electrons combine to form two molecules of hydrogen gas

Reference:
Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049

Further Reading
Oxidation States (Numbers)
Oxidation and Reduction
Balancing Half Equations
Electrolysis - Electrolytic Cells
Percentage Composition

Study Questions:

1. What is meant by the term oxidation state (or oxidation number)?
2. What is the oxidation state (or oxidation number) for each of the following:
   
   • Mn³⁺
   • Mn⁴⁺
   • manganese (II)
   • manganese (IV)
     
3. Write equations to represent each of the following:
   
   • The oxidation of manganese (II) to manganese (IV)
   • The reduction of manganese (IV) to manganese (II)
4. For each reaction in question 3 above, identify:
   • the oxidant
   • the reductant
     
5. Write an equation to represent the first step in the water splitting reaction.
6. Write an equation to represent the second step in the water splitting reaction.
7. Use the equations in question 5 and 6 above to write an overall reaction for the water splitting reaction.
8. For each equation in questions 5 and 6,
   • label the reaction as an oxidation or reduction reaction
   • identify the oxidizing agent(s)
   • identify the reducing agent(s)
9. In the formula of birnessite, (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O, what does the 1.5 H₂O mean?
   
10. Calculate the percentage composition of birnessite.
    
    

Fingerprint Powders

University of Queensland scientists have just completed a study revealing that the human factor involved in the process of identifying a set of fingerprints could lead to errors and false convictions of innocent people.
In the study 37 qualified fingerprint experts and 37 novices were given pairs of fingerprints to examine and decide whether a simulated crime scene matched a potential suspect or not. Some of the print pairs belonged to the "criminal" while others were highly similar but actually belonged to an "innocent" person.
The experts correctly matched just over 92 percent of the prints to the criminal. But, they mistakenly matched 0.68 percent of the prints to the innocent person.

Fingerprint powders are fine powders used in dusting for fingerprints by crime scene investigators. Fingerprint powders are often white or black in colour. White powders would be used on dark surfaces while black powders would be used on light coloured surfaces.

Examples of powders that have been used to dust for fingerprints:

White Powders Black Powders Calcium oxide Chalk Titanium dioxide White tempera (starch + titanium dioxide) Haddonite white (titanium dioxide + kaolin + chalk) Lanconide (zinc sulfide + zinc oxide + barium sulfate + titanium dioxide + bismuth oxychloride + calcium carbonate) Charcoal Graphite Lampblack Dragon's blood (Daemonorops draco plant resin) Haddonite black (lampblack + graphite + powdered acacia) Dactyl black (graphite + lampblack + gum acacia)

The fingerprint powder must be fine enough to show fingerprint details. The finer the powder is the better it should be.
To be a good fingerprint powder the powder must adhere to, or stick to, the fingerprint but not to the surface the fingerprint is on.


Reference:
Association for Psychological Science (2011, May 11). Dusting for fingerprints -- It ain't CSI. ScienceDaily. Retrieved May 15, 2011, from http://www.sciencedaily.com­ /releases/2011/05/110511162536.htm

Further Reading:
Pure Substances and Mixtures
Elements and Compounds
Allotropes
Writing Ionic Formula
Calculating Percentage Composition

Study Questions

1. What are the main chemical constituents of:
   
   • chalk
   • lampblack
   • charcoal
   • graphite
2. Draw up a table dividing the fingerprint powders into pure substances and mixtures.
3. For the pure substances listed in question 2, draw up another table dividing these up into elements and compounds.
   
4. Write the chemical formula for each of the following:
   
   • calcium oxide
   • titanium dioxide
   • zinc sulfide
   • zinc oxide
   • barium sulfate
   • calcium carbonate
5. Calculate the percentage composition of each of the following compounds:
   • calcium oxide
   • titanium dioxide
   • zinc sulfide
   • zinc oxide
   • barium sulfate
   • calcium carbonate
6. Titanium dioxide features in many of the recipes for making white fingerprint powders. What properties of titanium dioxide make it useful for this purpose?
7. Graphite is a common constituent in many black fingerprint powders. What properties of graphite might make it particularly useful as a fingerprint powder?
8. For the University of Queensland study, give examples of the following types of variables:
   
   • independent variable
   • dependent variable
   • controlled variables
9. Based on the information provided in the article, do you think the University of Queensland study was a fair test? Explain your answer.
10. Design an experiment to test the statement that graphite is a better powder than charcoal to use for detecting fingerprints on the surface of tiles.
    
    

Ocean pH

Coccoliths are very small shells of calcium carbonate that form around a number of species of algae. Algae play an important role in the global carbon-oxygen cycle and thus in our ecosystem. Scientists at the Nano-Science Center, University of Copenhagen, have measured how individual coccoliths react to water with different degrees of acidity.

Coccoliths which have a mass of about 500 pg (0.0000000005 g), were weighed before and after they had been immersed in water with different acidities. The results enable the scientists to say something about how important the water acidity is for the marine environment.

The world's oceans are acidifying due to our emissions of carbon dioxide. Over time the pH of the Earth's oceans is decreasing:

Time	pH
18th century	8.179
Recent past (1990s)	8.104
Present levels	~8.069
2050 (estimated)	7.949
2100 (estimated)	7.824

Coccoliths are protected from dissolution by a very thin layer of organic material that the algae form, even though the seawater is extremely unsaturated relative to calcite (calcium carbonate). The protection of the organic material is lost when the pH is lowered slightly. In fact, it turns out that the shell falls completely apart when experiments are done in water with a pH value of 7.8, the pH that many researchers believe will be the found in the world oceans in the year 2100.

Reference:
T. Hassenkam, A. Johnsson, K. Bechgaard, S. L. S. Stipp. Tracking single coccolith dissolution with picogram resolution and implications for CO2 sequestration and ocean acidification. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1009447108\

Further Reading:
Carbon Cycle
Combustion of Hydrocarbons
Acid Rain
Mass Conversions
pH Calculations
Acid Dissociation Constants (Ka)

Study Questions:

1. Write the formula for:
   
   • carbon dioxide
   • calcium carbonate
2. Calculate the percentage composition of
   
   • carbon dioxide
   • calcium carbonate
3. Write a balanced chemical equation to show the production of carbon dioxide from the combustion of methane (a fossil fuel).
4. Assuming 100 tonnes of methane gas were combusted under standard laboratory conditions:
   
   • What is the maximum mass of carbon dioxide that could be produced?
   • What volume would this mass of carbon dioxide occupy?
5. Coccoliths have a mass of about 500 pg. Convert this to a mass in:
   • milligrams
   • micrograms
   • nanograms
   • kilograms
6. The current pH of ocean water is approximately 8.069. Assuming the temperature of the oceans to be 25^oC, calculate the current:
   • hydrogen ion concentration of ocean water
   • hydroxide ion concentration of ocean water
   • pOH of ocean water
   
   
7. In 2100, the pH of ocean water is predicted to be 7.824. Assuming the temperature of the oceans to be 25^oC, calculate the:
   • hydrogen ion concentration of ocean water in 2100
   • the hydroxide ion concentration of ocean water in 2100
   • the pOH of ocean water in 2100
   • the increase in hydrogen ion concentration between now and 2100
     
   
   
   
8. For the reaction: H₂CO₃ HCO₃^- + H⁺ K_a = 4.5 x 10^-7
   
   • Is H₂CO₃ a strong acid or weak acid? Explain your answer.
     
   • Calculate the concentration of H⁺
   • Calculate the pH of the solution.
   • Calculate the pOH of this solution at 25^oC.
   • Calculate the concentration of hydroxide ions at 25^oC.
   • Explain what impact an increase in hydrogen ion concentration would have on this equation.
     

Making Methanol

Methanol as an energy source can be used as a fuel in the same way as petrol (gasoline), or it can be used in fuel cells. About 90% of the worldwide production of methanol is derived from methane, the main component of natural gas. Current methods for producing this methanol involve converting methane into syngas, a mixture of carbon monoxide and hydrogen, and then converting this syngas into methanol. Eliminating the syngas stage would dramatically reduce the cost of producing methanol.
But methane is not very reactive, and combines readily with oxygen only at high temperatures. A catalyst helps, but commonly used catalysts themselves work only at 300^oC or higher. At these temperatures, most of the methanol produced is oxidized to carbon dioxide and water. Indeed, methanol yields from such reactions can be as low as 2%.

A lower temperature catalyst such as platinum dissolved in concentrated sulfuric acid at 200^oC, has achieved a methanol yield of more than 70% in the laboratory, but platinum is an expensive metal.

Methane can also be converted to methanol in the laboratory using a halogen such as bromine. Using a suitable catalyst at 250^oC methane reacts with bromine to form bromomethane (methylbromide) and hydrogen bromide. Bromomethane (methyl bromide) then reacts with water to form methanol. The bromine from the hydrogen bromide can be recovered by reaction with air, and reused.

Methanol can be made by combining carbon dioxide and hydrogen. Such a process requires considerable energy just to harvest the hydrogen from water, for example. The carbon dioxide could be captured from flue gases, and even directly from the atmosphere.

Further Reading
Nomenclature
Alcohols
Balancing Chemical Equations
Combustion of Hydrocarbons
Halogenation of Hydrocarbons
Fuel Cells and Batteries
Temperature Conversions
Ideal Gas law
Yield

Study Questions

1. Write the chemical formula for each of the following:
   
   • methanol
   • methane
   • carbon monoxide
   • hydrogen gas
   • oxygen gas
   • carbon dioxide
   • water
   • bromine liquid
   • bromomethane (methylbromide)
   • hydrogen bromide
2. Write balanced chemical equations for each of these reactions:
   
   • carbon monoxide + hydrogen gas → methanol
   • carbon dioxide + hydrogen gas → methanol
     
   • methane + oxygen gas → carbon dioxide gas + water
   • methane + oxygen gas → methanol
   • methane + bromine liquid → bromomethane + hydrogen bromide
   • bromomethane + water → methanol + hydrogen bromide
     
   • water → hydrogen gas + oxygen gas
3. Convert the following temperatures in ^oC to Kelvin
   • 200^oC
   • 250^oC
   • 300^oC
   
   
4. For the reaction between methane and oxygen to produce methanol, calculate the theoretical yield of methanol that could be produced from 100kg of methane.
5. Using the platinum-based sulfuric acid catalyst at 200^oC, yields of 70% have been achieved for the above reaction.
   • What mass of methanol is actually produced during this reaction if you start with 100kg of methane?
   • Convert this mass to moles.
   • Calculate the volume of methanol gas produced.
     
6. At 300^oC the yield of methanol produced from the reaction between methane and oxygen is 2%. Assume the reaction starts with 100L of methane gas
   • Calculate the moles of methane gas in the reaction mixture
   • Calculate the theoretical yield of methanol that could be produced
   • Calculate the actual yield of methanol
7. Why do you think it is important for Chemists to continue to search for inexpensive catalysts for the methane to methanol reaction?