Calcite

The teeth and bones of mammals, the shells of marine organisms, even the sharp spines of sea urchins, are all made of the same material, calcite. Calcite is a carbonate mineral and is the most stable form of calcium carbonate.

Marine organisms use the most readily available elements in the sea water, that is calcium, oxygen and carbon, to produce their teeth, bones or shells. In seawater solutions, organisms can use the available calcium and carbonate ions to precipitate calcium carbonate.

Calcite is also commonly found in sedimentary rocks, and especially in limestone which is formed from the shells of dead marine organisms. Calcite is the mineral found in cave formations such as stalactites and stalagmites. Overlying limestones and marbles are dissolved away by slightly acidic groundwater which then percolates into caverns below. As the water evaporates, calcite is precipitated.


Calcite has some unusual properties.
It becomes less soluble in water as the temperature increases, a property known as retrograde solubility.
Single crystals of calcite display an optical property known as birefringence which causes objects viewed through a piece of calcite to appear to be doubled.

The sea urchin tooth is a biomineral composed of calcite crystals in both plate and fiber forms, arranged crosswise and cemented together with super-hard calcite nanocement. Between the crystals are layers of organic materials which are weaker than the calcite crystals. These weaker organic materials allow pieces of the tooth to break off in predefined places which means that the sea urchin tooth is self-sharpening!

Reference
Christopher E. Killian, Rebecca A. Metzler, Yutao Gong, Tyler H. Churchill, Ian C. Olson, Vasily Trubetskoy, Matthew B. Christensen, John H. Fournelle, Francesco De Carlo, Sidney Cohen, Julia Mahamid, Andreas Scholl, Anthony Young, Andrew Doran, Fred H. Wilt, Susan N. Coppersmith and Pupa U. P. A. Gilbert. Self-Sharpening Mechanism of the Sea Urchin Tooth. Advanced Functional Materials, 2010; DOI: 10.1002/adfm.2010015

Further Reading
Naming Ionic Compounds
Writing Ionic Formulae
Molecular Mass
Percentage Composition
Shapes of Molecules
Lewis Structures
Solubility Product

Study Questions

1. Give the chemical formula for calcium carbonate.
2. List the names and formulae of other compounds that can be made out of carbon, oxygen and calcium.
3. Calculate the percentage composition of calcium carbonate as well as the compounds you listed in question 2.
   
4. Write a balanced chemical equation to show the precipitation of calcium carbonate from ions in sea-water.
5. What shape do you expect the carbonate ion to take? Explain your answer.
6. Draw a Lewis Structure, electron dot diagram, for a "molecule" of calcium carbonate.
7. Why is it inaccurate to describe calcium carbonate as a molecule?
   
8. Describe the process by which calcium carbonate can dissolve in water.
9. Would calcium carbonate more readily precipitate from fresh water or sea-water at the same temperature? Explain your answer.
   

Palmitoleic Acid

Scientists have discovered that trans-palmitoleic acid or trans-9-hexadecenoic acid, a fatty acid found in milk, cheese, yoghurt and butter, can reduce the risk of type 2 diabetes. Trans-palmitoleic acid is not produced in the body whereas cis-palmitoleic acid is produced in the body and is already known from animal experiments to protect against diabetes.

Cis-palmitoleic acid is made in the body using palmitic acid and the enzyme delta-9 desaturase.

In 2001, scientists suggested that fatty acids, such as palmitoleic acid found on the surface of skin, decompose to form 2-nonenal which has an unpleasant smell, a bit like old books, and could be the cause of what is commonly referred to as "old person smell".

Palmitoleic acid is used by the key enzymes in the body that control fat oxidation at very high rates. It has been suggested that palmitoleic acid could therefore be used to combat obesity.

Reference
Dariush Mozaffarian, Haiming Cao, Irena B. King, Rozenn N. Lemaitre, Xiaoling Song, David S. Siscovick, and Gökhan S. Hotamisligil. Trans-Palmitoleic Acid, Metabolic Risk Factors, and New-Onset Diabetes in U.S. Adults. Annals of Internal Medicine, December 21, 2010

Further Reading
Lipids
Carboxylic Acids
Functional Groups
Enzymes

Study Questions

1. Write the molecular formula for palmitoleic acid
2. On the structural formula for palmitoleic acid shown above, locate and name two different functional groups.
3. Draw a structural formula for palmitic acid, CH₃(CH₂)₁₄COOH.
4. Locate and name the functional group present in palmitic acid.
5. Draw a structural formula for oleic acid, CH₃(CH₂)₇CH=CH(CH₂)₇COOH.
6. Locate and name the functional groups present in oleic acid.
7. In what ways is palmitoleic acid similar to palmitic acid and oleic acid?
8. In what ways is palmitoleic acid different to palmitic acid and oleic acid?
9. Draw a structural formula for 2-nonenal.
10. Write the molecular formula for 2-nonenal.
11. Locate and name the functional groups present in 2-nonenal.
12. How would cis-palmitoleic acid differ structurally from trans-palmitoleic acid?
    

New Nitrogen Oxide Compound

Chemists already know about a number of compounds composed of nitrogen and oxygen:

• Nitric oxide is a colourless gas which is produced during the combustion of fossil fuels. It has a melting point of -163.6^oC, a boiling point of -150.8^oC, and rapidly oxidizes in air to form nitrogen dioxide. Its solubility in water is 7.4ml/100ml.
  
• Nitrogen dioxide is a reddish-brown toxic gas. Its melting point is -11.2^oC and its boiling point is 21.1^oC. It reacts with water to produce nitric acid.
• Nitrous oxide is a colourless gas commonly referred to as laughing gas. It is a greenhouse gas and also causes ozone depletion. Its melting point is -90.86^oC and its boiling point is -88.48^oC. Its solubility in water is 0.15g/100ml.
• Dinitrogen trioxide is a deep blue liquid at low temperatures. It has a melting point of -100.1^oC, a boiling point of 3^oC , and is very soluble in water.
• Dinitrogen tetroxide is a colourless liquid which is a powerful oxidizing agent and highly toxic. Its melting point is -11.2^oC, its boiling point is 21.1^oC, and it reacts with water to produce both nitrous aid and nitric acid.
• Dinitrogen pentoxide exists as colourless crystals. It is unstable and potentially dangerous oxidizer. It has a melting point of 30^oC and it sublimes at 47^oC. Dinitrogen pentoxide reacts with water to produce nitric acid.
  

Chemists at the Royal Institute of Technology in Sweden have discovered a new molecule in this nitrogen oxide group. The new molecule, composed only of nitrogen and oxygen, is called trinitramid. Trinitramid has the chemical formula N(NO₂)₃ and is shaped like a propeller.

Reference
Martin Rahm, Sergey V. Dvinskikh, István Furó, Tore Brinck. Experimental Detection of Trinitramide, N(NO₂)₃. Angewandte Chemie International Edition, 2011; (forthcoming)

Further Reading
Molecular Mass
Percentage Composition
Definitions of a Mole
Mass-Mole Calculations
Lewis Structures
Intermolecular Forces

Study Questions

1. Give the chemical formula for each of the following molecules:
   • nitric oxide
   • nitrogen dioxide
   • nitrous oxide
   • dinitrogen trioxide
   • dinitrogen pentoxide
2. Calculate the molecular mass (formula weight) of each of the molecules above.
3. Calculate the percentage composition of each of the molecules above.
4. Draw a Lewis Structure (electron dot diagram) for each of the molecules above.
5. Use the information in the article above, as well as your calculations, tabulate the melting points, boiling points and solubility of each molecule.
6. Graph melting points and boiling points against molecular mass.
   
7. Can you identify any trends in the melting points, boiling points, or solubilities of these molecules? Explain any trends you identify.
8. So far, scientists have only produced enough trinitramid to detect it. What do you predict its melting point, boiling point and solubility in water to be? Explain your answer.
   

Atomic Weights to Change

The atomic weights of 10 elements are to be changed in order to more accurately reflect how these elements are found in nature. These 10 elements are:

• hydrogen
• lithium
• boron
• carbon
• nitrogen
• oxygen
• silicon
• sulfur
• chlorine
• thallium

The atomic weights of these 10 elements will now be expressed as intervals, having upper and lower bounds.
For example, sulfur is commonly known to have a standard atomic weight of 32.065. However, its actual atomic weight can be anywhere between 32.059 and 32.076, depending on where the element is found. In sports doping investigations, performance-enhancing testosterone can be identified in the human body because the atomic weight of carbon in natural human testosterone is higher than that in pharmaceutical testosterone.

Elements with only one stable isotope do not exhibit variations in their atomic weights. For example, the standard atomic weights for fluorine, aluminum, sodium and gold are constant, and their values are known to better than six decimal places.

IUPAC will feature the change in the standard atomic weights table as part of associated International Year of Chemistry activities in 2011.

Reference
Michael E. Wieser, Tyler B. Coplen. Atomic weights of the elements 2009 (IUPAC Technical Report). Pure and Applied Chemistry, 2010; 1 DOI: 10.1351/PAC-REP-10-09-14

Further Reading
http://www.ausetute.com.au/isotopes.html
http://www.ausetute.com.au/atomicmass.html

Study Questions

1. What is meant by the term isotope?
2. How is atomic weight calculated?
3. If naturally occurring hydrogen contains 99.99% hydrogen-1 and 0.01% deuterium, what is the atomic weight of naturally occurring hydrogen?
4. If you took an air sample from a planet on which there was 100 times more deuterium than on earth, what would you expect the atomic weight of hydrogen to be then?
5. Why is tritium not included in the calculation of the atomic weight of hydrogen?
   
6. Why would the atomic weight of an element with only one stable isotope be more consistent than the atomic weight of an element with two or more stable isotopes?
7. Why are unstable isotope abundances not used when calculating the atomic mass of a naturally occurring element?
8. Why would the atomic weight of carbon in natural human testosterone be higher than that in pharmaceutical testosterone?

Calcium Carbide

Pure calcium carbide, CaC₂, is a colourless compound. It is a very important compound because it is used to produce ethyne (acetylene) which is itself an extremely important industrial compound. The combustion of ethyne (acetylene) in oxyacetylene welding and cutting produces a flame of over 3300^oC, making it the third hottest natural chemical flame, it and releases 11.8kJ/g of energy.

Calcium carbide is produced in an electric arc furnace in which a mixture of lime, CaO, and coke, C, is heated to about 2000^oC. The products of the reaction are calcium carbide and carbon monoxide gas. This reaction produces a calcium carbide yield of approximately 80%.

Calcium carbide reacts with water to produce ethyne (acetylene) and calcium hydroxide. The ethyne produced in this reaction can be used directly in combustion reactions such as in oxyacteylene welding and cutting, as well as in carbide lamps which are still in use in mines in some less wealthy countries.

Further Reading
http://www.ausetute.com.au/namsynes.html
http://www.ausetute.com.au/intrafor.html
http://www.ausetute.com.au/balcheme.html
http://www.ausetute.com.au/combusta.html
http://www.ausetute.com.au/tempconv.html
http://www.ausetute.com.au/enthchan.html
http://www.ausetute.com.au/manienth.html
http://www.ausetute.com.au/percentc.html
http://www.ausetute.com.au/yield.html

Study Questions

1. Write the molecular and structural formula for ethyne (acetylene).
2. What type of bonding is present in the calcium carbide compound? Explain your answer.
3. Write a balanced chemical equation for the reaction between lime and coke.
   
4. Write a balanced chemical equation for the production of ethyne (acetylene) from calcium carbide.
5. Convert these temperatures in ^oC to Kelvin:
   • 3300^oC
   • 2000^oC


6. Is the combustion of ethyne (acetylene) an endothermic or an exothermic reaction? Explain your answer.
7. Calculate the energy released during the combustion of :
   • 5kg of ethyne (acetylene)
   • 500g of ethyne (acetylene)
8. Calculate the energy released from the combustion of
   • 5 moles of ethyne (acetylene)
   • 0.1moles ethyne (acetylene)
9. Calculate the percentage composition of
   • calcium carbide
   • ethyne (acetylene)
10. Explain what is meant by the statement below:
    "This reaction produces a calcium carbide yield of approximately 80%".

Reaction Rates and Evolution

University of North Carolina scientists have been studying the effect of temperature on extremely slow chemical reactions in order to determine whether life on Earth originated in a hot or cold environment and whether enough time has passed in order for life to have evolved to its current complexity. Their investigations suggest that the time required for evolution on a warmth earth is shorter than critics might expect.

They found that the influence of temperature on reaction rates varies dramatically. In one slow reaction, raising the temperature from 25 to 100^oC increased the rate 10 million fold!
High temperatures were probably a crucial influence on reaction rates when life began forming in hot springs and submarine vents. Later, the cooling of the earth provided elective pressure for primitive enzymes to evolve and become more sophisticated.

Using two different reaction catalysts which are not protein enzymes but that resemble the precursors to enzymes, they found that the catalyzed reactions were indeed less sensitive to temperature.

Reference
R. B. Stockbridge, C. A. Lewis, Y. Yuan, R. Wolfenden. Impact of temperature on the time required for the establishment of primordial biochemistry, and for the evolution of enzymes. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.1013647107

Further Reading
http://www.ausetute.com.au/reactrate.html
http://www.ausetute.com.au/enerprof.html
http://www.ausetute.com.au/enzymes.html
http://www.ausetute.com.au/proteins.html

Study Questions

1. Explain why an increase in temperature generally speeds up the rate of a chemical reaction.
2. Define both the following terms :
   • catalyst
   • protein
   • enzyme
3. Draw an energy profile diagram to show the effect of a catalyst on a reaction.
   
4. Why would the researchers choose to use a non-protein based catalyst to study reactions that possibly occurred early on in the Earth's history?
5. Why is the study of catalyzed reactions, especially enzyme catalyzed reactions, important when studying the origins of life on Earth?
6. There is an enzyme, catalase, present in liver that speeds up the rate of decomposition of hydrogen peroxide. Design an experiment to demonstrate the effect of temperature change on this reaction.
   

Graphene: AUS-e-NEWS December 2010

Excitement is growing in the scientific community about the possible uses for graphene.
This simple, naturally occurring allotrope of carbon could revolutionize our world.
The December 2010 issue of AUS-e-NEWS, AUS-e-TUTE's quarterly newsletter, takes a look at the chemistry of graphene, and at its possible future uses.

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