Oxygen Evolution Reactions

Oxygen Evolution Reactions (OER) produce molecular oxygen via chemical reactions. These reactions are being studied because of their importance to the development of energy-storage systems.

During photosynthesis, molecular oxygen is produced from water as shown in the equation below:
2H₂O → 4e + 4H⁺ + O_2(g)so photosynthesis is an Oxygen Evolution Reaction.

The electrolysis of water, is another Oxygen Evolution Reaction. Oxygen gas is produced according to the equation shown below:
2H₂O_(l) → O_2(g) + 2H_2(g)
In either case, the chemical reaction to split water in order to produce oxygen gas is not spontaneous, it requires the addition of energy. In the case of the electrolysis of water in the laboratory, electrical energy is supplied from a battery or a power pack. In the case of photosynthesis, sunlight provides the energy for the reaction in the form of photons.
A catalyst can be used to speed up the reaction. In the case of photosynthesis, chlorophyll is the catalyst present in green plants. Scientists are continually working to find a good catalyst that will work just like chlorophyll. TiO₂, SrTiO₃, and BaTiO₃ have all been investigated as possible catalysts.

A team of MIT researchers have just found one of the most effective catalysts ever discovered for an Oxygen Evolution Reaction. The new catalyst is composed of cobalt, iron, oxygen and some other metals.

Reference
J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. Shao-Horn. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science, 2011; DOI: 10.1126/science.1212858

Further Reading
Carbon Cycle
Electrolysis
Mass-Mole Calculations
Mole Calculations
Gas Tests

Suggested Study Questions

1. Assuming 18g of water is to be split to provide oxygen gas. Calculate the maximum mass of oxygen gas that could be obtained.
2. Assuming 1L of water is to be electrolyzed at 25^oC and 1 atmosphere pressure. What is the maximum volume of oxygen gas that could be produced?
3. During the electrolysis of water, what is produced at the
   • cathode
   • anode
4. During the electrolysis of water, what is the charge on the
   • cathode
   • anode
5. During electrolysis name the electrode at which each of the processes below occurs:
   • oxidation
   • reduction
6. Write balanced equations to represent the reaction that occurs at the
   • cathode
   • anode
7. Calculate the minimum voltage that would be required in order for the electrolysis of water to produce both hydrogen gas and oxygen gas.
8. Describe a test that you could conduct in the laboratory that would allow you to say which of the gases evolved during the electrolysis of water is oxygen and which is hydrogen.
   
   
   
   
   

Defining Electronegativity

Electronegativity is defined as the power of an atom to attract electrons to itself, but it cannot be directly measured so it must be calculated using other properties of atoms. There are many different ways of calculating electronegativity.

Pauling Electronegativity
In 1932 Pauling proposed the concept of electronegativity to explain why the covalent bond formed between two different atoms, eg A-B bond, is stronger than you might expect if you were to take an average of the A-A and B-B bond strengths. Since Pauling's electronegativities are based on the differences in bond strengths between atoms, it is necessary to choose an arbitrary reference point so that these electronegativities can be compared and used to make predictions. Hydrogen, with an electronegativity of 2.20, has been used as the reference to build up a table of electronegativities. Pauling electronegativities have no units.

Mulliken Electronegativity
Mulliken electronegativities are based on the mean of the first ionization energy and the electron affinity and has the units kJ/mol or eV.

Allred-Rochow Electronegativity
Allred-Rochow electronegativity is related to the charge experienced by an electron on the 'surface' of an atom and is calculated using the ratio of effective nuclear charge experienced by the valence electrons and the square of the covalent radius.

Sanderson Electronegativity
Similar to the Allred-Rochow electronegativity, Sanderson's calculations use atomic volume instead of the square of the covalent radius.

Allen Electronegativity
Allen electronegativity is related to the average energy of the valence electrons in a free atom.

Solid State Energy Scale
In 2011, Oregon State University scientists created a new method to describe electronegativity. In this approach, electronegativity is characterised as the solid state energy of elements in a compound, and shows that electrons simply move from a higher energy to a lower energy.

Reference
Brian D. Pelatt, Ram Ravichandran, John F. Wager, Douglas A. Keszler. Atomic Solid State Energy Scale. Journal of the American Chemical Society, 2011; : 111003131629001 DOI: 10.1021/ja204670s

Further Reading
Electronegativity Trends
Electronegativity and Bond Polarity
Interactive Periodic Table of the Elements
Ionization Energy and Electron Configuration

Suggested Study Questions

1. Use the Interactive Periodic Table to find the Pauling Electronegativity for each of the following elements:
   • sodium
   • potassium
   • magnesium
     
   • calcium
   • oxygen
   • sulfur
   • chlorine
   • fluorine
2. Use the values for the electronegativities above to describe each of the following compounds as ionic or covalent:
   • sodium chloride
   • magnesium oxide
   • potassium fluoride
   • calcium oxide
   • sulfur dioxide
   • chlorine dioxide
   • ozone
3. For each of the covalent compounds above, describe the compound as non-polar or polar covalent.
4. What is meant by the term ionization energy?
5. What is meant by the term electron affinity?
6. How is electronegativity differen to electron affinity?
7. If an element has a very low first ionization energy, do you expect it to have a high of low Pauling electronegativity? Explain your answer.
8. If an element has a high Pauling electronegativity, do you expect it to have a high or low value for its electron affinity? Explain your answer.
   
   

Sunken Silver

In 1941, the British cargo ship SS Gairsoppa was carrying 7,000 tonnes of cargo from Calcutta. The cargo included pig iron, tea and about 200 tonnes of silver. A German U-boat torpedo sunk the SS Gairsoppa as it made its way to Ireland in stormy weather. While only one person out of the 85 crew members on board survived the attack, it is believed its cargo of silver is still at the bottom of the ocean.

Silver is unique because it has the highest electrical and thermal conductivity of any known element. It is a soft metal, just a little harder than gold, and is extremely ductile and malleable meaning it can be bent or beaten into almost any shape.
There were many uses for silver during World War II :

• many electrical connectors and switches were silver plated
• silver bus bars were needed for the new aluminum plants (aluminium aircraft)
• silver replaced large amounts of tin in solder
• silver was used in the reflectors in lights

There are a number of silver alloys:

• Fine silver contains 99.9% by mass silver
• Britannia silver contains 95.84% by mass silver with copper making up the remaining mass
  
• Sterling silver contains 92.5% by mass silver and 7.5% by mass copper.
• Argentium sterling silver is a modern alloy containing 92.5% silver and 7.5% by mass of copper and germanium
• Electrum is a natually occurring alloy of gold and silver. The % by mass of gold can be within the range of 70-90%.

And, silver is present in most coloured carat gold alloys:

• 9 carat gold contains 62.5% silver and 37.5% gold
• 22 carat gold contains 91.7% gold with the remaining mass made up of silver and/or copper
  

Silver is stable in pure air and water, but tarnishes when exposed to air or water containing ozone or hydrogen sulfide. In the presence of oxygen gas and hydrogen sulfide gas, elemental silver forms the dark-coloured silver (I) sulfide and water.

Reference
Shipwreck of SS Gairsoppa reveals £150m silver haul
BBC News Online
http://www.bbc.co.uk/news/uk-15061868

Further Reading
Properties of Metals and Non-metals
Percent by Mass
Writing Ionic Formula
Balancing Chemical Equations

Suggested Study Questions

1. For the element silver, give the
   • chemical symbol
   • atomic number (Z)
   • atomic mass
     
2. For each of the uses given for silver in the article above, explain which physical and/or chemical properties of silver make it ideal for that use.
3. Using the composition of the various silver alloys provided in the article above, place the alloys in order of increasing mass of silver present in a 1kg sample.
4. Using the information contained in the article above, describe the relationship between the mass of gold in a sample and the use of the term carat.
5. Place the following terms in order of decreasing mass of gold: 9 carat gold, 18 carat gold, 22 carat gold and 24 carat gold.
6. Write a word equation to describe the process of tarnishing in air that contains some hydrogen sulfide.
7. Write a balanced chemical equation for the tarnishing of silver in air that contains some hydrogen sulfide.
8. It is thought that the silver being carried by the SS Gairsoppa contained some gold. Which alloy of silver could this be? Explain your answer.
9. The SS Gairsoppa is resting 4,700m below the ocean's surface. Do you expect the silver to be tarnished? Explain your answer.
   

Skutterudites and Thermoelectric Generators

More than 60 percent of the energy produced by cars, machines, and industry around the world is lost as waste heat. If we could use this wasted energy we could improve the efficiency with which we use fuels, and benefit the environment.

Thermoelectric generators are devices which convert heat energy directly into electrical energy. Semi-conducting bismuth telluride, Bi₂Te₃, can be used to convert heat into electrical energy, but it is only about 5% efficient, too low to be useful in practical thermoelectric generators.
A number of scientists have been working with skutterudites to see if they can be used to increase the efficiency of thermoelectric generators.
Skutterudites have the general formula MX₃ in which M can be cobalt, rhodium or iridium, and X can be phosphorus, arsenic or antimony. The most promising of these compounds have been the CoSb₃. These compounds have 32 atoms in the unit cell and can be represented with the Co atoms occupying the corners of cubes.
The thermal conductivity of CoSb3 is too high for them to be used effectively.

So scientists have tried adding fillers to the structure to reduce the thermal conductivity. Rare earth elements and alkaline earth metals have been used as fillers.

Until recently these compounds have taken many days to make and have been expensive to produce. Oregan State University scientists have found a way to use microwaves to turn powdered metals into skutterudites in a few minutes and at a much lower cost. The first compound they produced using this technique was an indium cobalt antimonite compound in which indium is the filler.

Reference
Krishnendu Biswas, Sean Muir, M. A. Subramanian. Rapid Microwave Synthesis of Indium Filled Skutterudites: An energy efficient route to high performance thermoelectric materials. Materials Research Bulletin, 2011; DOI: 10.1016/j.materresbull.2011.08.058

Further Reading
Periodic Table
Writing Ionic Formula
Naming Ionic Compounds

Suggested Study Questions

1. Complete the following sentences:
   • A thermoelectric generator converts heat energy into ? energy.
   • A battery converts ? energy into electrical energy.
   • In a torch, the ? energy in the battery is converted into ? energy when the torch is turned on.
     
   • The ? energy in petrol (gasoline) is converted into ? energy when the fuel is combusted.
   • The ? energy released during combustion of a fuel can be converted into ? energy to move a car forward.
2. Skutterudites have the general formula MX₃. Write the formula of the skutterudite formed in each of the following situations:
   • M = cobalt and X = antimony
   • M = rhodium and X = phosphorus
   • M = iridium and X = arsenic
3. Give the name for each of the compounds formed in question 2.
4. For each of the following pairs of atoms, determine which is the most electronegative:
   • cobalt and antimony
   • rhodium and phosphorus
   • iridium and arsenic
5. Locate the elements cobalt, rhodium and iridium in the Periodic Table. In what ways do you expect these elements to be similar? Explain your answer.
6. Locate the elements phosphorus, arsenic and antimony in the Periodic Table. In what ways do you expect these elements to be similar? Explain your answer.
7. Give the names and chemical symbols of four examples of rare earth elements.
8. Give the names and chemical symbols of four examples of alkaline earth metals.
9. Write a possible formula for the skutterudite indium cobalt antimonite.
10. One structure has been represented as In_xCe_yCo₄Sb₁₂. Explain why this is an example of a skutterudite.
    
    
    
    

Measuring Iodine in Water

The recommended daily intake of iodine for a typical adult is about 150μg per day, of which the thyroid gland uses about half. Iodine deficiency results in a condition known as hypothyroidism which produces symptoms such as extreme fatigue, goitre, mental slowing, depression, weight gain, and low basal body temperatures. Excess iodine intake results in symptoms similar to those of iodine deficiency.
Iodine is a known disinfectant and is used to inhibit the growth of microorganisms in the drinking water stored at the International Space Station. Scientists at Iowa State University have developed a simple test to measure the concentration of iodine in this drinking water.

A 10-milliliter water sample is run through a thin, one-centimeter disk that changes colour from white to yellow to orange to rust-red as the concentration of iodine increases. A handheld device, a diffuse reflectance spectrometer, can read the disk's color changes and precisely measure the concentration of molecular iodine, or I₂. The whole process is called colorimetric solid phase extraction.

After a series of successful space tests in 2009 and 2010, this water-testing equipment is now certified operational hardware and is part of the space station's environmental monitoring toolbox.

Reference:
Iowa State University (2011, September 15). Chemists help astronauts make sure their drinking water is clean. ScienceDaily. Retrieved September 20, 2011, from http://www.sciencedaily.com­ /releases/2011/09/110914171753.htm

Further Reading
Mass Conversions
Mole Definitions
Molarity
w/v %
ppm

Suggested Study Questions

1. Convert 150μg to a mass in
   • grams
   • milligrams
   • kilograms
   • nanograms
2. Convert 10 milliliters to a volume in
   • liters
   • microliters
   • nanoliters
   • kiloliters
     
   
   
3. For 150μg of iodine, find the
   
   • moles of molecular iodine
   • moles of iodine atoms
   • number of iodine atoms
   • number of molecules of molecular iodine
     
   
   
4. In the Space Station the iodine is dissolved in the water supply. If an astronaut must consume 3L of water a day in order to consume the recommended daily allowance of iodine, calculate the concentration of the iodine in the water in
   • g/mL
   • ppm
   • M
   • % by mass of iodine
5. Assuming 10mL of the iodine solution from question 4 is run through the water-testing disk as described in the article above. Calculate the mass of iodine present in this sample in
   
   • grams
   • milligrams
   • micrograms
   • nanograms
6. Some water is held in contingency water containers (CWCs) which look like duffle bags and hold about 40kg of water each. Assuming each bag is treated with enough iodine to provide the recommended daily intake of iodine if an adult consumes 3L per day, calculate
   
   • the mass of iodine in each CWC in grams
   • the mass of iodine in each CWC in milligrams
   • the mass of iodine in each CWC in micrograms
     
   • the concentration of iodine in each CWC in g/mL
   • the concentration of iodine in each CWC in ppm
     
   • the concentration of iodine in each CWC in mol/L
     
   
   

Heavy Metal Pollution

Hard water contains calcium and magnesium cations which can be removed using a suitable ion exchange resin.
Heavy metal ions such as cadmium, copper, chromium, mercury, nickel, lead and zinc, may also be present in water at unacceptable levels. This is referred to as heavy metal pollution of water.
Run off from roads is a large source of heavy metal contamination in water:

• Lead: leaded petrol (gasoline), tire wear, lubricating oil and grease, bearing wear
• Zinc: tire wear, motor oil, grease, brake emissions, corrosion of galvanized parts
• Iron: car body rust, engine parts
• Copper: bearing wear, engine parts, brake emissions
• Cadmium: tire wear, fuel burning, batteries
• Chromium: air conditioning coolants, engine parts, brake emissions
• Nickel: diesel fuel and petrol (gasoline), lubricating oil, brake emissions

Many of these heavy metals form negatively charged complex ions in water.
For example, chromium can form the chromate ion, CrO₄²⁻, and the dichromate ion, Cr₂O₇^2-.
Existing water treatment processes to remove these negatively charged ions can be inefficient and expensive.

Chemists at the University of California, Santa Cruz, have now developed a new type of material that can soak up negatively-charged pollutants from water. The new material called SLUG-26 contains copper hydroxide ethanedisulfonate.

Copper hydroxide ethanedisulfonate has a layered structure of positively-charged two-dimensional sheets with a high capacity for holding onto negative ions. These positively charged sheets are made up of [Cu4(OH)6]2+ ions as shown to the right. Copper atoms are shown in green and oxygen atoms are shown in red. The ethanedisulfonate exists as a counteranion.

This material could be used to treat polluted water through an ion exchange process similar to water softening. SLUG-26 provides a positively-charged substrate that can exchange a nontoxic negative ion for the negatively-charged pollutants.
The researchers are currently focusing on the use of SLUG-26 to trap the radioactive metal technetium, which is a major concern for long-term disposal of radioactive waste. Technetium is produced in nuclear reactors and has a long half-life of 212,000 years. It forms the negative ion pertechnetate in water and can leach out of solid waste, making groundwater contamination a serious concern.

Reference
Honghan Fei, Scott R. J. Oliver. Copper Hydroxide Ethanedisulfonate: A Cationic Inorganic Layered Material for High-Capacity Anion Exchange. Angewandte Chemie, 2011; DOI: 10.1002/ange.201104200

Further Reading
Periodic Table
Oxidation States
Zeolites

Suggested Study Questions

1. Give the chemical symbol for each of the following elements:
   • calcium
   • magnesium
   • cadmium
   • copper
   • chromium
   • mercury
   • nickel
   • lead
   • zinc
   • technetium
2. Identify which group of the periodic table each of the metals above belongs to.
3. Give the oxidation state for the metal atom in each of the following compounds:
   • CrO₄²⁻
   • Cr₂O₇^2-
   • [Cu₄(OH)₆]²⁺
4. Zeolites can be used to soften hard water. Write an equation to describe the process in which calcium ions in hard water are exchanged for sodium ions.
5. Suggest some possible non-toxic anions that could be exchanged for the toxic pertechnetate anion using SLUG-26.
6. Write an equation to show how the pertechnetate anion could be exchanged for one of the non-toxic anions in question 5.
7. Describe some other possible uses, apart from the removal of pertechnetate anions, for SLUG-26.
   
   
   

Stable Foam

Foams often have detergent properties due to their particular texture and the molecules that make up the foam. These molecules, which must be dispersed in water to create foam, are called "surface-active." They are located spontaneously in water and air, so that very thin films of water can stabilize around air bubbles of foam with a special architecture. Due to such properties, various foams have numerous applications in cleaning, decontamination, cosmetics, battling pollution and Scientists have been studying a particular surface-active molecule known as 12-hydroxystearic acid which is produced from castor oil.

This molecule is insoluble in water but it becomes water soluble when a suitable salt is added. This surfactant is very special because even in small quantities, it produces abundant foam and, above all, remains stable for more than six months, in contrast with traditional surfactants that stabilize foams for only several hours.

At temperatures between 20 and 60°C, the surfactant disperses in water in the form of tubes that are several microns in size. The tubes form a structure that is perfectly stable and rigid in very thin films of water located between air bubbles, which explains the foam's resistance.
Above 60°C, the tubes merge into micelles, spherical assemblies that are a thousand times smaller (several nanometers). The previously stable foam then collapses because the rigid structure disappears. The researchers have demonstrated that this transition from an assembly of tubes to an assembly of micelles is reversible. If the foam's temperature is increased, its volume will diminish when micelles start to form, and if the temperature is again reduced to between 20 and 60°C, the tubes will form again and the form will re-stabilize (to regain the initial volume of the foam, air must be re-injected).

Reference
Anne-Laure Fameau, Arnaud Saint-Jalmes, Fabrice Cousin, Bérénice Houinsou Houssou, Bruno Novales, Laurence Navailles, Frédéric Nallet, Cédric Gaillard, François Boué, Jean-Paul Douliez. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angewandte Chemie, 2011; DOI: 10.1002/ange.201102115

Further Reading
Synthetic Detergents
Soaps and Saponification
Functional Groups
Lipids

Suggested Questions:

1. Give the molecular formula for 12-hydroxystearic acid.
2. On the structural formula of 12-hydroxystearic acid identify the:
   • carboxyl functional group
   • hydroxyl functional group
3. Is 12-hydroxystearic acid a saturated or unsaturated fatty acid. Explain your answer.
4. Draw 2 structural isomers of 12-hydroxystearic acid.
5. Explain why 12-hydroxystearic acid is not very soluble in water.
6. Draw a structural formula for lithium 12-hydroxystearate, the lithium salt of 12-hydroxystearic acid.
7. What properties of lithium 12-hydroxystearate make it a common component in greases used in motor vehicles, aircraft and heavy machinery?
8. Explain how the properties listed above in question 7 relate to the chemical structure of lithium 12-hydroxystearate.
9. Design experiments to test:
   
   • the stability of the foams formed by a range of household detergents
   • the stability of foam at different temperatures
   • the stability of foam in the presence of different salts