Liquid Chlorine?

What is wrong with this picture?

Is it possible for an ordinary plastic bottle with a screw cap to contain liquid chlorine?
Probably not!

Chlorine exists as a diatomic yellow-green gas at room temperature and pressure, that is, chlorine exists as Cl_2(g).
In order to produce liquid chlorine we could:

• lower the temperature of the bottle to change the gas into a liquid at atmospheric pressure.
• raise the pressure within the bottle to change the gas into a liquid at room temperature.
• lower the temperature and raise the pressure at the same time.
  

At 1 atmosphere pressure, the melting point of chlorine is about -101^oC and its boiling point is about -34^oC. So, chlorine will be a liquid at temperatures between -34^oC and -101^oC.
For comparison, your refrigerator is probably set to maintain a temperature of about 4^oC while the freezer has a temperature of around 0^oC, not cold enough to liquefy chlorine! A plastic bottle sitting on the shelf in your garage is not going to be cold enough to store chlorine as a liquid!

Gaseous chlorine could also be changed into a liquid by applying pressure. At room temperature this can be achieved with a pressure about 8 times that of atmospheric pressure, which is highly unlikely to occur in our plastic bottle with the screw cap.

So, the fluid in the plastic bottle labelled "liquid chlorine" is not chlorine. What is it?
It is most likely to be an aqueous solution of sodium hypochlorite, NaClO_(aq).
Aqueous solutions of sodium hypochlorite are produced by bubbling chlorine gas, Cl_2(g), through an aqueous solution of sodium hydroxide, NaOH_(aq) at room tmeperature:
Cl_2(g) + 2NaOH_(aq) → NaClO_(aq) + NaCl_(aq) + H₂O_(l)
When the aqueous sodium hypochlorite solution is mixed with dilute acid, chlorine gas is released:
2H⁺_(aq) + OCl^-_(aq) + Cl^-_(aq) → Cl_2(g) + H₂O_(l)

The chlorine gas that is released can kill bacteria and other microbes, so aqueous solutions of hypochlorites are often used as disinfectants.

Further Reading
Chemical and Physical Changes
Kinetic Theory of Gases

Suggested Study Questions:

1. Identify each of the changes below as either a chemical change or a physical change:
   • freezing water in a freezer
   • boiling water in a kettle
   • cooling chlorine gas to make chlorine liquid
   • boiling liquid chlorine to make chlorine gas
   • bubbling liquid chlorine though aqueous sodium hydroxide solution to form a solution of sodium hypochlorite
     
   • bubbling chlorine gas through water to make hypochlorous acid
2. Name each of the physical changes above.
3. Use the kinetic theory of matter to explain what happens to chlorine molecules when:
   • chlorine gas is cooled to produce liquid chlorine at 1 atm pressure
     
   • chlorine gas is subjected to a pressure of more than 8 atmospheres at 25^oC
   • chlorine gas is cooled to 4^oC
4. Sodium hydroxide has a melting point of 319^oC and a boiling point of 1390^oC at 1 atm pressure. Describe how you could produce sodium hydroxide liquid.
5. Which of the following pure substances could be kept in an ordinary plastic bottle with a screw cap on a shelf in your garage?
   
   • ozone (melting point -192^oC, boiling point -110⁰C)
   • potassium chloride (melting point 772^oC, boiling point 1407^oC)
   • sulfur dioxide (melting point -75^oC, boiling point -10^oC)
   • ethanol (melting point -114^oC, boiling point 78^oC)
     
   
   
   

World's Lightest Material?

UC Irvine, HRL Laboratories and the California Institute of Technology have announced that they have succeeded in making the world's lightest material, with a density of 0.9 mg/cm³.

This material is made up of a metallic lattice of interconnected hollow tubes with walls a thousand times thinner than a human hair. Because of this open lattice structure, the material is actually made up mostly of air, 99.99% air .

But what is the metal making up this new material?

We know the density of the new material, so we can calculate the mass of a 1cm cubed volume of this material:

1cm³ of the new material would have a mass of 0.9 mg = 0.0009g.

If 99.99% of the mass of this material is made up of air, then

the mass of air = 99.99/100 x 0.0009 = 8.991 x 10^-4g (0.8991 mg)

and the mass of metal in the new material = 0.0009 - 8.991 x 10^-4 = 9 x 10^-7g (9 x 10^-4 mg)

If we assume that 0.00001% of the volume of the new material is metal, then

the volume of metal = 0.00001/100 x 1cm³ = 1 x 10^-7cm³

So, the density of the pure metallic solid would be 9 x 10^-7g/10^-7cm³ = 9g/cm³

If we compare this calculated density of the metal to a list of common metals as shown below,

Pure Substance	State	Density (g/cm3) at 25oC and 1atm
gold	solid	19.3
mercury	liquid	13.6
lead	solid	11.4
silver	solid	10.5
copper tin	solid solid	9.0 7.3
zinc	solid	7.1
aluminium	solid	2.7

then we see it is possible that the new material is made up of copper.

Reference

T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit, W. B. Carter.Ultralight Metallic Microlattices. Science, 2011; 334 (6058): 962 DOI: 10.1126/science.1211649

Further Reading

Density Calculations

Suggested Study Questions

1. Using the table of densities above, calculate the mass in grams of a
   • cubic centimetre of gold
   • a cubic metre of copper
   • a cubic millimetre of silver
   • a cubic kilometre of zinc
2. Using the table of densities above, calculate the volume in cubic centimetres of
   • 1g of copper
   • 100mg of lead
   • 4500μg of aluminium
   • 2kg of silver
3. Brass is a mixture of copper and zinc. A sample of brass has a density of 8.5g/cm³
• What is the mass a cubic centimetre volume of this brass sample?
• If the sample were made up of equal masses of copper and zinc, what is the mass of copper in the sample?
4. A sample of brass was produced using 500cm³ of each of copper and zinc.
• What mass of copper is present in the brass?
• What mass of zinc is present in the alloy?
• Assuming additivity of volumes, what is the density of this brass sample?
5. Cymbals are commonly made of bronze which is a mixture of about 10% (by mass) tin and 90% (by mass) copper. For a 100g sample of bronze, calculate
• the mass of copper present in the sample
• the volume of copper this mass represents
• the mass of tin present in the sample
• the volume of tin this mass represents
• the density of the bronze sample assuming additivity of volumes
6. Typically, bronze contains copper and about 12% (by mass) tin. Calculate the density of a sample of this bronze.
7. Bronze coins often contain copper and about 5% tin. Calculate the density of the bronze used to make coins.
8. The brass used to make springs and screws is often 65% (by mass) copper and 35% (by mass) zinc. Calculate the density of the alloy in a brass screw.

Newest AUS-e-TUTE Resources

New AUS-e-TUTE Resources for All Members:

Electrolyisis of Molten Salts Tutorial : http://www.ausetute.com.au/elymsalt.html

Electrolyisis of Molten Salts Game

Electrolyisis of Molten Salts Test

Electrolyisis of Molten Salts Game

Electrolyisis of Aqueous Salt Solutions Tutorial : http://www.ausetute.com.au/elyqsalt.html

Electrolyisis of Aqueous Salt Solutions Game

Electrolyisis of Aqueous Salt Solutions Test

Electrolyisis of Aqueous Salt Solutions Exam

New AUS-e-TUTE Resources for Teachers:

Learning Activities (now listed under tutorials in the Teachers Only section of the website):

-Electrolysis of Molten Salts

-Electrolysis of Aqueous Salt Solutions

-Periodic Table Trends : Atomic Radius

-Periodic Table Trends : First Ionization Energy

-Periodic Table Trends : Group 1

Stimulus Resources:

-Defining Electronegativity http://auseblog.blogspot.com/2011/10/defining-electronegativity.html

-Oxygen Evolution Reactions http://auseblog.blogspot.com/2011/10/oxygen-evolution-reactions.html

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.