Shapes of Melting Ice

 What shape is submerged ice as it melts?

That depends on temperature apparently ..

 Which suggests that we can infer water temperature in nature by observing the shape of its melting ice.

Scott Weady, Joshua Tong, Alexandra Zidovska, and Leif Ristroph (2022); Anomalous Convective Flows Carve Pinnacles and Scallops in Melting Ice. Phys. Rev. Lett 128(4)  https://doi.org/10.1103/PhysRevLett.128.044502

How to Kill the COVID-19 Virus

By the middle of 2020 millions of people had been infected with a virus which causes a disease known as COVID-19 and hundreds of thousands of people had died.

So I was intrigued when I read that Professor Mary-Louise McLaws, an infection control expert from the University of New South Wales, had stated that, "it's relatively easy to kill compared to some other viruses".

Why is the COVID-19 virus easy to kill and how do you kill it? 

Read this edition of AUS-e-NEWS to find out more.

Subscribe to AUS-e-NEWS at https://www.ausetute.com.au/ausenews.html

Nitrogen vs Air in Car Tyres

The tread on my car tyres had worn down, so I popped into my local tyre retailer to buy 4 new tyres.

"We can inflate your new tyres with air, or, for an extra $5 per tyre we can fill them with nitrogen gas", the sales person told me, " Nitrogen gas doesn't react with tyre and rim material so your tyres will last longer, it will help maintain the pressure in your tyres so you won't need to check your tyre pressure as often, and it reduces the running temperature of the tyres so your tyres are less likely to explode."

Is this just marketing hype or are there good reasons for choosing to inflate your car tyres with nitrogen instead of air?

Read the March 2020 edition of AUS-e-NEWS to find out more.

Subscribe to AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry teachers and students, at https://www.ausetute.com.au/ausenews.html

Gibbs Free Energy Calculations

For a chemical system, either a chemical reaction or a physical change, at a constant temperature and pressure we define a function called the Gibbs Free Energy (G) so that we can determine whether the system will be spontaneous or non-spontaneous:

• spontaneous if ΔG < 0 (ΔG is negative)
• non-spontaneous if ΔG > 0 (ΔG is positive)

In this new tutorial we will calculate the change in Gibbs free energy of a reaction at constant temperature and pressure (ΔG) using:

• standard absolute entropy of each reactant and product (S°)
• standard enthalpy of formation of each reactant and product (ΔH_ƒ°)

For a chemical system under standard conditions, we can calculate the change in standard Gibbs free energy using the equation shown below:

ΔG° = ΔH° - TΔS°

AUS-e-TUTE members can access the new Gibbs free energy calculations tutorial, game, test and exam when they log-in (Go to Physical Chemistry Heading, then "Thermodynamics").

If you are not an AUS-e-TUTE member, there is a "free-to-view" Gibbs free energy calculations tutorial currently available for evaluation purposes at https://www.ausetute.com.au/gibbscalc.html

Introduction to Entropy

What is entropy?
What is meant by a chemical system having low entropy or high entropy?
What is the relationship between disorder, energy and entropy?

If you are asking these questions, then you will find AUS-e-TUTE's new entropy introductory tutorial, game and test very helpful! AUS-e-TUTE Members should log in to use these new resources (under the topic heading Thermodynamics in the Test Centre).

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Sticky Surfactant?

 How often have you found that, no matter how hard you try, it is impossible to get that last bit of detergent out of the plastic bottle? Do you turn the bottle upside down and wait, letting the cleaning stuff flow to the bottom, but then find there is still some left no matter how hard you squeeze the bottle? Do you then try adding a bit of water and shaking it so that you can extract just a little bit more of it out of the bottle? And then, do you eventually give up and finally chuck the bottle away, still containing a very small amount of the cleaning product?
Well, if you find this sticky surfactant problem unsatisfactory, you aren't alone!
 The reason why it is so hard to remove ALL the detergent from the bottle is the same reason why the detergent makes a good cleaning product, that is, surfactant molecules have a long non-polar chain that attracts other non-polar substances like oils and grease, and a polar or ionic head that attracts other polar substances like water. So, if you pour a detergent, containing surfactant molecules, into a non-polar plastic container like polyethylene or polypropylene, the non-polar parts of the surfactant molecule will be attracted to the non-polar surface of the bottle making it hard to get all the surfactant molecules out of the bottle.

But scientists at The Ohio State University have now developed a way to make the plastic bottles so that ALL your shampoo or "liquid" detergent will flow out of the bottle. It involves spray-coating the surface of the plastic with a solvent and ultrafine silica nanoparticles. The solvent softens the plastic enabling the silica to be embedded in the  surface formed "Y" shaped channels a few micrometers high and a few micrometers apart. The branches of the "Y" shapes overhang the plastic surface at an angle of less than 90 degrees resulting in trapped air. Surfactant molecules are then in contact with air rather than plastic so that they can form spherical beads that will roll off.

The university hopes to further develop this process and license the coating technique to manufacturers, not just for shampoo bottles, but for other plastic products that have to stay clean, such as biomedical devices or catheters.

Reference: 

Ohio State University. "Shampoo bottle that empties completely, every last drop." ScienceDaily. ScienceDaily, 27 June 2016.

Further Reading:

Soaps
Detergents
Wetting
Intermolecular Forces
Nanoparticles and Nanotechnology
Molecular Formula
2-Dimensional Structural Formula
Condensed (semi-structural) formula
Skeletal Formula
Introduction to Functional Groups
Carboxylic Acids


Suggested Study Questions

1. A typical soap molecule, sodium stearate is shown below:
   
   • Draw the full 2-dimensional structural formula for this molecule
   • Write the condensed (semi-structural) formula for this molecule
   • Write the molecular formula for this molecule
2. Draw the skeletal formula for potassium stearate:
   • draw a ring around the functional group
   • name the functional group
   • describe the non-polar part of the molecule
   • describe the polar part of the molecule
3. Draw a structural formula for stearic acid.
4. Write a chemical equation for the neutralisation of stearic acid using sodium hydroxide in aqueous solution.
5. Describe how soap removes dirt during washing.
6. Sodium dodecyl sulfate shown below is a common surfactant molecule found in detergents
   
• Draw the full 2-dimensional structural formula for this molecule
• Write the condensed (semi-structural) formula for this molecule
• Write the molecular formula for this molecule
7. Draw the skeletal formula for potassium dodecyl sulfate:
   • draw a ring around the functional group
   • name the functional group
   • describe the non-polar part of the molecule
   • describe the polar part of the molecule
8. Explain why sodium dodecyl sulfate is classified as an anionic detergent.
9. Compare molecules of sodium dodecyl sulfate and sodium stearate
   • Desribe any similarities between the two molecules
   • Describe any differences between the two molecules
   • Explain how both molecules can be used to remove dirt during washing
10. Consider the problem of detergent sticking to the inside walls of the plastic bottle.
    • Describe the physical properties of the plastic bottle that enable this to happen
    • Use a diagram to help explain why the detergent molecules can "stick" to the plastic bottle
    • Use a diagram to explain why adding water to the not-quite-empty plastic bottle allows more of the detergent to be removed

---

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Equilibrium Constants and Balanced Chemical Equations

Consider the reaction in which carbon monoxide gas reacts with oxygen gas to produce carbon dioxide gas:

CO_(g) + ½O_2(g) ⇔ CO_2(g)   with equilibrium constant = K₍₁₎ (at temperature = T)

and this reaction:
2CO_(g) + O_2(g) ⇔ 2CO_2(g)   with equilibrium constant = K₍₂₎ (also at temperature = T)

 Is the value of K₍₁₎ the same as the value of K₍₂₎ ?
 Not sure?
That's why AUS-e-TUTE has just added a new set of resources to help you understand this concept!

AUS-e-TUTE members should log-in to view the new Members Only resources.

 If you are not an AUS-e-TUTE member, there is a "free-to-view" tutorial available for evaluation purposes at http://www.ausetute.com.au/kstoichio.html

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Chemical Equilibrium

To understand chemical equilibrium you need to realise that

1. chemical reactions that are in equilibrium DO NOT go to completion. 
2. "reactants" and "products" are all present at equilibrium
3. the "things" that are in equilibrium are the "reactants" and "products" whose concentrations can be changed (that is, substances that exist in solution).
4. equilibrium is the state in which the rate of the forward reaction (reactants → products) is the same as the reverse reaction (products → reactants

Knowing this, we can then write:

• a mass-action expression (http://www.ausetute.com.au/massaction.html)

And then

• the equilibrium law expression (http://www.ausetute.com.au/equilibriumlaw.html)

Which at equilibrium, results in an expression for the

• equilibrium constant (http://www.ausetute.com.au/equicons.html)

Which can be used to:

• calculate concentration (http://www.ausetute.com.au/ricetable.html)
• determine the extent of a reaction (http://www.ausetute.com.au/kextent.html)
• determine the equilibrium constant for the reverse reaction (http://www.ausetute.com.au/kreverse.html)
• determine whether a chemical reaction is exothermic or endothermic (http://www.ausetute.com.au/ktemperature.html)

I.C.E., R.I.C.E and Equilibrium

Need to calculate an equilibrium constant, but, you haven't been given all the equilibrium concentrations?
Then you need an I.C.E. Table (also known as a R.I.C.E. Table, an ICE chart, a RICE chart), and you can find out all about that at AUS-e-TUTE's new tutorial:
http://www.ausetute.com.au/ricetable.html

Now, if you are an AUS-e-TUTE Member, not only can go to the great member's only tutorial, but you can also:

• play the R.I.C.E. game
• answer the R.I.C.E. test questions and get immediate feedback
• do the R.I.C.E. exam
• make a R.I.C.E. worksheet (if you are a Teacher Member)
• do a R.I.C.E. quiz (if you a student with a Class or School Group)

And if you still haven't joined AUS-e-TUTE, you can sign up right here !

Effect of Temperature on Solubility

What happens to the solubility of a salt or a gas at constant pressure when the temperature of the solution changes?
This can be treated as just a special case of Le Chatelier's Principle .....

And AUS-e-TUTe has just added new resources to help you understand the effect of temperature on solubility.
AUS-e-TUTE Members should log-in to use the new tutorial (includes a drill), game, test and exam.

If you are not an AUS-e-TUTE member, there is non-interactive "free-to-view" tutorial currently available for evaluation purposes at http://www.ausetute.com.au/solubilitylcp.html


Get a better understanding of concepts in chemistry, and improve your problem solving ..... join AUS-e-TUTE today!

Henry's Law and the Solubility of Gases

What is the relationship between the solubility of a gas in a solvent, and the pressure of the gas?
What happens to the solubility of gases as the temperature changes?
If you are asking these questions, you need AUS-e-TUTE's new Henry's Law resources!

AUS-e-TUTE Members will find the new resources listed under Gas Laws in the Members Only Test Centre. Where relevant to your syllabus, links to these resources have also been added to your Syllabus Study Guide.

If you are not an AUS-e-TUTE member, there is a "free-to-view" tutorial available for evaluation purposes at http://www.ausetute.com.au/henryslaw.html

Then, when you are ready to start improving your chemistry results, you can join AUS-e-TUTE at
http://www.ausetute.com.au/register.html

Ammonium Nitrate Explodes

On Friday 6^th September 2014, a truck carrying 56 tonnes of ammonium nitrate exploded after rolling over near a bridge on the Mitchell Highway between Cunnamulla and Charleville in the Australian State of Queensland. Eight people were injured, and the Mitchell Highway has been closed "indefinitely".

The most important use for ammonium nitrate is as a fertilizer, but it can also be used as an explosive. An explosion results when ammonium nitrate is brought into contact with heat, a source of ignition such as a spark, or, with reducing agents. Unfortunately, explosions of ammonium nitrate are not rare. A massive explosion of ammonium nitrate on 17^th April 2013 resulted in the death of 15 people, and the destruction of the West Fertilizer Company storage and distribution facility in Texas, USA.

Ammonium nitrate can be produced in commercial quantities by reacting ammonia gas with  nitric acid.

Reference: 
http://www.brisbanetimes.com.au/queensland/truck-explosion-injures-eight-closes-mitchell-highway-20140906-10dam3.html

Further Reading
Naming Ionic Compounds 
Writing Ionic Formula 
Mole Definitions 
Molar Gas Volumes

Suggested Study Questions:

1. Write the chemical formula for ammonium nitrate 
2. Explain why this compound is considered to be an ionic compound.
3. Write a word equation for the reaction between ammonia gas and nitric acid to produce ammonium nitrate
4. Write a balanced chemical equation for the reaction in question 3.
5. Ammonium nitrate decomposes in to N₂O and H₂O when heated. Write a balanced chemical equation for this reaction.
6. Calculate the molar mass of ammonium nitrate.
7. Calculate the moles of ammonium nitrate present in the truck.
8. Assuming that the decomposition of ammonium nitrate results only in gaseous products, what volume of gas at 25^oC and 100 kPa would be released by ammonium nitrate in the truck?
9. Assume the truck could carry a load with a volume of 30,000 L. Can you suggest a reason why rapidly decomposing ammonium nitrate could explode, destroying the truck, the bridge and the road?

Sulfuric Acid - Sodium Hydroxide Titrations

Why does a titration of sulfuric acid using sodium hydroxide have only one equivalence point?
Why isn't the pH 7 at the equivalence point?

These are both excellent questions.

So, we've written a set of resources to help you understand!

AUS-e-TUTE Members should log-in and go to the new tutorial at:
http://www.ausetute.com.au/members/titrh2so4.html
and you can follow the links to the game and test from this page.

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Part of this tutorial is currently available free to non-members for evaluation purposes at
http://ausetute.com.au/titrh2so4.html

Surface Tension

Why are rain drops spheres?
How can you float a steel needle on water?
AUS-e-TUTE has just added new Surface Tension resources.
AUS-e-TUTE members should log-in to use the new tutorial, game, test.

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

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.
   
   

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
     
   
   

Catalysis by Gold Nanoclusters

Since the early 1980s, experiments have indicated that gold nanoparticles exhibit unexpected catalytic activity towards many industrially important chemical reactions that involve activation of atomic bonds inside oxygen or hydrocarbon molecules. Room-temperature formation of carbon dioxide, CO₂, from carbon monoxide, CO, and oxygen molecule, O₂, is one of the most extensively studied processes. A number of different factors have been suggested to contribute to the ability of gold particles to activate the O-O bond, which is considered to be the key reaction step.

Finnish scientists recently exposed monolayer-thick gold clusters to a variable number of oxygen molecules. It was found that even one gold cluster can effectively adsorb multiple oxygen molecules at the boundaries of the cluster, simultaneously weakening, stretching, the O-O bond by transferring electrons to the oxygen molecules. Taking into account the effects of temperature and ambient pressure, the calculations predicted that the oxygen molecules will completely dissociate and the oxygen and gold atoms will form one-dimensional alternating chains at the cluster boundary. The oxygen atoms in these chains are negatively charged and the gold atoms positively charged, creating a system that is reminiscent of a one-dimensional gold-oxide chain. These chains are expected to be the highly catalytically active part towards conversion of carbon monoxide to carbon dioxide at room temperature.

At room temperature and pressure, it appears that gold can catalyse an oxidation reaction by first oxidizing itself to gold oxide, which seems to contradict the known properties of gold in the macroscopic level.

References

1. Pentti Frondelius, Hannu Häkkinen and Karoliina Honkala. Formation of Gold(I) Edge Oxide at Flat Gold Nanoclusters on an Ultrathin MgO Film under Ambient Conditions. Angewandte Chemie International Edition, 2010; DOI: 10.1002/anie.201003851
2. X. Lin, N. Nilius, H.-J. Freund, M. Walter, P. Frondelius, K. Honkala, H. Häkkinen. Quantum Well States in Two-Dimensional Gold Clusters on MgO Thin Films. Physical Review Letters, 2009; 102 (20) DOI: 10.1103/PhysRevLett.102.206801

Further Reading
Naming Compounds
Writing Formula
Balancing Chemical Equations
Oxidation States
Transition Metals
Energy Profiles
Reaction Rate

Study Questions:

1. Write a balanced chemical equation for the formation of carbon dioxide from carbon monoxide and oxygen.
2. For the reaction above, what other possible steps in the reaction mechanism could be rate determining steps?
3. Why do you think that scientists believe that the activation of the O-O bond is the key reaction step in the reaction mechanism for this reaction?
4. What is meant by the term catalysis?
5. Why is gold described as a catalyst for the reaction described in the article?
   
6. What is meant by the term dissociate?
7. Describe how oxygen molecules can dissociate.
8. What is meant by the term oxidize?
9. Given the position of gold in the Periodic Table, what oxidation states are possible?
10. Give the formula for two possible oxides of gold.
11. Name each of the oxides above.
    

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?

Reaction Mechanism for Ammonium Sulfates's Phase Transition

During a chemical reaction, the atoms in the reactants are rearranged to form new compounds. On a molecular level, the spatial arrangement of electrons and nuclei changes. While the structure of the reactant and product molecules can be measured the reaction mechanism, or the transient structures and molecular motions during a reaction, have remained unknown in most cases, but, this knowledge is a key element needed to understand the reaction.

Scientists at the Max-Born Institute in Berlin have now succeeded in making a "molecular movie" of the thermal phase transitions of ammonium sulfate which is a reversible reaction.

Using an advanced femtosecond laser system which generates a blue pulse of 50 femtosecond duration, they initiated the chemical reaction and then probed the structure of the excited material with high spatial resolution using a synchronised X-ray flash of 100 femtosecond duration. The X-ray pulse is diffracted off a powder made of small crystals, this is known as the Debye-Scherrer method. By simultaneously measuring the many different X-ray reflections they reconstructed the transient distances of atomic lattice planes and in turn the three dimensional distribution of electronic charge within the crystal. The "molecular movie" was created by taking X-ray snap shots at various times after triggering the reaction.

What they found is that the blue flash caused a release of both a proton from the ammonium ion and an electron from the sulfate ion. The proton and the electron then merged to form a hydrogen atom which jumped back and forth between two distant spatial positions.

Reference:
Michael Woerner, Flavio Zamponi, Zunaira Ansari, Jens Dreyer, Benjamin Freyer, Mirabelle Prémont-Schwarz, Thomas Elsaesser. Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction. The Journal of Chemical Physics, 2010; 133 (6): 064509 DOI: 10.1063/1.3469779

Study Questions

1. Give the molecular formula for ammonium sulfate.
2. What is the oxidation state (oxidation number) for nitrogen in the ammonium ion?
3. What is the oxidation state (oxidation number) for sulfur in the sulfate ion?
   
4. Write a chemical equation for the overall reaction for the thermal phase transition of ammonium sulfate.
5. What is meant by the term reversible reaction? Explain your answer using the chemical equation above.
6. Draw Lewis structures (electron dot diagrams) for the ammonium ion and the sulfate ion.
7. Draw Lewis structures (electron dot diagrams) for each of the ions above immediately after the laser's blue flash initiates the reaction.
8. Using the new species above, give the oxidation state (oxidation number) for nitrogen and sulfur after the reaction is initiated. Compare these oxidation states to those in questions 2 and 3. Is this an example of a redox reaction? Explain your answer.
9. Define the terms Bronsted-Lowry acid and Bronsted-Lowry base.
10. Are any of the species described in the reaction mechanism for the thermal phase transition of ammonium sulfate acting as Bronsted-Lowry acids or Bronsted-Lowry bases. Explain your answer.
11. Define the terms Lewis acid and Lewis base.
12. Are any of the species described in the reaction mechanism for the thermal phase transition of ammonium sulfate acting as Lewis acids or Lewis bases. Explain your answer.

Casting : Changes of State

The question of what happens when a material composed of more than one phase or state is heated or cooled is very important.
Many metal parts, for example, are made by casting. In the casting process liquid metal is poured into a mold and solidifies into the shape of the mold. As the liquid metal solidifies it forms tree-like structures called dendrites, and, if one of the dendrites breaks off it can lead to a change in the properties of the solidified material. The airplane industry has spent a long time developing solidification methods to avoid this problem when casting jet turbine blades.
Polymer solar cells use a complicated mixture of two polymers. When heated, the mixture evolves by a process that involves pinching which ultimately alters the properties of the mixture and the efficiency of the solar cell.
Scientists have been observing the heating process during which a rod-like phase or state embedded in another will break up into smaller domains just like droplets at the end of a stream of water, resulting in changes to the properties of the material. They have found that the shape of the interfaces during break up becomes universal, independent of the material used. This now allows them to predict the dynamics of the break-up process in a vast array of materials such as steel and polymers.

Reference:
Aagesen et al. Universality and self-similarity in pinch-off of rods by bulk diffusion. Nature Physics, 2010; DOI: 10.1038/nphys1737

Study Questions

1. Name the phase changes (changes of state) that can occur in each of the following situations:


• heating a solid


• heating a liquid


• cooling a liquid


• cooling a gas




2. Draw a sketch of the temperature-time graph expected for each of the following situations involving pure substances:
   


• heating a solid


• heating a liquid


• cooling a liquid


• cooling a gas




3. Explain why the temperature-time graph for the melting of ice differs from the temperature-time graph for freezing water.
4. Explain why the purity of a solid substance can be determined using its melting point.
5. Do you think the purity of a liquid substance could be determined using its freezing point? Explain your answer.
   
6. Explain what is meant by the term sublimation.
7. Give two examples of pure substances that undergo sublimation.