6 Random Questions Answered

Every so often, AUS-e-TUTE randomly selects 6 questions that have been asked in the last few hours by people who are not AUS-e-TUTE members.
Today's selection is:

1/ Question from Canada, "ammonia gas undergoes a combustion to produce nitrogen dioxide gas and water vapour. write and balance the equation"

Answer: 2NH₃ +⁷/₂O₂ → 2NO₂ + 3H₂O

2/ Question from USA, "How do you convert weight percent to mass fraction?"

Answer:  percent by mass (weight) = mass (weight) fraction x 100
 reference: http://www.ausetute.com.au/weightpc.html

3/ Question from USA, "what are the physical properties of metals due to metallic bonding?"
Answer:  The physical properties of solid metals are:

• conduct heat
• conduct electricity
• generally high melting and boiling points
• strong
• malleable (can be hammered or pressed out of shape without breaking)
• ductile (able to be drawn into a wire)
• metallic lustre
• opaque (reflect light)

Reference: http://www.ausetute.com.au/metallic.html

4/ Question from USA, "Can air be separated by physical means?"
Answer: Yes. Air is a mixture so it can be separated by physical means.
Reference: http://www.ausetute.com.au/chemphys.html

5/ Question from USA, "How do you calculate atomic mass from relative abundance?"
Answer: Given the relative atomic mass (r.a.m.) of an element and the mass of each of its isotopes, the relative abundance of each isotope can be calculated:
let y/100 = abundance of isotope 1
and (100 - y)/100 = abundance of isotope 2
then, r.a.m = [y/100 x mass isotope 1] + [(100 - y)/100 x mass isotope 2]
and solve for y
Reference: http://www.ausetute.com.au/atomicmass.html

6/ Question from USA, "How do you turn empirical formula into molecular formula?"
Answer: molecular formula = whole number x empirical formula
Reference: http://www.ausetute.com.au/empirical.html

Azobenzene: cis-trans isomers

A new soluble, light sensitive polymer has been created at the University of Helsinki. The polymer partially dissolves in water or certain alcohols, and, when exposed to light, the polymer switches to its cis isomer and dissolves completely.

This new polymer chain is known to contain azo compounds, and azo compounds are known for this ability to switch between cis and trans isomers using light.

Azo compounds are organic molecules in which two benzene rings (known as phenyl groups) are linked by two nitrogen atoms.
Azobenzene has the IUPAC name diphenyldiazene and is the simplest of the azo compounds.
When the 2 benzene rings (phenyl groups) are on the same side of the N=N bond this is known as the cis isomer. The structural formula for cis-azobenzene is shown below:

When the 2 benzene rings (phenyl groups) are on different sides of the N=N bond this is known as the trans isomer. The structural formula for trans-azobenzene is shown below:

The cis isomer is less stable than the trans isomer. In the presence of light (hν) , the trans isomer will convert to the cis isomer:

hν →

but because the cis isomer is less stable, it will relax back to the trans isomer over time:

→

Cis-azobenzene is more polar than trans-azobenzene so cis-azobenzene is more water soluble than trans-azobenzene.

Reference:
Szymon Wiktorowicz, Heikki Tenhu, Vladimir Aseyev. Using Light To Tune Thermo-Responsive Behavior and Host–Guest Interactions in Tegylated Poly(azocalix[4]arene)s. Macromolecules, 2013; 46 (15): 6209 DOI: 10.1021/ma4011457

Further Reading:
Cis-trans (geometric) Isomers

Suggested Study Questions:

1. Give the molecular formula for cis-azobenzene and trans-azobenzene.
2. Explain why azobenzene displays cis-trans (geometric) isomerism.
3. If the N=N bond in azobenzene was replaced with N-N, would the resulting molecule display cis-trans (geometric) isomerism? Explain your answer.
4.  On a drawing of the structural formula of trans-azobenzene, circle a phenyl group.
5. Diazene (diimine) has the molecular formula (NH)₂ . It contains the same N=N bond as azobenzene or diphenyldiazene. Diazene also displays cis-trans (geometric) isomerism. Draw both cis-diazene and trans-diazene.
6. Hydrazine has the molecular formula N₂H₄. Draw a possible structural formula for hydrazine. 
7. Do you expect hydrazine to display cis-trans (geometric) isomerism? Explain your answer.
8. Draw the molecular formula for the molecule trans-dibutyldiazene.
9. Dibutyldiazene undergoes an addition reaction with hydrogen gas. Draw the structural formula for the product of this reaction.

New AUS-e-TUTE Resources

AUS-e-TUTE has added new tutorials, games, tests and exams on the following chemistry topics:

• Types of Chemical Reactions

• Chemical and Physical Properties of Alkenes

• cis-trans (geometric) Isomers of Alkenes

• Structural (constitutional) Isomers of Haloalkanes (alkyl halides)

• Thin Layer Chromatography

The following syllabus study guides have also been updated:

• HSC chemistry
• VCE chemistry
• SACE chemistry
• AP chemistry
• A.C.T. chemistry
• QLD chemistry

Year 12 students preparing for the HSC, VCE or WACE  should also log-in to the Test Centre and use the exam preparation resources available for these courses.

6 Random Questions Answered

1/ Question From USA, "How do you calculate density for chemistry?"
Answer: density = mass/volume
(http://ausetute.com.au/density.html)

2/ Question from Canada, "What is the main functional group in aspirin?"
Answer: Aspirin (acetylsalicylic acid) contains a carboxyl and an ester functional group. The carboxyl group is responsible for the acidic properties of aspirin.
(http://ausetute.com.au/aspirin.html)

3/ Question from USA, "What is the magnetic quantum number for iron?"
Answer: Magnetic quantum number, m_l,  has a value from -l to +l
For iron (Z=26), m_l can have values of -2, -1, 0, +1, +2
(http://ausetute.com.au/quantum.html )

4/ Question from Australia, "What is the enthalpy of combustion of ethanol?"
Answer: 1367 kJ/mol
(http://ausetute.com.au/members/alkanolp.html)

5/ Question from Australia, "What is enthalpy of combustion?"
Answer: Enthalpy (or heat) of combustion of a substance is the amount of energy released when 1 mole of the substance combusts (burns) in excess oxygen at constant pressure.
(http://ausetute.com.au/members/heatcomb.html)

6/ Question from Australia, "How do you tell a non reducing sugar?"
Answer: Reducing sugars can be oxidised by mild oxidising agents, non-reducing sugars can not be oxidised by mild oxidising agents.
(http://ausetute.com.au/members/redsugar.html)

Spelling Mistakes vs Incorrect Science

It's that time of year in Australia when tens of thousands of students get ready to sit their final exams.
For many of these students it will be the first time they have sat an external assessment, that is, these exams will not be marked by teachers who know them. This time there will be no kindly teacher marking the exam paper and thinking, "oh dear, well I know what Chris really meant ....", this time a complete stranger will be sitting in front of your paper, and not only that, a complete stranger who has by now had to try to read and mark thousands of papers, and this stranger is only going to give you a mark based on exactly what you did write NOT on what he/she thinks you might have meant!

Which brings me to the subject of this discussion. 
When is a spelling mistake (typo) going to be marked as wrong on a Chemistry exam paper?
A spelling mistake will be marked wrong if it introduces ambiguity into your answer or if it makes what you are saying scientifically incorrect.

Spelling mistakes that will be probably not be marked as wrong include misplaced apostrophes, for example it's instead of its, or misspelling non-science-specific-words like using 'their' instead of 'there', as long as this does not alter the scientific meaning of what you have written.

Spelling mistakes that will definitely be marked wrong are those that make your answer scientifically "wrong".
In chemistry, changing just one letter in a word can have an enormous impact!
For example, ethane has the molecular formula C₂H₆ and is a saturated, and not very reactive, hydrocarbon. On the other hand, ethyne has the molecular formula C₂H₂ and is an unsaturated, and very reactive, hydrocarbon. Changing one letter, an a to a y, has a huge impact on the formula and properties of the compound! If you start writing about the addition of bromine to ethane instead of ethyne you will, and certainly should be, marked wrong.
Then there is the problem of capitalization, which is very important when writing chemical formula.
If you write Co then you have given the chemical symbol for the transition metal cobalt, but if you write CO you have given the molecular formula for the covalent compound carbon monoxide. Just using a lowercase letter instead of a capital (uppercase) letter  has changed the substance from a metallic element to a covalent compound! So, if you start talking about the metallic bonding of CO in the solid state at room temperature and pressure, you will be marked wrong.

After you have written an answer to an exam question, read the question again. Then read your answer. Make sure you have written an answer to the question that was asked, and, make sure that your answer says what you meant it to say! Be on the look-out for those spelling mistakes (typos) that could lose you marks.

Nobel Prize in Chemistry 2013

Martin Karplus, Michael Levitt and Arieh Warshel have been awarded the 2013 Nobel Prize in Chemistry "for the development of multiscale models for complex chemical systems", that is, they have made it possible to model complex chemical reactions using computers.

When light hits the retina in your eye, the free electrons in retinal are filled with energy, which changes the shape of the molecule. This is the first stage in the process of your sense of sight.
In 1972, Karplus and Warshel modelled retinal by developing a computer program that used quantum  physics when it performed calculations on free electrons and then used classical physics for all electrons and all atomic nuclei. This was the first time that anyone had managed to bring about a chemically relevant collaboration between classical and quantum physics.

Enzymes are crucial to life, but in order to simulate the reactions of enzymes, a computer program would need classical and quantum physics to collaborate more smoothly. In 1976 Levitt and Warshel successfully simulated an enzymatic reaction, and this program would work for any kind of molecule.

The use of computers to simulate chemical reactions is increasingly important as a tool for understanding how reactions occur and the paths the reactants take. Computer programs can be used to optimize chemical process in order to produce better solar cells, improve catalysts in motor vehicles, develop more effective drugs, and so much more.

Reference:
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2013/popular-chemistryprize2013.pdf

Further Reading:
Enzymes: http://ausetute.com.au/members/enzymes.html 
Proteins: http://ausetute.com.au/members/proteins.html
 

Nobel Prize Facts 2013

Alfred Nobel was a Swedish Chemist and the inventor of dynamite. The fortune that he made from this during his lifetime, about SEK 31 million (today about SEK 1,702 million or about $265 million), was left in his will to institute the Nobel Prizes. One of these prizes is awarded to "the person who shall have made the most important chemical discovery or improvement". The announcement regarding the award of Nobel Prizes is made in October each year.
While we wait for this years announcement, here are a few facts about the Nobel Prize in Chemistry:

• 104 Nobel Prizes in Chemistry have been awarded since 1901.
• A Nobel Prize in Chemistry was not awarded in 1916, 1917, 1919, 1924, 1933, 1940, 1941 and 1942
• 63 Chemistry Prizes have been given to one Laureate only
• 23 Chemistry Prizes have been shared between 3 Laureates. 
• A Nobel Prize in Chemistry can not be awarded to more than 3 people in each year.
• 162 individuals have been awarded the Nobel Prize in Chemistry (these people are called Laureates).
• 1 person, Frederick Sanger, has been awarded the Nobel Prize in Chemistry twice.
• 4 women have been awarded the Nobel Prize in Chemistry:
  • Marie Curie 1911
  • Irene Joliot-Curie 1935
  • Dorothy Crowfoot Hodgkin 1964
  • Ada Yonath 2009
• 2 Chemistry Laureates have been awarded Nobel Prizes in other areas as well:
  • Marie Curie (Physics 1903, Chemistry 1911)
  • Linus Pauling (Chemistry 1954, Peace 1962)
•  1 person, Linus Pauling, is the only person who has been awarded 2 unshared Nobel Prizes.
• 4  Nobel Prize Families:
  • Marie and Pierre Curie - husband and wife (Physics 1903)
  • Irene Joilot-Curie and Frederic Joliot - husband and wife (Chemistry 1935)
  • Hans von Euler-Cheplin (father, Chemistry 1929) and Ulf von Euler (son, Medicine 1970)
  • Arthur Kornberg (father, Medicine 1959) and Roger D. Konberg (son, Chemistry 2006)
• Average age of all Chemistry Laureates is 57
• Youngest Chemistry Laureate was Frederic Joliot who was 35 years old when awarded his Nobel Prize in 1935 (together with his wife Irene Joliot-Curie)
• Oldest Chemistry Laureate was John B. Fenn who was 85 years old when he was awarded the Nobel Prize in Chemistry in 2002.
• 0 posthumous Nobel Prizes in Chemistry. A Nobel Prize cannot be awarded posthumously unless death occurred after the announcement of the Nobel Prize.
• 2 Nobel Laureates in Chemistry have been forced by authorities to decline the Nobel Prize. Adolf Hitler forbade Richard Kuhn (Chemistry 1938) and  Adolf Butenandt (Chemistry 1939) from receiving their Nobel Prizes.

Reference:
http://www.nobelprize.org/nobel_prizes/facts/chemistry/