We
use electronic devices every day; computers, drones, home security systems,
mobile phones, smart speakers, smart watches, etc. If the device stops working,
or we want to upgrade to a newer version, we throw away the old device
resulting in electronic waste or e-waste. Each of these devices contains a
printed circuit board (PCB) and the metal content of each PCB can be as high as
40% by mass.
Recycling 1 tonne of mobile phones alone could produce more than
130 kg metal, including about 340 g of gold. The value of just the gold in that
tonne of rubbish is over $25,000(AUD). Recovering metal from waste electronics
and electrical equipment is becoming big business, but what impact does this
have on us and our environment?
Read this edition of AUS-e-NEWS to find
out more ...
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Lots of common batteries like alkaline batteries and NiCad batteries use oxidation and reduction reactions in alkaline, or basic, conditions.
Learn how to write half-equations for these reactions at https://www.ausetute.com.au/halfeqtnb.html
AUS-e-TUTE Members should log-in to access the new tutorial, game and test with worked solutions to help you learn how to write these half-equations for basic conditions.
Some batteries, like lead-acid batteries used in cars, and some fuel cells like those that use alcohols, rely on oxidation and reduction reactions that occur in aqueous solution under acidic conditions.
You will need to be able to write half-equations for these reactions.
How do you write balanced chemical half-equations for these reactions?
When you walk around having a chat to your friends on your mobile
phone, or watch a show on your tablet, or do your homework on your laptop in a
cosy cafe, have you ever stopped to wonder about the amazing revolution in
chemistry that allows you to do these "every day" things?
Batteries that are small, that can store enough electrical energy
so that they can be used continuously for hours, can be quickly recharged, and
can be discharged and recharged many, many times, are a very recent
development. Without these batteries your life-style would be a lot less
mobile!
Somewhere around the 8th century, an Arab alchemist produced white arsenic trioxide (As2O3) from realgar, a naturally occurring arsenic sulfide mineral, As4S4:
As4S4 + 7O2 → 2As2O3 + 4SO2
Arsenic trioxide became one of the most widely administered poisons in history, and therefore, it became necessary to find reliable tests to show whether or not arsenic was present in a sample of food or drink, or whether it was present in a corpse.
In 1775, Carl Wilhelm Scheele treated arsenic trioxide with nitric acid and zinc which resulted in arsine gas (AsH3), zinc nitrate and water:
As2O3 + 6Zn + 12HNO3 → 2AsH3 + 6Zn(NO3)2 + 3H2O
Arsenic was said to be present if an odour of garlic was produced because arsine gas smells a lot like garlic.
In 1787, Johann Metzger used a carbon reduction method to produce carbon dioxide gas and solid arsenic from arsenic trioxide and carbon:
2 As2O3 + 3 C → 3 CO2 + 4 As
As the arsenic trioxide is heated with charcoal, an "arsenic mirror" forms on the charcoal.
In 1785, Samuel Hahnemann produced a bright yellow precipitate of arsenic trisulfide, As2S3, by passing a stream of hydrogen sulfide gas, H2S, through an acidified arsenic solution.
In 1836, James Marsh designed an apparatus that would detect and measure arsenic.
The sample is placed in a flask with arsenic-free zinc and sulfuric acid.
Arsine gas forms:
As2O3 + 6 Zn + 6 H2SO4 → 2 AsH3 + 6 ZnSO4 + 3 H2O
The arsine gas is fed through a drying tube to a glass tube which is heated.
Arsine deomposes on heating, forming elemental arsenic which is a shiny black substance:
2 AsH3 → 3 H2 + 2 As
By placing a cold surface at the end of the heated tube it is possible to condense this arsenic, which forms a black "mirror".
Today, these "wet" methods of chemical detection have been superceded by instrumental analysis. e Atomic absorption spectroscopy (AAS) can now be used to determine not only the presence of arsenic, but also how much arsenic is present.
Give the oxidation state (number) for arsenic in each of the following:
As2O3
As4S4
AsH3
As
For the reaction: As4S4 + 7O2 → 2As2O3 + 4SO2
which element, or elements, have been oxidized?
Consider the reaction: 2 As2O3 + 3 C → 3 CO2 + 4 As Is arsenic likely to be more or less active than carbon?
Write a possible net ionic equation for the reaction in which arsenic ions react with hydrogen sulfide gas to produce a precipitate of arsenic trisulfide.
Consider the reaction: 2 AsH3 → 3 H2 + 2 As which results in the formation of a shiny black "mirror" of arsenic.
Has arsenic been oxidized or reduced?
Has hydrogen been oxidized or reduced?
If 1 mole of arsine gas decomposes completely, how many moles of hydrogen gas would be produced?
At 0oC and 100 kPa, what volume of gas would be produced by the thermal decomposition of 25 moles of arsine?
At 0oC and 100 kPa, what mass of arsenic would be deposited after the thermal decomposition of 150 mg of arsine.
At 25oC and 100 kPa, what mass of arsine will decompose to produce 0.05 L of hydrogen gas?
A sample of gas containing arsine produces an "arsenic mirror" containing 0.02 g of arsenic. What mass of arsine was present in the gas sample?
New teaching and learning resources have been added to AUS-e-TUTE on the following topics:
Batteries (student learning resources)
Lead-Acid Battery Case Study (student learning resources)
Fuel
Cells (student learning resources)
Electrical Energy Calculations (student learning resources)
Half-equations for Ions (teaching resources)
Redox
Reaction Concepts (teaching resources)
Standard Electrode Potentials for Oxidation and Reduction Reactions (teaching resources)
Become an AUS-e-TUTE member and get the full benefit of using teaching and learning resources developed by experienced science teachers.
Visit http://www.ausetute.com.au.com.au to find out more.
AUS-e-TUTE has been updating its electrochemistry section.
The latest additions have been tutorials, games, tests, exams and drills on the following topics:
To connect solar and wind energy sources to the electrical grid, grid managers require batteries that can store large amounts of energy created at the source. Lithium ion rechargeable batteries, common in consumer electronics and electric vehicles, perform well, but are too expensive for widespread use on the grid because many batteries will be needed, and they will likely need to be large. Sodium is the next best choice, but the sodium-sulfur batteries currently in use run at temperatures above 300oC (572oF), making them less energy efficient and safe than batteries that run at ambient temperatures.
The electrodes in lithium rechargeable batteries that interest scientists are made of manganese oxide. The atoms in this metal oxide form many holes and tunnels that lithium ions travel through when batteries are being charged or are in use. The free movement of lithium ions allows the battery to hold electricity or release it in a current. But there are problems with simply replacing the lithium ions with sodium ions because sodium ions are 70 percent bigger than lithium ions and don't fit in the crevices as well. So, the scientists needed to find a way to make the holes in the manganese oxide larger. They have done this by mixing different kinds of manganese oxides at different temperatures. The manganese oxide formed after mixing at 750oC (1382oF) created a material that had the best crystals, high capacity, and held up well to cycles of charging and discharging. Unfortunately, the scientists found that the faster they charged the sodium ion batteries, the less electricity these batteries could hold, and grid batteries need fast charging so they can collect as much newly made energy coming from renewable sources as possible.
Reference Yuliang Cao, Lifen Xiao, Wei Wang, Daiwon Choi, Zimin Nie, Jianguo Yu, Laxmikant V. Saraf, Zhenguo Yang, Jun Liu. Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Advanced Materials, 2011; DOI: 10.1002/adma.201100904
The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.
Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts. The formula for birnessite is (Na0.3Ca0.1K0.1)(Mn4+,Mn3+)2O4 · 1.5 H2O As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.
The water splitting reaction has two steps:
Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
The protons and electrons combine to form two molecules of hydrogen gas
Reference: Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049
Vincent van Gogh used a pigment known as chrome yellow to achieve the intensity of colour present in such famous 19th century works of art as his Sunflowers paintings. Chrome yellow is made up of lead (II) chromate, PbCrO4, and can be produced by mixing solutions of lead (II) nitrate and potassium chromate, then filtering off the lead (II) chromate precipitate.
Unfortunately, chrome yellow paint darkens in the presence of sunlight as Cr(VI) changes to Cr(III). The Cr(III) compounds form as a nanometer-thin coating over the pigment particles that make up the paint.
Because chrome yellow darkens in the presence of sunlight, and because it contains toxic lead, it was replaced with cadmium yellow by the 1950s. Cadmium yellow is actually cadmium sulfide. While cadmium yellow does not tend to change colour in sunlight, it does contain toxic cadmium.
Cadmium pigments are slowly being replaced by azo dyes which are of the general formula R-N=N-R', where R usually contains a benzene ring within its structure .
Reference Letizia Monico, Geert Van der Snickt, Koen Janssens, Wout De Nolf, Costanza Miliani, Joris Dik, Marie Radepont, Ella Hendriks, Muriel Geldof, Marine Cotte. Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 2. Original Paint Layer Samples. Analytical Chemistry, 2011; 83 (4): 1224 DOI: 10.1021/ac1025122
While noble metals such as platinum and palladium are becoming increasingly important, the world has limited supplies of these metals so that it is vitally important that industry can recycle these metals efficiently.
Many of the transition metals have negative standard reduction potentials, indicating that these metals will dissolve in dilute acid, eg, clean chromium will dissolve in dilute hydrochloric acid
2 x [Cr(s) ----> Cr3+ + 3e] Eo = 0.74V 3 x [2e + 2H+ ----> H2(g)] Eo = 0.00 _____________________________________________ 2Cr(s) + 6H+ -----> 2Cr3+ + 3H2(g) Eo = 0.74V
Some transition metals have positive reduction potentials, so they are poorer reducing agents than hydrogen, and are difficult to dissolve in acid. These metals are referred to as the noble metals and include silver, gold and platinum as well as ruthenium, rhodium, palladium, osmium, and iridium. Dissolving the noble metals requires the use of an oxidizing agent and sometimes a complexing agent. The most common reagent used to dissolve noble metals is aqua regia.
Aqua regia is a highly corrosive, fuming yellow or red solution formed when 1 part of concentrated nitric acid is added to 3 parts of concentrated hydrochloric acid. The nitric acid part is a powerful oxidizer, it oxidizers the noble metal atoms to cations. The hydrochloric acid provides a supply of chloride anions which react with the noble metal cations. For example, gold can be dissolved in aqua regia:
gold reacts with nitric acid to form gold (III) ions:
gold (III) ions react with chloride ions to form chloroaurate anions:
Au3+(aq) + 4Cl-(aq) -----> AuCl4-(aq)
However, aqua regia will dissolve all the metals together which introduces impurities into the recycling process. Georgia Institute of Technology scientists have developed a new organic solvent process that may solve this problem since the concentration of each component of the solvent can be adjusted to preferentially dissolve gold or palladium, but will not dissolve platinum. This solvent has been dubbed organic aqua regia. Reference Wei Lin, Rong-Wei Zhang, Seung-Soon Jang, Ching-Ping Wong, Jung-Il Hong. 'Organic Aqua Regia'-Powerful Liquids for Dissolving Noble Metals. Angewandte Chemie, 2010; 122 (43): 8101 DOI: 10.1002/ange.201001244
Only small amounts of hydrogen occur naturally on Earth, yet the US Department of Energy estimates that the USA uses about 9 million tons per year, and, that this is set to grow if the "hydrogen economy" ever eventuates.
About 95% of the hydrogen in use is produced through steam reforming of natural gas, a catalytic process in which steam reacts with methane to yield carbon monoxide and hydrogen. This mixture is known as synthesis gas, or syngas, and is an intermediate in production processes for synthetic fuels, ammonia, methanol and other compounds.
Hydrogen is a high energy density fuel that is being considered as a cleaner source of future energy, particularly for low-temperature fuel-cell powered devices including vehicles. Fuel cells use electrochemical process to convert hydrogen and oxygen into water, producing current that powers a motor. Fuel cell vehicles require highly purified hydrogen such as is produced in the water-gas-shift reaction. This reaction strips residual carbon monoxide from the hydrogen generated through steam reforming of fossil fuels. Water-gas-shift catalysts decrease the amount of carbon monoxide in hydrogen and increase the hydrogen content by harvesting hydrogen from water molecules.
Currently, copper-based catalysts supported on zinc oxide and alumina are in use. Copper is pyrophoric, it can spontaneously ignite when exposed to air, so researchers have been looking for other more stable catalysts.
Platinum supported on cerium oxide is known to work, but platinum is expensive and cerium occurs in only a few places around the world. Scientists have discovered that sodium improves the platinum activity in the water-gas-shift reaction, which can now take place at low temperatures, even on inert materials such as silica. Less platinum is required, so the cost of hydrogen production should decrease.
Reference: Yanping Zhai, Danny Pierre, Rui Si, Weiling Deng, Peter Ferrin, Anand U. Nilekar, Guowen Peng, Jeffrey A. Herron, David C. Bell, Howard Saltsburg, Manos Mavrikakis, and Maria Flytzani-Stephanopoulos. Alkali-Stabilized Pt-OHx Species Catalyze Low-Temperature Water-Gas Shift Reactions. Science, 24 September 2010: Vol. 329. no. 5999, pp. 1633 - 1636 DOI: 10.1126/science.1192449
Lithium-ion batteries are used to power electronic devices such as mobile phones (cell phones) and are widely used because of their low weight, high energy density and recharging ability. If scientists could see the batteries working at the nanoscale, observing the functionality of the batteries at the level of a single grain or an extended defect, they could determine what makes one battery work and another one fail. Department of Energy's Oak Ridge National Laboratory (ORNL) scientists have developed a new type of scanning probe microscopy called electrochemical strain microscopy (ESM) to examine the movement of lithium ions through a battery's cathode material. They showed that the lithium ion flow could concentrate along grain boundaries, leading to cracking and battery failure.
Reference: N. Balke, S. Jesse, A. N. Morozovska, E. Eliseev, D. W. Chung, Y. Kim, L. Adamczyk, R. E. GarcÃa, N. Dudney, S. V. Kalinin. Nanoscale mapping of ion diffusion in a lithium-ion battery cathode. Nature Nanotechnology, 2010; DOI: 10.1038/nnano.2010.174
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