24 Carat Chemistry

I was trying to buy a present for my mum for Mothers' Day. "A gold chain," I thought, "would be a nice present". But when I got to the jewellery shop I found that there is a huge variety of "gold chains", 24 carat gold, 18 carat gold, 9 carat gold, 995, 750, 375, white gold, rose gold...

So what is a carat?
What do those numbers mean?
How can "gold" be different colours?

Read all about it in the June 2019 issue of AUS-e-NEWS

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Titanium Gold Alloy

Titanium is the metal used to replace hip and knee joints because it is strong, resistant to wear, and, is nontoxic.

Before titanium was used to replace hips and knees, stainless steel was used.
The density of stainless steel used to make replacement hips and knees is about 7.8 g cm³. The density of titanium metal is 4.506 g cm³, about half the density of stainless steel. This means that a replacement joint made out of titanium will weigh less than the same replacement joint made out of stainless steel.
Titanium is also strong. The strength of the material used to replace hip and knee joints is important because you do not want your new joint to bend or break or under strain.

Titanium has a melting point of 1670°C and a boiling point of 3287°C, which means it remains solid over the range of temperatures humans are exposed to. This is important because you want your new hip or knee replacement to remain a solid!
Naturally occurring titanium is made up of a number of isotopes, all of which are stable so they do not undergo nuclear decay. This is important because it means that there is no loss of titanium mass due to radioactive decay, and there is no fear of damage to cells from the emission of radiation.

isotope	atomic mass	abundance
46Ti	45.953	8.25
47Ti	46.952	7.44
48Ti	47.948	73.72
49Ti	48.948	5.41
50Ti	49.945	5.18

Titanium metal will react with water, halogens and dilute hydrochloric acid, but only if the temperature is elevated well above body temperature. Similarly, titanium metal will react with oxygen in a combustion reaction at elevated temperatures. Titanium metal does not appear to react with bases at all. Therefore, titanium is unlikely to react with substances found in the human body.

Researchers at Rice University have found that alloying titanium with gold can produce an even better material to use for replacement hips and knees. Mixing titanium and gold in the ratio of 3:1 at high temperature produces an alloy that is 3 times harder than steel and 4 times harder than the pure titanium commonly in use for hip and knee replacements. The atoms of titanium and gold in this alloy are packed in a cubic arrangement, an arrangement that is usually associated hardness. The structure of this alloy is shown below:

This titanium gold alloy has been found to be even more biocompatible that pure titanium.
The researchers intend to undertake further studies to investigate whether using chemical dopants might improve the alloy's hardness even further.

Reference:
Rice University. "Titanium and gold equals new gold standard for artificial joints: Titanium-gold alloy that is 4 times harder than most steels." ScienceDaily. ScienceDaily, 20 July 2016. 

Further Reading
Metals and Non-metals 
Density
Isotopes
Relative Atomic Mass
Alloys

Suggested Study Questions

1. Titanium and gold are both metallic metallic elements.
   • What are the physical properties common to most metallic elements?
   • What are the chemical properties common to most metallic elements?
2. Draw up a table of the physical properties of titanium.
3. A typical knee replacement made out of titanium has a mass of 560 g.
   • Calculate the volume of the titanium knee replacement.
   • Calculate the mass of the same knee replacement if it were made out of stainless steel
4. Define the term isotope.
5. Determine the number of protons in the nucleus of an atom of each of the isotopes of titanium listed in the article above.
6. Determine the number of neutrons in the nucleus of an atom of each of the isotopes of titanium lists in the article above.
7. Which is the most abundant isotope of titanium? Explain your answer.
8. Use the data in the article above to calculate the relative atomic mass of naturally occurring titanium.
9. Given the atomic radius of titanium is  176 pm (1.76 x 10^-10 m) and the atomic radius of gold is 174 pm (1.74 x 10^-10 m), do you think the alloy of titanium and gold discussed in the article above is an interstitial alloy or a substitutional alloy? Explain your answer.
10. Consider the structure of the titanium gold alloy shown in the diagram in the article above.
    • The blue balls represent which atoms of which element?
    • The red balls represent which atoms of which element?

Alloys

What is an alloy?
What kinds of alloys are there?
Are steel, brass and bronze alloys?
How do the properties of an alloy differ from the properties of the elements making up the alloy?

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What's $1 Really Worth?

Have you ever wondered what the value of the metal used to make a coin is?
The Australian $1 coin has a mass of 9 grams and is composed of:

• 92% copper
• 6% aluminium
• 2% nickel

 If we convert each percentage to a mass, then the Australian $1 coin is composed of:

• 92/100 x 9 = 8.28 g of copper
• 6/100 x 9 = 0.54 g of aluminium
• 2/100 x 9 = 0.18 g of nickel

The following are the approximate costs per tonne for each metal:

• copper: $ 7000 per tonne
• aluminium: $ 1750 per tonne
• nickel: $ 19 000 per tonne

Now we convert the cost of each metal per tonne to a cost per gram:

• copper: $7000/1000 = $7 per kg = 7/1000 = $0.007 per gram = 0.7 cents per gram
• aluminium: $1750/1000 = $1.75 per kg = 1.75/1000 = $0.00175 per gram = 0.175 cents per gram
• nickel: $19000/1000 = $19 per kg = 19/1000 = $0.019 per gram = 1.9 cents per gram

Then calculate the value of each of metal used to make an Australian $1 coin:

• copper: 8.28 g x 0.7 cents per gram = 5.796 cents
• aluminium: 0.54 g x 0.175 cents per gram = 0.0945 cents
• nickel: 0.18 g x 1.9 cents per gram = 0.342 cents

The value of an Australian $1 coin is
5.796 cents + 0.0945 cents + 0.342 cents = 6.2325 cents!

Further Reading:
Percentage Composition Calculations
Mass Conversions

Calculate the value of each of the following coins:

1. Australian $2 coin has a mass of 6.60 g and is composed of  92% copper, 6% aluminium, 2% nickel
2. Australian 50 cent coin has a mass of 15.55 grams and is composed of 75% copper, 25% nickel
3. Australian 20 cent coin has a mass of 11.31 grams and is composed of 75% copper, 25% nickel
4. Australian 10 cent coin has a mass of 5.66 grams and is composed of 75% copper, 25% nickel
5. Australian 5 cent coin has a mass of 2.83 grams and is composed of 75% copper, 25% nickel
6. An American nickel (5 cent coin) has a mass of 5.00 grams and is composed of 75% copper, 25% nickel
7. An American dime (10 cent coin) has a mass of 2.268 grams and is composed of 91.67% copper, 8.33% nickel
8. An American quarter (25 cent coin) has a mass of 5.67 grams and is composed of 91.67% copper, 8.33% nickel

Olympic Medal Metals

Metallic medals have been awarded to 1^st, 2^nd and 3^rd place Olympic athletes since the 1900 Paris Olympic Games. The medals awarded at the 2012 London Olympic Games are 7mm thick, 85 mm in diameter, and, weigh 400g.
While we happily refer to these olympic medals as gold, silver and bronze, is this chemically accurate?

"Bronze medals" are often made of bronze, an alloy of copper and tin.
At the 2012 London Olympic Games, the bronze medals are made up of a mixture of 97% copper, 2.5% zinc and 0.5% tin. This composition is actually much closer to the composition of brass which is the term used to refer to an alloy of copper and zinc.

"Silver medals" contain at least 92.5% silver. The silver medals awarded in the 2012 Olympic Games were composed of 92.5% silver and 7.5% copper.

"Gold medals" must also contain at least 92.5% silver, but they are plated with at least 6g of gold so they look like "gold" medals. The 2012 Olympic gold medals are made up of 92.5% silver, 6.16% copper and 1.34% gold.

Reference

www.olympic.org/Assets/OSC%20Section/pdf/QR_1E.pdf

Further Reading
Periodic Table
Percentage Composition
Mass-mole Calculations
Moles-Number of Particle Calculations
Density Calculations

Suggested Study Questions

1. Give the chemical symbol for each of the following elements:
   • gold
   • silver
   • copper
   • tin
   • zinc
2. Explain why chemists refer to bronze and brass as alloys.
3. Calculate the mass of each element present in the bronze olympic medals awarded in 2012.
4. Calculate the mole of each element present.
5. For the 2012 Olympic gold medal, calculate the mass of silver present.
6. Calculate the number of silver atoms present in a 2012 olympic gold medal.
7. Calculate the mass of gold present in a 2012 olympic gold medal.
8. Calculate the volume of an olympic medal and use this to calculate the density of an olympic medal.
9. Calculate the surface area of a 2012 Olympic gold medal.
10. Assuming the 2012 gold medal is coated evenly with gold, what thickness is the layer of gold?

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.

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.
   

Multiferroic Alloy

University of Minnesota scientists have discovered a new alloy that converts heat directly into electricity. In theory, an alloy like this could be used to capture waste heat from a car's exhaust and use it to produce electricity to charge the car's battery. Similarly, a thin film of this alloy could be used to convert waste heat from computers into electricity.
The alloy is made out of nickel, cobalt, manganese and tin and has the formula Ni₄₅Co₅Mn₄₀Sn₁₀.
This new alloy undergoes a highly reversible phase transformation in which one solid turns into another solid with very different magnetic properties. This means that the new alloy begins as a non-magnetic material, then suddenly becomes strongly magnetic when the temperature is raised a small amount. When this happens, the material absorbs heat and spontaneously produces electricity in a surrounding coil.
Substances which combine unusual magnetic and electric properties such as this alloy are called multiferroic materials. Other multiferroic materials include TbMnO₃, HoMn₂O₅, LuFe₂O₄, BiFeO₃ and BiMnO₃.

In the demonstration below, "University of Minnesota researchers show how a new multiferroic material they created begins as a non-magnetic material then suddenly becomes strongly magnetic as the piece of copper below is heated a small amount. When this happens, it jumps over to a permanent magnet. This demonstration represents the direct conversion of heat to kinetic energy."




Reference
Vijay Srivastava, Yintao Song, Kanwal Bhatti, R. D. James. The Direct Conversion of Heat to Electricity Using Multiferroic Alloys. Advanced Energy Materials, 2011; 1 (1): 97 DOI: 10.1002/aenm.201000048

Further Reading
Metals and Non-metals
Physical and Chemical Changes
Pure Substances and Mixtures
Percentage Composition

Study Questions

1. What is meant by the term "alloy"?
2. What is meant by the term "multiferroic material"?
   
3. Calculate the percentage composition of the following multiferroic materials:
   
   • TbMnO₃
   • HoMn₂O₅
   • LuFe₂O₄
   • BiFeO₃
   • BiMnO₃
   • Ni₄₅Co₅Mn₄₀Sn₁₀
4. Which of the above substances would be classified as alloy(s)? Explain your answer.
5. Which of the substances in question 3 would be classified as mixture(s)? Explain your answer.
   
6. What similarities can you see in the multiferroic materials listed in question 3?
   
7. How do you think these similarities contribute to their magnetic and electrical properties?
   

Hard Metal

Hard metal is a mixture of a hard carbide phase, tungsten carbide, and a tougher metal phase, cobalt. It is produced by sintering, a process in which fine powders of tungsten carbide and cobalt are heated up so that the cobalt melts and the material is pulled together by capillary force. This results in a solid material consisting of hard tungsten carbide grains surrounded by the tougher cobalt-rich cement phase.

The size of the tungsten carbide grains determines the hardness of the hard metal.
Scientists know that by doping the material, that is, by adding another substance in tiny amounts, they can limit the size of the grains. For example, adding a tiny amount of vanadium can limit the growth of the grains, instead of growing grains 1/1000 mm in diameter, the addition of vanadium results in grain sizes about 1/10,000 mm. Scientists at the Chalmers University of Technology in Sweden have just used high-resolution electron microscopy to observe an extremely thin layer, only 2 atom layers thick, of a cubical structure on the tungsten carbide grains which they believe is affecting the growth of the grains.

Reference:
Expertanswer (2010, June 14). Materials researchers micromanage atoms in hard metal. ScienceDaily. Retrieved June 16, 2010, from http://www.sciencedaily.com­ /releases/2010/06/100614093343.htm

Study Questions:

1. Write the symbol for each of the following elements: tungsten, carbon, cobalt, vanadium.
2. To which group of the Periodic Table do tungsten, cobalt and vanadium belong?
3. Give possible oxidation states (numbers) for tungsten, cobalt, vanadium and carbon.
   
4. Suggest a formula for tungsten carbide.
5. Would you expect larger or smaller grains of tungsten carbide to grow at higher temperatures?