Chapter 14 - Making Use of Metals Flashcards
Where do we find most metals?
In the earth’s crust
What is the crust mainly made up of?
Compounds, some elements such as:
- Cu, Ag, Hg, Pt, Au - occur native
- 45% O2
- 27% silicon
Why do some elements occur native in the crust?
They’re uncombined because they are “unreactive”
What are the two main non-metal elements in the crust?
What compounds do they occur in?
Si and O - make up nearly 3/4
- occur together in compounds e.g silicon dioxide (silica or sand)
- O2 found in compounds such as iron(III) oxide, aluminium oxide, calcium carbonate
What percentage of the earth’s crust is metal?
2%
What metals are classified as “precious metals” and why?
Gold, silver, platinum and palladium
- scarce, expensive, often kept as a store of wealth
Definition of ore
a rock from which metals are obtained / extracted
What ore is sodium chloride found in?
rock salt
What ore is aluminium oxide found in? What other elements are in this ore?
bauxite
- contains impurities such as sand and iron oxide
6 questions mining companies must consider before digging up ore?
- how much ore is there
- how much metal will we get from it
- are there any special problems getting the ore out
- how much will it cost to mine the ore and extract the metal from it
- how much will we be able to sell the metal for
- will they make profit
what do the costs of mining consist of? 8
roads, buildings, mining equipment, the extraction plant, transport, fuel, chemicals and wages
Can the prices of an ore change?
Yes - even low-quality / low-grade ore may be valuable one year
why may local people object to mining operations?
What are the pros?
- may spoil an area
- air and rivers can become polluted
P - may welcome new jobs
How would you extract metals such as gold, silver or platinum from the earth?
They are native - occur in their ore as elements
All you need to do is separate the metal from sand and other impurities
DOES NOT INVOLVE CHEMICAL REACTIONS
What does extract mean?
To remove a metal from its ore
How are metals less reactive than carbon extracted?
They’re heated with carbon (or CO) or a suitable reducing agent so that they’re reduced
They’re less stable
How are metals more reactive than carbon extracted?
Their compounds are very stable
- need electrolysis to reduce them
- costs a lot because it uses electricity
In what form is carbon used during extraction?
in the form of coke (coal)
What 3 metals are below C in the reactivity series?
Zn, Fe, Pb
What 5 metals are above C in the reactivity series?
K, Na, Ca, Mg, Al
What form is the metal in when its added to carbon in a furnace?
In its metal oxide
What does the actual reducing during the process using carbon for extraction?
If carbon reacts with the limited amount of oxygen, this may bring about the actual reduction
Give the basic chemical equation for the extraction of iron ore
Fe2O3 + 3CO —> 2Fe + 3CO2
iron (III) oxide
Give the basic chemical equation for the extraction of aluminium ore
(extracted by electroysis)
2Al2O3 —> 4Al + 3O2
What does zinc blende consist of?
Mainly zinc sulfide - ZnS
What is the first step in extracting zinc from zinc blended?
Give the equation
First, zinc blende is roasted in air giving zinc oxide and sulfur dioxide.
2ZnS + 3O2 —> 2ZnO + SO2
In which two ways can zinc oxide be turned into zinc
- Using carbon monoxide
2. By electrolysis
Explain how zinc is extracted from zinc oxide by using carbon monoxide
ZnO + CO —> Zn + CO2
What does the final mixture contain? How is zinc extracted from this? (zinc extraction using CO)
Zinc and a slag of impurities
Separated by fractional distillation (907 degrees C)
What is zinc oxide dissolved in so it can be extracted by electrolysis? Why?
Dissolved in dilute sulfuric acid (sulfur dioxide made during roasting stage)
To reduce its melting point (1975 degrees C) (its also insoluble in water)
What is formed when the zinc oxide is dissolved in dilute sulfuric acid? Why?
Zinc sulfate - zinc oxide is a base so it neutralises the sulfuric acid giving a solution of zinc sulfate
What is the result fo the electrolysis of zinc sulfate?
half equation
Zinc is deposited at the cathode
Zn2+ + 2e- —> Zn (reduction)
Why is the method of electrolysis most commonly used to extract zinc?
- gives zinc of a very high purity
What other metals are found in zinc blende and what happens to them?
- calcium and lead appear as impurities in the zinc blende and these metals are recovered and sold too
What happens to the zinc deposited onto the cathode?
Scraped off the cathode and melted into bars to sell
4 uses of Al
overhead electricity cables (with a steel core for strength)
cooking foil and food cartons
drinks cans
coating CDs and DVDS
3 reasons why Al is used for overhead electricity cables
a good conductor of electricity (not as good as copper, but cheaper and much lighter);
ductile,
resists corrosion
3 reasons why Al is used for cooking foil and food cartons
non-toxic,
resistant to corrosion,
can be rolled into thin sheets
3 reasons why Al is used tor drink cans
light,
non-toxic,
resistant to corrosion
2 reasons why Al is used for coating CDs and DVDS
can be deposited as a thin film;
shiny surface reflects laser beam
2 uses of Cu
electrical wiring
saucepans and saucepan bases
2 reasons why Cu is used for electrical wiring
one of the best conductors of electricity,
ductile
4 reasons why Cu is used for saucepans and saucepan bases
malleable,
conducts heat well, unreactive,
tough
3 uses of zinc
protecting steel from rusting
coating or galvanising iron and steel
for torch batteries
Reason why Zn is used for protecting steel from rusting
offers sacrificial protection to the iron in steel
2 reasons why Zb is used for coating or galvanising iron and steel
resists corrosion, but offers sacrificial protection if coating cracks
2 reasons why Zn is used for torch batteries
gives a current when connected to a carbon pole, packed into a paste of electrolyte
Definition of alloy
A mixture where at least one other substance is added to a metal, to improve its properties; the other substance is often a metal too
Why do metals have to be molten to make an alloy?
So that the atoms of the second metal can entre the lattice
Why is an alloy better than just a pure metal?
Turning a metal into an alloy changes its properties and makes it more useful.
Why is an alloy stronger than a pure metal?
- Metal atoms are arranged in a lattice (in a pure metal)
- When pressure is applied, e.g hammering, the layers can slide over each other easily
- When the metal is turned into an alloy, new atoms of different sizes enter the lattice. The layer can NO LONGER slide easily
What is another property besides strength which might change in an alloy?
Could be more resistant to corrosion
Name 2 alloys
Brass and Aluminum alloy 7075 TF
What is brass made from?
70% Copper
30% Zinc
What are the special properties of brass?
Harder than copper
Does not corrode
What are the 3 uses of brass?
Musical instruments
Ornaments
Doorknobs
What is aluminium alloy made from?
90.25 % Aluminium
6% Zinc
2.5% Magnesium
1.25% Copper
What are the special properties of Al alloy?
light but very strong
does not corrode
What is the use of Al alloy?
aircraft
Why is pure iron not good for building things?
too soft
stretches quite easily
rusts very easily
What does mild steel contain?
99.7% iron 0.3% carbon
What is mild steel used for and why is it good for these things?
Hard and Strong
- buildings
- ships
- car bodies
- machinery
What does stainless steel contain?
20% chromium, 10% nickel and 70% iron
What is stainless steel used for and why is it good for these things?
Hard and Rustproof
- cutlery
- equipment in chemical factories
- surgical tools (easy to sterilise)
Basic summary of what goes on in a blast furnace
A mixture called the charge, containing iron ore is added through the top of the furnace.
Hot air is blasted in through the bottom.
After a series of reactions, liquid iron collects at the bottom of the furnace.
What 3 things are in the charge?
- Iron ore
- Limestone (CaCO3 - calcium carbonate)
- Coke (made from coal - almost pure carbon)
What are the 3 main reactions that occur in a blast furnace?
- Coke burns, giving off heat
- Carbon monoxide is made
- Iron (III) oxide is reduced
Give the chemical equation for stage 1 - coke burning with an explanation of what’s happening
The blast of air starts the coke burning - reacts with oxygen in the air to give carbon dioxide
C + O2 —> CO2
Is stage 1 a redox reaction? How?
yes
- C is oxidised to carbon dioxide
Where is the oxygen which reacts with C from? (stage 1)
The blast of hot air
What’s special about the reaction between C and O2?
It’s EXOTHERMIC - gives off heat, which helps to heat the furnace
Give the chemical equation for stage 2 - CO is made with an explanation of what’s happening
Carbon dioxide reacts with more coke:
C + CO2 —> 2CO
Is stage 2 a redox reaction? How?
Yes
- CO2 loses oxygen and is reduced
How does stage 2 reaction differ from stage 1?
It’s ENDOTHERMIC - takes in heat from the furnace
- good as stage 3 needs a lower temperature
Give the chemical equation for stage 3 - reduction of iron oxide with an explanation of what’s happening
- where extraction occurs
- CO reacts with iron ore to give liquid iron
- this trickles to the bottom of the furnace
Fe2O3 + 3CO —> 2Fe 3CO2
What is special about carbon monoxide in the blast furnace?
Acts as a REDUCING AGENT
- reduces iron (III) oxide to the metal
- CO oxidised to CO2
What is the limestone for?
To remove impurities
1.Give the chemical equation for limestone in a blast furnace - with an explanation of what’s happening + purpose
Limestone thermally decomposes in the heat
CaCO3 —> CaO + CO2
- to produce CaO which will remove sand that was present in the ore
2.Give the chemical equation for calcium oxide in a blast furnace - with an explanation of what’s happening
Calcium oxide reacts with the sand, which is mainly silicon dioxide/silica.
CaO + SiO2 —> CaSiO3
calcium + silica + calcium silicate
oxide
What happens to calcium silicate in a blast furnace?
Forms a slag which runs down the furnace and floats on the iron
What type of reaction is the reaction between CaO and SiO2
CaO - basic oxide SiO2 - acidic oxide CaSiO3 - salt
NEUTRALISATION
What is done with the slag from the blast furnace?
Molten slag is drained off
When it solidifies, it is sold, mostly for road building
What 2 waste gases are produced by the blast furnace? What are they used for?
Why are they waste gases?
carbon dioxide and nitrogen (come out the top of the furnace)
Heat is transferred from them to heat the incoming blast of air
Have not reacted with anything CO2 - stage 3 N - air
What is the iron from a blast furnace called?
Pig iron
Is iron from a blast furnace pure? Why?
NO
- carbon and sand are the main impurities
- phosphorus and sulfur compounds are also present from the iron ore
What is the pig iron used for?
Some is run into moulds to make cast iron
Most made into steels
What is cast iron used for and why?
Used for things like canisters for bottled gas and drain covers
- hard but brittle due to its high carbon content
What do scientists do to the rocks surrounding the area (where they want to mine) and why?
They test rocks and analyse the results in order to find out how much bauxite there is.
If the tests are satisfactory, mining begins.
What colour is bauxite?
Red-brown
Is it easy to mine bauxite? Why?
Yes - found near the surface so it’s easy to dig up
Where is the ore of Al taken to and what is it processed to?
Taken to a bauxite plant, where impurities are removed.
The result is white aluminium oxide or alumina.
Where is the aluminium oxide taken to be processed?
Alumina is taken to another plant for electrolysis. May even be sent yo another country where electricity is cheaper.
What happens to the extracted aluminium?
Made into sheets and blocks and sold to other industries.
Describe the basic structure of where Al2O3 is electrolysed
Electrolysis carried out in large steel tank
- lined with carbon which acts as a cathode (-)
- big blocks of carbon hang in the middle of the tank and act as the anode (+)
What is aluminium oxide dissolved in for the process of electrolysis and why?
Dissolved in molten cryolite.
- alumina melts at 2045 degrees C
- impossible to keep the tank that hot
- dissolved to decrease melting point as cryolite has lower mp
What is cryolite also called?
sodium aluminium fluoride
Give the half equation for the cathode and a brief explanation (Al extraction)
At cathode - reduction - Al ions gain electrons
Al3+ + 3e- –> Al
What happens to the aluminium when it is reduced?
Drops to the bottom of the cell as molten metal. This is run off at intervals.
What happens to the molten aluminium? (AL extraction)
Some will be mixed with other metals to make alloys.
Some is run into moulds, to harden into blocks.
Give the half equation for the anode and a brief explanation (Al extraction)
At anode - oxidation - oxygen ions lose electrons
2O2- —> O2 + 4e-
Why do the carbon anodes need to be replaced? Give a chemical equation (Al extraction)
Oxygen gas bubbles off and reacts with the anode
C + O2 —> CO2
Carbon block gets eaten away and needs to be replaced
What is the overall reaction for the electrolysis of alumina?
Alumina is broken down giving aluminium
2Al2O3 —> 4Al + 3O2
7 properties of aluminium
- bluish-silver shiny metal
- Has a low density
- good conductor of heat and electricity
- Malleable and ductile
- resists corrosion (fine coat of Al2O3 forms)
- Not very strong when pure - made stronger when mixed with other metals to form alloys
- Generally non-toxic (not harmful to health)
What are the 2 main steps for making steels?
- Unwanted impurities are removed from the iron
2. Other elements are added
What is used to remove impurities from iron? (making of steels)
Molten iron from a blast furnace is poured into an oxygen furnace, and a jet of oxygen is turned on
What does the oxygen do in an oxygen furnace (making of steels)?
Oxygen reacts with carbon, forming carbon monoxide and carbon dioxide gases, which are carried off.
Reacts with other impurities to form acidic oxides.
What is added after the oxygen has done its job in an oxygen furnace? (making of steels)
Calcium oxide
- it is a basic oxide
- reacts with silicon dioxide and other acidic oxides to give a slag that is skimmed off
Are all impurities removed from iron in the steel making process?
Depends on the type of steel
- for some yes, for some they just contain iron and carbon so carbon doesn’t need to be removed
What beneficial qualities does carbon give to iron in a steel?
Carbon makes steel stronger
What negative qualities can carbon give to iron in a steel?
Too much can make the steel brittle
- therefore the carbon content is checked continually.
- when it is correct the oxygen is turned off.
