Using Materials (chemistry only)
Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting is an example of corrosion. Both air and water are necessary for iron to rust.
Rusting is a specific example of corrosion, and requires both air and water for iron to rust. There are three ways to prevent iron from rusting:
- exclusion of oxygen (by either using an atmosphere of nitrogen or argon)
- exclusion of water (by storing with a desiccant)
- sacrificial protection (when a more reactive metal than iron is attached to the object to prevent iron from rusting)
Corrosion can be prevented by using a physical barrier (e.g. applying a coating such as greasing, painting, or plastic). Aluminium has an oxide coating that protects the metal from further corrosion.
Some coatings are reactive and contain a more reactive metal to provide sacrificial protection, eg zinc is used to galvanise iron.
Electrolysis can be used to put a thin layer of a metal on an object that you don't want to corrode. An example is steel cutlery being electroplated with silver. It requires:
- the cathode to be the object you want electroplated (e.g. steel cutlery)
- the anode to be the metal you are plating the object with (e.g. silver)
- the electrolyte to contain ions of the metal found at the anode (e.g. silver nitrate solution)
Electroplating is used to improve the corrosion resistance of metal objects, but also is used to improve appearance (e.g. gold-plating jewellery).
Different conditions demonstrating if an iron nail would corrode.
Most metals in everyday use are alloys. An alloy is a mixture of two or more elements, where at least one of the elements is a metal. Bronze is an alloy of copper and tin. Brass is an alloy of copper and zinc.
Gold used as jewellery is usually an alloy with silver, copper and zinc. The proportion of gold in the alloy is measured in carats. 24 carat being 100% (pure gold), and 18 carat being 75% gold.
Pure metals are generally too soft to use as they have a regular lattice structure. When a force is applied to the metal, the regularly aligned layers are able to easily slide over
each other (this makes them malleable).
Alloys are made of different sized atoms, distorting the regular layers so that they are not able to easily slide over each other any more - increasing the strength of the metal (they are less malleable, and more brittle).
Iron is able to be alloyed with other elements to produce a variety of alloy steels. These alloys have different properties depending on which elements, and how much of each element, is present.
|low carbon||carbon||softer, more easily shaped||car body parts|
|high carbon||carbon||strong, but brittle||railways, cutting tools|
|stainless||chromium and nickel||hard, resistant to rusting||taps, cutlery|
Uses of aluminium alloys
Aluminium does not react with water, as its surface is protected by a naturally formed layer of aluinium oxide (allows the metal to resist corrosion). Aluminium foil is used domestically to wrap/store food, because:
- it does not react with the substances in food
- it is malleable and so can easily be folded around the food
- it has a low density so it is very lightweight
Ceramics, Polymers and Composites
|appearance||transparent||opaque and dull||opaque and shiny||transparent or opaque|
|melting point||high||high||high||high or low|
|malleable or brittle?||brittle||brittle||malleable||malleable or brittle|
Glass ceramics are made by melting sand with other substances (normally metal oxides), then allowing the liquid to cool and solidify.
Most of the glass we use is soda-lime glass, made by heating a mixture of sand, sodium carbonate and limestone. Borosilicate glass, made from sand and boron trioxide, melts at higher temperatures than soda-lime glass.
Clay ceramics, including pottery and bricks, are made by shaping wet clay and then heating in a furnace.
Polymers can be transparent or opaque and they are often tough and flexible, but some are hard and brittle.
Most composites are made of two materials, a matrix or binder surrounding and binding together fibres or fragments of the other
material, which is called the reinforcement.
Properties of Polymers
The properties of polymers depend on what monomers they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are both produced from ethene.
Low density poly(ethene) has a branched structure meaning that the molecules are arranged randomly.
High density poly(ethene) has less (or no) branching of the polymer chains, so the molecules line up closer together.
Thermosoftening and thermosetting
Thermosoftening plastics melt when heated, meaning that they can be recycled, as this involves melting them before making a new product. This is because thermosoftening plastics do not have branches (made
of covalent bonds) between polymer molecules, so the molecules are able to move over each other when heated.
Thermosetting plastics don't melt when heated, and just tend to char/burn instead. However, they are more resistant to higher temperatures than thermosoftening plastics so are used to make electrical plugs.
|low density poly(ethene), LDPE||flexible, unreactive, can be made into films||carrier bags, bubble wrap|
|high density poly(ethene), HDPE||strong, flexible, resists shattering, resists chemical attack||plastic bottles, pipes, buckets|
The Haber Process
nitrogen + hydrogen ⇌ ammonia
N2 + 3H2 ⇌ 2NH3
The Haber Process is used to produce ammonia, which can be used to manufacture nitrogen-based fertilisers. This process is a reversible reaction between nitrogen (extracted from the air) and hydrogen (obtained from
natural gas). This reaction can reach a dynamic equilibrium.
On cooling, the ammonia liquefies and is removed. The remaining hydrogen and nitrogen are recycled.
The conditions for the Haber Process are:
- temperature of 450 °C
- pressure of 200 atmospheres (200 atm)
- iron catalyst (doesn't change the equilibrium, just increases rate of both forward and backwards reactions)
These conditions are a compromise to keep the costs down, but also to be the right conditions to achieve an acceptable yield.
During industrial reactions, including the Haber process, conditions used are related to:
- the availability and cost of raw materials and energy supplies
- the control of temperature, pressure and catalyst used produce an acceptable yield in an acceptable time
Temperature of 450 °C
- forward reaction is exothermic, so a lower temperature favours the forward reaction to maximise yield
- through heating particles will have more energy, so more frequent, successful collisions between particles (so a faster rate)
- a compromise is made on the temperature
Pressure of 200 atm
- the forward reaction turns 4 moles ('volumes') of gas into 2 moles of gas
- increasing the pressure will favour the forward reaction to maximise the yield (as this forces the particles to take up less volume)
- to maintain this high pressure, lots of energy is needed - so high costs
Compounds of nitrogen, phosphorus and potassium are used as fertilisers to improve agricultural productivity. NPK fertilisers contain compounds of all three elements.
Industrial production of NPK fertilisers can be achieved using a variety of raw materials in several integrated processes. NPK fertilisers are formulations of various salts containing appropriate percentages of the
Ammonia is made in the Haber process, and is used to manufacture ammonium salts as well as nitric acid.
Potassium chloride, potassium sulfate and phosphate rock are obtained by mining, but phosphate rock cannot be used directly as a fertiliser.
Phosphate rock is treated with nitric acid or sulfuric acid to produce soluble salts that can be used as fertilisers. Reacting phosphate rock with:
- nitric acid makes phosphoric acid and calcium nitrate
- sulfuric acid makes single superphosphate (this is a mixture of calcium phosphate and calcium sulfate)
- phosphoric acid makes triple superphosphate (calcium phosphate)
Making ammonium sulfate in the lab
Ammonium sulfate can be made in the lab using dilute ammonia solution and dilute
sulfuric acid. Both reactants are soluble, so a titration must be used to carry out this reaction:
ammonia + sulfuric acid → ammonium sulfate
2NH3(aq) + H2SO4(aq) → (NH4)2SO4(aq)
The lab preparation of ammonium sulfate is a batch process. This means that only a small amount of product at any one time, and the apparatus needs to be cleaned before each new batch.
- pour dilute sulfuric acid into a beaker
- add a few drops of methyl orange indicator
- add dilute ammonia solution drop by drop, stirring in between each addition
- continue step 3 until the colour permanently changes from red to orange
- pour the reaction mixture into an evaporating basin, and heat carefully over a boiling water bath
- stop heating before all the water has evaporated. Leave aside for crystals to form
- pour away excess water and leave the crystals to dry in a warm oven (or pat dry with filter paper)
Industrial production of ammonium sulfate
The industrial production of ammonium sulfate happens on a much larger scale than its production in the lab. A fertiliser factory begins with the raw materials needed to make ammonia and sulfuric acid,
rather than buying these two reactants from elsewhere.
The industrial production of ammonium sulfate is a continuous process. The product is made quickly all the time, as long as raw materials are provided. There are four stages:
- sulfur + oxygen → sulfur dioxide
- sulfur dioxide + oxygen ⇌ sulfur trioxide
- sulfur trioxide + water → sulfuric acid
- ammonia + sulfuric acid → ammonium sulfate