Separate Chemistry 2

THIS UNIT HAS THREE MAIN SECTIONS: ORGANIC CHEMISTRY, QUALITATIVE ANALYSIS AND MATERIALS. THE CORE PRACTICAL FROM THE QUALITATIVE ANALYSIS SECTION HAS MANY PARTS TO TEST FOR DIFFERENT TYPES OF METHODS. THE CHEMICALS USED IN THESE TESTS AND THE POSITIVE RESULTS FOR EACH ION MUST BE LEARNT

Qualitative Analysis

It is important that the test for any ion is unique so that the ion can be identified. Two ions that produce the same coloured precipitate cannot be distinguished without further tests.

Instrumental methods of analysis improve speed of tests, accuracy by giving values closer to their true values and sensitivity by detecting small quantities. A flame photometer measures light intensity of flame colours and can determine the concentration of an ion by locating it on a calibration curve of standard solutions with known concentrations. The flame photometer separates out the colours to produce a spectrum of light. Each metal produces a different spectrum and if it matches part of the spectra for an unknown solution then this ion is present in that solution

CORE PRACTICAL: Testing for Ions

• If a white precipitate forms, add excess hydroxide to see if it redissolves

Test	Method	Results
Flame test for metal cations	Light a Bunsen burner and open the air hole for hot blue flame Pick up a small sample of solid salt using a nichrome wire loop cleaned in HCl(aq) Hold sample in the edge of the flame Observe and record the flame colour	Li+	Red Flame
		Na+	Yellow Flame
		K+	Lilac Flame
		Ca2+	Orange-Red
		Cu2+	Blue-Green
Hydroxide precipitate tests for metal cations     Ammonium ions	Dissolve a small amount of salt in a test tube using distilled water Add dilute sodium hydroxide and warm gently  Record the colour of any precipitate formed If a white precipitate forms, add excess hydroxide to see if it redissolves	Fe2+	Green
		Fe3+	Brown
		Ca2+	White (Stays white in excess)
		Al3+	White (Redissolves in excel)
		Cu2+	Blue
		Turns damp litmus paper blue as ammonia is been produced when ammonium ions are warmed
Carbonate Ions	Add solution containing ions to test tube then add a few drops of acid Check for bubbling and use limewater to confirm that it is CO2	Bubbling indicates carbonate ions. CO2 should turn limewater cloudy
Sulfate Ions	Add a few drops of hydrochloric acid to remove other ions like carbonate ions Add a few drops of barium chloride	White precipitate indicates sulfate ions are present as insoluble barium sulfate is formed
Halide Ions	Add solution containing ions to test tube Add a few drops of dilute nitric acid to acidify the solution and remove carbonates  Add a few drops of silver nitrate	Cl–	White Precipitate
		Br–	Cream Precipitate
		I–	Yellow Precipitate
Using these result, you can predict the identity of a salt. Use a flame test or precipitate reaction for the cation, then the halide test, sulfate test or carbonate test for the anion

Materials

Material	Properties – data will be provided in the Paper 2 Exam if questioned
Glass and Clay Ceramics	Durable compounds that change very little when heated; unreactive, hard and stiff Poor conductors and high melting points due to giant structures Clay ceramics are made from clay moulded into shape then heated to form crystals Glazes are hard, waterproof and smooth  Glass is made by melting sand then allowing it to cool and solidify Glass atoms not arranged regularly so glass is transparent
Polymers	High Mr, made from chain of monomers Moulded into complex shapes Strong and unreactive; poor conductors PVC is used in pipes and window frames as it is rigid PVC is made softer using plasticisers in manufacture to be flexible for use in indoorpipes and waterproof flooring
Metals	Strong, hard, shiny solids with high melting points Good conductors which are malleable  Alloys can be made for strengthening Electroplated to improve appearance or resistance to corrosion
Composites	Mixture of two or more materials of contrasting properties Concrete is made by mixing cement, sand, aggregate and water. As it hardens, reactions hold the components together. The sand and aggregate form the reinforcement of the concrete, while the cement which bonds the two together is the matrix In fibreglass, thin glass fibres form the reinforcement while polymer resin forms the matrix, producing a lightweight fibre with high tensile strength Wood is a natural composite material with cellulose fibres in a matrix of lignin

Nanoparticles

Nanoparticles consist of a few hundred atoms, about 1-100nm in size, therefore they are larger than molecules and atoms but smaller than cells.

Nanoparticles have a small size and a large surface area to volume ratio, which makes them very useful. Nanoparticulate titanium dioxide is transparent as particles are so small, but absorbs harmful UV radiation, making them useful in sunscreens. The surface area to volume ratio makes them efficient catalysts, so stain-resistant clothing is treated with nanoparticles to catalyse the breakdown of dirt.

Scientists are worried that nanoparticulate materials pose hazards to health and the environment. Their small size allows them to be breathed in and pass through membranes. The SA:V ratio may allow them to catalyse harmful reactions or to carry toxic substances across surfaces. The risks are difficult to tell as nanoparticles have not been used extensively

Organic Chemistry

Hydrocarbons

Hydrocarbons are molecules containing only carbon and hydrogen. Alkanes are hydrocarbons based on a chain of carbon atoms, with a general formula CnH2n+2. The suffix ‘–ane’ indicates that a molecule is an alkane. The prefix ‘meth-’ means there is one carbon atom, ‘eth-’ means there are two, ‘prop-’ means three atoms and ‘but-’ means that there are four carbon atoms in the chain.

Alkanes are saturated hydrocarbons because all of the carbon-carbon bonds are single bonds. Alkenes, with the general formula CnH2n, are unsaturated hydrocarbons with a carbon-carbon double bond present in the molecule. This double bond is the functional group, the group of atoms responsible for the molecule’s chemical properties.

A functional group can be in different places but the molecules have the same formula. Molecules with the same molecular formula but different arrangements of atoms are called isomers. The image to the right is but-1-ene, and below it is but-2- ene. The two molecules have the same molecular formula, but are arranged in different ways

Bromine water is orange-brown and is used to indicate alkenes. When bromine water reacts with an alkene, it becomes colourless. This is because the C=C double bond reacts with the bromine to form colourless dibromoethane. The bromine is therefore removed from the solution, causing the loss of colour. Since alkanes are saturated, they contain no C=C bonds and cannot react with bromine atoms. This is an addition reaction as two reactants react to form a single product.

When hydrocarbons burn, carbon dioxide and water are formed in plentiful supply of oxygen. If all the atoms in the hydrocarbon are fully oxidised, then the CO2 and H2O are the only products. If there is a limited supply, then incomplete combustion occurs.

Polymers

A polymer is a substance of high average relative molecular mass made up of repeating units called monomers. Monomers join together in a polymerisation reaction. The name of the polymer is poly(name of repeating unit) e.g. poly(ethene).

Ethene molecules have a C=C bond. One of the bonds breaks open and another ethene molecule adds on. This process repeats, creating a long chain in a process called addition polymerisation

The structure shows how to convert from monomer to polymer. Note the bond lines passing out of the brackets to show it is repeating, and the single bond. When converting backwards, remember the double bond. Keep the units on the four extensions the same

Polymer	Poly(ethene)	Poly(propene)	Poly(chloroethene)	Poly(tetrafluoroethene)
Properties	Flexible, cheap, good insulator	Flexible, does not shatter	Tough, good insulator, can be hard or flexible	Tough, slippery
Uses	Plastic bags, bottles, cling film	Buckets, bowls, crates, rope	Window frames, gutters, pipes	Non-stick coating, burette taps, carpets
Monomer

Polyesters are synthetic polymers made by condensation polymerisation in which monomers join together and eliminate a small molecule, such as water.

Carboxylic Acid + Alcohol → Ester + Water

A polyester is a long-chain molecule with many ester links. To form the link, each monomer must have two functional groups, one at each end of the molecule.

Polyesters are used to make synthetic fibres and bottles

DNA is a polymer made from four different monomers called nucleotides. Starch is a polymer based on sugars. Proteins are polymers based on amino acids

Many polymers originate from fractions of crude oil, a finite resource, and therefore chemists must find a new source of monomers soon. Polymers are not biodegradable as they cannot be broken down by microorganisms. Therefore, they must be disposed in one of three ways:

	Landfill	Burning	Recycling
Advantages	Cheap process	No space taken Energy generation	Less space wasted Reduced emissions Less resources wasted making new plastics e.g.crude oil Saves money and creates jobs
Disadvantages	Land is used  Plastics are not biodegradable so remain for 1000s of years	Toxic gasesproduced  Harmful to health Global Warming  Ash produced	Separation can be difficult and expensive  Quality of recycled polymer reduced due to mixing Strength decreases so can only be recycled acertain number of times  Melting plastics emits harmful gases

Alcohols and Carboxylic Acids

The alcohols have the functional group -OH and have the general formula CnH2n+1OH. All alcohols have similar reactions because their molecules contain the same functional group. They all produce carbon dioxide and water when burnt, and react with reactive metals.

Carboxylic acids have the general formula CnH2n+1COOH, the functional group being -COOH. Carboxylic acids are formed when alcohols are oxidised gently using oxidising agents like hot copper oxide. They react as all acids do with metals, bases and carbonates. The salt produces will be an -oate salt e.g. ethanoate. Ethanoic acid is what gives vinegar its distinctive taste

Methanol	CH3OH		Methanoic Acid	HCOOH
Ethanol	C2H5OH		Ethanoic Acid	CH3COOH
Propanol	C3H7OH		Propanoic Acid	C2H5COOH
Butanol	C4H9OH		Butanoic Acid	C3H7COOH

Ethanol is produced in fermentation, as enzymes in yeast turn sugars into ethanol and carbon dioxide. Carbohydrates are dissolved in water before the yeast is added. Fermentation is carried out at 30-40°C at atmospheric pressure. The rate of reaction is slow. During fermentation, the temperature and pH must be controlled to ensure that enzyme activity is optimum. Oxygen must be kept out as fermentation is a type of anaerobic respiration. The ethanol produced is impure, and is separated from water by fractional distillation, as its boiling point is lower at 78°C. The first distillate will be concentrated ethanol.

CORE PRACTICAL: Combustion of Alcohols

A – Measure mass of alcohol burner and cap. Record mass and name of alcohol

B – Place burner in the centre of heat-resistant mat

C – Use measuring cylinder to add 100cm3 of water to a flask. Record the initial temperature and clamp flask above the burner

D – Light the wick and allow water to heat for a period of time. Stir occasionally

E – Replace the cap and record final temperature

F – Measure mass of burner to get mass lost

G – Calculate mass of alcohol burnt to produce a 1°C rise in temperature

H – Repeat using different alcohols

I – Plot a graph of mass per 1°C rise against length of carbon chain. Overall, the longer the chain, the more efficient the alcohol is because a lower mass produced a greater relative rise in temperature