Separate Chemistry 2

Question 1

Alkanes

Answer 1

Alkanes are a group of saturated hydrocarbons.
The term saturated means that they only have single carbon-carbon bonds, there are no double bonds.
The general formula of the alkanes is CnH2n+2.
They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases.
Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and can react with halogens in the presence of light.
Methane is an alkane and is the major component of natural gas.

Question 2

Examples of alkanes

Answer 2

methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane.

Question 3

Alkenes

Answer 3

Alkenes are a homologous series of hydrocarbons. They’re more reactive than alkanes. All alkenes contain the functional group C=C, a double covalent bond between two of the carbon atoms in their chain.
Alkenes are known as unsaturated because they contain a C=C double bond. This can open up to become a single bond, allowing the two carbon atoms to bond with other atoms.

Question 4

Examples of alkenes

Answer 4

ethene, propene, butene, pentene, hexene, heptene, octene, nonene, decene, and undecene.

Question 5

Bromine and alkenes

Answer 5

Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
The reaction between bromine and ethene is an example of an addition reaction.
The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond.

Question 6

Distinguishing between alkanes and alkenes

Answer 6

Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond).
Br2(aq) is an orange-yellow solution, called bromine water.
The unknown compound is shaken with the bromine water.
If the compound is unsaturated, an addition reaction will take place and the coloured solution will become clear.

Question 7

Combustion of alkanes

Answer 7

Alkanes and alkenes undergo combustion in the presence of air
Complete combustion occurs to form water and carbon dioxide gas
Hydrocarbon + oxygen ——- carbon dioxide + water

Gasoline is largely composed of isomers of octane, C8H18 , which requires large amounts of oxygen to combust fully.
The efficiency of car engines does not usually enable all the gasoline to burn, so car exhaust will contain small amounts of unburnt hydrocarbons as well as other products such as carbon monoxide and soot which lead to environmental problems.
The carbon dioxide produced is a major contributor to global warming and the replacement of combustion engines with electric vehicles is a major on-going challenge for all countries.

Question 8

Combustion of alkenes

Answer 8

These compounds undergo complete and incomplete combustion, but because of the higher carbon to hydrogen ratio they tend to undergo incomplete combustion, producing a smoky flame in air.

Question 9

Polymers

Answer 9

Polymers are large molecules of high relative molecular mass and are made by linking together large numbers of smaller molecules called monomers.
Each monomer is a repeat unit and is connected to the adjacent units via covalent bonds.
Polymerisation reactions usually require high pressures and the use of a catalyst.
Many everyday materials such as resins, plastics, polystyrene cups, nylon etc. are polymers.
These are manufactured and are called synthetic polymers.
Nature also produces polymers which are called natural or biological polymers.

Question 10

Formation of poly(ethene)

Answer 10

Addition polymers are formed by the joining up of many monomers and only occurs in monomers that contain C=C bonds.
One of the bonds in each C=C bond breaks and forms a bond with the adjacent monomer with the polymer being formed containing single bonds only.
Many polymers can be made by the addition of alkene monomers
Poly(ethene) is formed by the addition polymerisation of ethene monomers and is most commonly called polythene.

Question 11

Other addition polymers

Answer 11

Other addition polymers are made from alkene monomers with different atoms attached to the monomer such as chlorine, fluorine or a methyl group.
The name of the polymer is deduced by putting the name of the monomer in brackets and adding poly- as the prefix.
For example if propene is the alkene monomer used, then the name is poly(propene).

Question 12

Deducing the monomer from the polymer

Answer 12

Polymer molecules are very large compared with most other molecule.
Repeat units are used when displaying the formula.
To draw a repeat unit, change the double bond in the monomer to a single bond in the repeat unit.
Add a bond to each end of the repeat unit.
The bonds on either side of the polymer must extend outside the brackets (these are called extension or continuation bonds).
A small subscript n is written on the bottom right hand side to indicate a large number of repeat units.

Question 13

Deducing the monomer from the polymer

Answer 13

Identify the repeating unit in the polymer.
Change the single bond in the repeat unit to a double bond in the monomer.
Remove the bond from each end of the repeat unit and the subscript n (which can be placed in front of the monomer).

Question 14

Use of poly(ethene)

Answer 14

Properties - Flexible, cheap and electrically insulating..
Use - Plastic Bags (Low density polythene).
Plastic Bottles (high density polythene).

Question 15

Use of poly(propene)

Answer 15

Properties - Flexible and strong.
Use - Food Packaging
Ropes
Carpets

Question 16

Use of poly(chloroethene) (PVC)

Answer 16

Properties - Long-lasting, tough and cheap.
Use - Plastic Sheets
Artificial Leather
Drainpipes and Gutters
Insulation on Wires

Question 17

Use of poly(tetrafluoroethene) (PTFE)

Answer 17

Properties - Very tough, non-stick and resistant to high temperatures.
Use - Cookware (non stick pans)
Pipework

Question 18

Condensation polymers

Answer 18

Condensation polymers are formed when two different monomers are linked together with the removal of a small molecule, usually water.
The monomers have two functional groups present, one on each end.
The functional groups at the ends of one monomer react with the functional group on the end of the other monomer, in so doing creating long chains of alternating monomers, forming the polymer.
Polyesters are formed from two different monomers and produce water.
For every ester linkage formed in condensation polymerisation, one molecule of water is formed from the combination of a -H and an -OH group.

Question 19

Condensation polymer vs addition polymer

Answer 19

Addition polymerisation forms the polymer molecule only.
Condensation polymerisation forms the polymer molecule and one water molecule per linkage.

Question 20

Polyester

Answer 20

Dicarboxylic acid monomers can react with diol monomers to form ester links this is a condensation reaction. The dicarboxylic acid monomers contain two carboxylic acid groups (-COOH) and the diol monomers contain two alcohol groups (-OH).
The molecule with the ester link has a functional group at each end. These can then react in condensation reactions, making the chain longer. The series of reactions together is known as condensation polymerisation and the resultant polymer is called a polyester.

Question 21

Problems with polymers

Answer 21

Polymers are formed by the joining up of many small molecules with strong covalent bonds.
This makes polymers unreactive and chemically inert so they don’t easily biodegrade.

Question 22

Disposing polymers landfill sites

Answer 22

Waste polymers are disposed of in landfill sites but this takes up valuable land, as polymers are non-biodegradable so micro-organisms such as decomposers cannot break them down.
This causes sites to quickly fill up.

Question 23

Disposing polymers incineration

Answer 23

Polymers release a lot of heat energy when they burn and produces carbon dioxide which is a greenhouse gas that contributes to climate change.
Polymers that contain chlorine such as PVC release toxic hydrogen chloride gas when burned.
If incinerated by incomplete combustion, carbon monoxide will be produced which is a toxic gas.

Question 24

Disposing polymers recycling

Answer 24

Polymers can be recycled but different polymers must be separated from each other.
This process is difficult and expensive.

Question 25

Advantages of recycling polymers

Answer 25

Recycling is a more economically viable process than manufacturing from scratch.
It decreases the use of crude oil which allows it to be kept for other purposes.
It is better for the environment as plastic waste is being collected and reused, hence recycling reduces the emissions of greenhouse gases and other toxic gases produced during the manufacturing process.
It also reduces the amount of landfill sites needed.
Recycling is itself an entire industry which creates employment and economic growth.

Question 26

Disadvantages of recycling polymers

Answer 26

Sorting plastics by type of polymer is a tedious and labour intensive process which is costly.
Recycling counts on what is collected in as the raw material, therefore production of certain types of polymers may not be possible due to a lack of starting ingredients.
Melting polymers produces toxic gases that are harmful to plants and animals
Polymers can only be recycled a number of times before they lose their properties and become useless.
Recycling runs the risk of mixing different polymers together, which again will affect their properties. This is particularly risky for polymers designed for specialist use such as aircraft or automobile parts, where safety is of utmost importance.

Question 27

Raw materials of polymers

Answer 27

Plastics are made from crude oil. Crude oil is a finite resource, eventually it will all get used up and run out. The more we use up our crude oil resources, the more expensive crude oil will become. This will then increase the price of crude oil products.
Crude oil isn’t just used to make plastics, we need it for lots of different things, such as petrol for cars and heating our homes. As resources dry up, we will face the dilemma of how to use the remaining oil. One way we can help delay this problem is by recycling our polymers.

Question 28

DNA (natural polymer)

Answer 28

DNA (deoxyribonucleic acid) is found in every living thing and many viruses. It contains genetic instructions that allow the organism to develop and operate. It’s a large molecule that takes a double helix structure.
DNA is made of two polymer chains of four different types of monomers called ‘nucleotides’. The order of the nucleotides acts as a code for the organism’s genetic information.

Question 29

Starch (natural polymer)

Answer 29

Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula Cx(H2O)y.
There are simple carbohydrates and complex carbohydrates.
Simple carbohydrates are called monosaccharides and are sugars such as fructose and glucose.
Complex carbohydrates are called polysaccharides such as starch
The monomers from which starch is made are sugars.
Starch is used to store energy.
Complex carbohydrates are condensation polymers formed from simple sugar monomers and, unlike proteins, are usually made up of the same monomers.
An H2O molecule is eliminated when simple sugars polymerise
The linkage formed is an -O- linkage and is called a glycosidic linkage.

Question 30

Protein (natural polymer)

Answer 30

Proteins are condensation polymers which are formed from amino acid monomers joined together by peptide bonds.
Amino acids are small molecules containing amine (-NH2) and carboxylic acid (-COOH) functional groups.

Question 31

Alcohol

Answer 31

All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions.
The general formula of an alcohol is CnH2n+1OH.
Alcohols are colourless liquids that dissolve in water to form neutral solutions.
In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’.

Question 32

Examples of alcohol

Answer 32

methanol, ethanol, propanol and butanol.

Question 33

Carboxylic acids

Answer 33

Carboxylic acids is the name given to compounds containing the functional group carboxyl, -COOH.
The naming of a carboxylic acid follows the pattern alkane + oic acid.

Question 34

Examples of carboxylic acids

Answer 34

Methanoic acid, ethanoic acid, propanoic acid, butanoic acid.

Question 35

Making ethanoic acid

Answer 35

Alcohols undergo oxidation to produce carboxylic acids when treated with oxidising agents.
When ethanol is heated with acidified potassium dichromate solution the ethanol oxidises to ethanoic acid.
The equation for the reaction is:
CH3CH2OH + [O] → CH3COOH + H2O

The oxidising agent is represented by the symbol for oxygen in square brackets.
The reaction is slow so the mixture is heated to its boiling point for about an hour; to avoid the substances evaporating a condenser is placed above the reaction flask that prevents volatile liquids from escaping.
During the reaction the potassium dichromate turns from orange to green.

Question 36

Predicting products

Answer 36

Organic molecules that belong to the same homologous series react in the same way, so the products of those reactions can be predicted.
Homologous series are families or groups of organic compounds that have similar features and chemical properties due to them having the same functional group.
All members of a homologous series have:
The same general formula.
The difference in the molecular formula between one member and the next is CH2.
Gradation in their physical properties.
Same functional group.
Similar chemical properties.
The chemistry of homologous series is therefore determined by the functional group.
We can use this to predict how other molecules in a homologous series will react.
Although the homologous series allows us to predict what the reaction products should be, it tells us nothing about the rate or extent of the reaction.
For example, as the chain length increases in alcohols the combustion or oxidation reactions may be slower or incomplete as the carbon chain influences the reactivity of the functional group.

Question 37

Fermentation

Answer 37

Ethanol (C2H5OH) is one of the most important alcohols.
It is used as fuel (for vehicles in some countries) and as a solvent.
It is the type of alcohol found in alcoholic drinks such as wine and beer.
It can be produced by fermentation where sugar or starch is dissolved in water and yeast is added.
The mixture is then fermented between 15 and 35 °C with the absence of oxygen for a few days.
Yeast contains enzymes that break down sugar to alcohol.
If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured.
The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide.

Question 38

Fractional distillation of ethanol

Answer 38

Fermentation produces a dilute solution of ethanol which needs to be separated from the reaction mixture.
This is done using fractional distillation.
The mixture is heated to 78 ºC which is the boiling point of ethanol but below that of water (100 ºC).
The ethanol evaporates and its vapours pass through a condenser, where they cool and condense, forming liquid ethanol.
The water and any other impurities remain behind in the reaction flask.
When the temperature starts to increase to 100 ºC heating should be stopped. The water and ethanol have now been separated.

Question 39

Nano particles

Answer 39

Nanoparticles are between 1 and 100 nanometres in size and usually contain only a few hundred atoms.
Atoms and simple molecules are around 100 times smaller than this.
Nanoparticles are much smaller than fine particles which have diameters of between 100 and 2500 nm.
The research into the production and application of nanoparticles is called nanoscience.

Question 40

Properties of nanoparticles

Answer 40

One of the most interesting features of nanoparticles is their very high surface area to volume ratio.
As particles decrease in size, their surface area increases in relation to their volume.
This is why nanoparticles may have properties different from those for the same materials in bulk.
It may also mean that smaller quantities are needed to be effective than for materials with normal particle sizes.
Fullerenes (nanoparticles made of carbon) behave very differently to larger compounds of carbon like diamond and graphite.
The surface area to volume ratio is an important feature in catalysis and surface chemistry.
The higher the ratio then the more surface area is available for reaction, hence the better the catalyst.

Question 41

Use of nanoparticles

Answer 41

The main industrial application of nanoparticles is in catalysis due to their high surface area to volume ratios.
Titanium dioxide is a good example of how the same chemical has different properties in bulk and nanoparticle form.
Titanium dioxide in nanoparticle form is used in sunscreens as it blocks UV light but leaves no white marks on the skin.
The same chemical in bulk form is used as a white pigment in paints
Fullerenes are used in the medicine and drug design as they are more easily absorbed than other particles and can deliver drugs to target areas more effectively.
Fullerenes are also used in electronic circuitry and as coatings for artificial limbs and joints.
Nanoparticles of silver are sprayed onto the fibres of medical clothing and surgical masks which gives them the flexibility of a material but with the added benefit of the antibacterial properties of silver metal.

Question 42

Risks of nanoparticles

Answer 42

Nanoparticles have widespread uses and applications that can provide an immense advance in materials technology.
The use of nanoparticles in science is in its early stages so there are still a lot of unknown factors and potential risks.
In particular there is a lack of understanding on how they may affect health.
Although there haven’t been any serious short term side effects, there could be long term side effects which we haven’t detected yet as they haven’t been in use long enough.
Even a small amount of toxicity in a particular nanoparticle would be multiplied due to the high surface area to volume ratio.
This coupled with the fact that they are not easily disposed of by the body are a cause for caution in the medical application of nanoparticles.

Question 43

Physical Glass ceramics

Answer 43

Transparent and strong, glass insulates against heat.
Glass ceramics are also more durable than other materials hence they are better suited for use in windows than plastic.
Most of the glass produced is soda-lime glass which is made by heating a mixture of limestone, sand and sodium carbonate (soda) until it melts.
On cooling it solidifies to form glass.
A variation is borosilicate glass which is made using sand and boron trioxide and has a higher melting point than soda-lime glass.

Question 44

Physical properties Clay ceramics

Answer 44

These are hardened materials that resist compressive forces.
Clay is a soft material dug up from the earth which hardens at high temperatures and when it is fired, produces a very strong and hard material.
This allows bricks to be used to build walls which withstand the weight and pressure of the material bearing downwards on itself.

Question 45

Physical properties of polymers

Answer 45

Can be tailor designed to have specific properties for specific uses.
Can be made opaque or transparent.
Usually tough and flexible, some specialist polymers can be brittle.
Poor conductor of heat and electricity.

Question 46

Physical properties of composites

Answer 46

Made from two components: reinforcement and matrix.
The matrix is what binds the reinforcement together.
Common examples include fibreglass and steel reinforced concrete.
The properties of composites depend on the reinforcement and matrix used so composites can be tailor engineered to meet specific needs.

Question 47

Physical properties of metals

Answer 47

Shiny, malleable and ductile so can be hammered into different shapes.
Can be mixed with other elements to form alloys, which have different properties to the elements they contain.
Corrosion resistant metals can be produced which last longer than other metals.
Good conductors of heat and electricity.

Question 48

Use of glass and metals

Answer 48

Glass and steel are extremely useful building materials.
Apart from its transparency, the hardness and the high compressive strength of glass makes it an ideal material for making walls and windows.
Metals are used extensively in electrical cabling and in electronics due to their ability to conduct electricity.
Copper is the most frequently used as it is a good conductor and is very malleable and easy to thread into cables.
Aluminium is a very strong metal but is also very light.
This makes it ideal for use in the construction of airplanes as it has a high strength-to-weight ratio.

Question 49

Use of composites

Answer 49

Steel reinforced concrete has immense tensile and compressive strength allowing it to be used as columns and supporting structures in construction.
Carbon fibres composites are extremely strong and low weight, hence they are used in aviation, aeronautics and for making professional racing bicycles.

Question 50

Use of polymers

Answer 50

As they are poor conductors of heat and electricity, this makes polymers good thermal and electrical insulators.
These properties are extremely useful for insulating electrical wiring as they prevent electric shocks and overheating.
The low melting points and flexibility of polymers enable them to be moulded easily into an infinite variety of shapes.

Question 51

Flame tests

Answer 51

Metal ions produce a colour if heated strongly in a flame.
Ions from different metals produce different colours.
The flame test is thus used to identify metal ions by the colour of the flame they produce.

Question 52

Flame test for Lithium, sodium, potassium, calcium and copper

Answer 52

lithium ion, Li+ (red)
sodium ion, Na+ (yellow)
potassium ion, K+ (lilac)
calcium ion, Ca2+ (orange-red)
copper ion, Cu2+ (blue-green)

Question 53

Test for cations

Answer 53

Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide.
If only a small amount of NaOH is used then normally the metal hydroxide precipitates.
In excess NaOH some of the precipitates may dissolve.
For this reason just a few drops of NaOH are added at first and very slowly
If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate which dissolves as quickly as it forms if excess solution is added.
A small amount is thus added, very gradually and any colour changes or precipitates formed are noted.
Most transition metals produce hydroxides with distinctive colours.

Question 54

Flame test procedure

Answer 54

Dip the loop of an unreactive metal wire such as nichrome or platinum in dilute acid, and then hold it in the blue flame of a Bunsen burner until there is no colour change.
This cleans the wire loop and avoids contamination.
This is an important step as the test will only work if there is just one type of ion present.
Two or more ions means the colours will mix, making identification erroneous
Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame.
Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour.

Question 55

Test for aluminium, calcium, copper, iron

Answer 55

Aluminium - White precipitate, dissolves in excess NaOH to form a colourless solution.
Calcium - White precipitate, insoluble so remains in excess NaOH.
Copper (Il) - Light blue precipitate, insoluble in excess NaOH
Iron (Il) - Green precipitate, insoluble in excess NaOH
Iron (III) - Red-brown precipitate, insoluble in excess NaOH

Question 56

Test for ammonium

Answer 56

Ammonium ions are also tested with sodium hydroxide, but not by a precipitation reaction.
To test for the ammonium ion, gentle heating is required after adding the NaOH solution.
If the ammonium ion is present, ammonia gas is produced which can be tested with damp red litmus paper turning blue.

Question 57

Test for carbonate

Answer 57

Add dilute acid and test the gas released.
Effervescence should be seen and the gas produced is CO2 which forms a white precipitate of calcium carbonate when bubbled through limewater.

Question 58

Test for sulphate

Answer 58

Acidify with dilute hydrochloric acid and add aqueous barium chloride.
A white precipitate of barium sulphate is formed.

Question 59

Test for halide

Answer 59

Acidify with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution (AgNO3).
This forms a silver halide precipitate.
Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion.
Silver chloride is white, silver bromide is cream and silver iodide is yellow.

Question 60

Advantages of instrumental technology

Answer 60

Advancements in technology and computing have allowed for the development of instruments designed to analyse chemical substances.
Methods of analysis include X-ray, Infra-Red and Mass Spectroscopy, Gas Chromatography and Flame Photometry.
These analytical techniques require modern day instruments which are a vital part of chemistry laboratories.
The advantage of using these instruments over more traditional methods include:
They provide greater accuracy.
They are faster and easier to use.
They are automated and can perform multiple simultaneous sampling and testing.
Modern instruments are very sensitive and can work with very small sample sizes.

Question 61

Flame photometry

Answer 61

This technique is used to analyse metal ions in solution.
When substances are heated they often emit energy in the form of light
This is due to electrons falling back to their original energy levels after becoming energised which causes them to jump up one or more energy levels.
Flame emission spectroscopy works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted.
The output is an emission spectrum in which different elements produce lines in different parts of the spectrum.
The emission spectrum consists of brightly coloured thin lines on a dark background and each element ion produces a unique spectrum.
Flame emission spectroscopy also works for mixtures of ions.
This is a major advantage over flame testing which can only analyze one ion at a time.
The intensity of the light produced is proportional to the number of ions vaporised, so the technique can be used to determine the concentration of metal ions in a solution by reference to a standard solution of known concentration.

Question 62

Reference data

Answer 62

Ions in unknown samples can be identified by comparing the sample spectrum to reference spectra.
This is particularly useful if the sample contains a number of different ions.