PAPER 2 CHEMISTRY

Question 1

Polystyrene can be used for screwdriver handles and car battery cases, whereas polyethene can be used in cling film and milk bottles.

Describe and explain the uses of polystyrene and polyethene in terms of their structure and properties(4)

Answer 1

-polystyrene consists of long carbon chains with benzene rings attached along the chain
-these large benzene rings create a much stiffer, more rigid structure
-making it more suitable for hand wearing materials. Polyethene consist of long carbon chains with no side groups
-these chains are more flexible
and polyethene is better suited for more bendable materials.

Question 2

define addition polymerisation(1)

Answer 2

addition polymerisation is the process of unsaturated monomers joining together to produce long saturated chains with no double bond

Question 3

explain how poly(chloroethene), PVC, can be modified using plasticisers. Give one use of PVC that has been modified with plasticisers. (4)

Answer 3

• plasticisers are small molecules that force polymer chains apart
  -so the intermolecular forces between chains are weakened
  -chains can then slide past one another more easily, making the polymer more flexible
  -some uses of plasticised PVC include electrical cable insulation
  flooring tiles and clothes

Question 4

suggest how these tripeptides could be broken down to form the original amino acids. Include any reagents and conditions in your answer (2)

Answer 4

• the tripeptides could be hydrolysed

- by heating with concentrated HCl (aq0

Question 5

state a technique that could be used to separate amino acids (1)

Answer 5

-chromatography

Question 6

describe the structure of DNA (6)

Answer 6

1-a single strand of DNA (deoxyribonucleic acid) is a polymer of nucleotides
2-each nucleotide is made up of a phosphate ion bonded to 2-deoxyriboose
3-which is in turn bonded to 1 of the 4 bases: A,C,T,G (full names)
4-the nucleotides are liked by covalent bonds
5-between the phosphate group of one nucleotide and the 2-deoxyribose of another nucleotide. This results in a sugar-phosphate polymer chain. DNA exists as 2 complementary strands joined by H bonds
6- and arranged in the form of a double helix

Question 7

Describe the key features of enzymes and how they control reaction in living things (5)

Answer 7

1-enzymes are protein- based catalysts
2-an enzyme increases the rate of reaction in living organisms by providing an alternative reaction route with a lower activation energy
3-enzymes are not used up in the process
4-enzymes are stereospecific
5-this is because a reactant (substrate) must be the correct shape to bind with the active site in order to react
-this is often referred to as a lock and key mechanism
6-enzymes are specific as they optimised for one particular reaction

Question 8

describe evidence that prove benzene doesn’t have the cyclohexatriene structure (3)

Answer 8

• cyclohexatriene has alternating single and double bonds have different lengths. However, all bonds in benzene are the same length
• the experimentally observed enthalpy change for the hydrogenation of benzene is lower than the calculated enthalpy change for the theoretical hydrogenation of cyclohexatriene
• benzene is less reactive than cyclohexatriene, e.g. cyclohexatriene would be expected to decolourise benzene water however benzene does not

Question 9

describe the structure and bonding of benzene (5)

Answer 9

• C6H6
• with each carbon covalently bonded to 2 other carbon atoms and 1 hydrogen atom
• the 4th valence electron of each carbon atom is in a p-orbital, not held within a bond. The vertical p-orbitals (perpendicular to the plane of the ring) overlap, meaning the electrons they contain are delocalised
• and form regions of electron density above and below the ring. Benzene has a planar hexagonal structure
• with all bonds the same length (in between a single and double carbon- carbon bond)

Question 10

why is benzene more stable than cyclohexatriene?

Answer 10

due to the delocalisation of p electrons

Question 11

explain why benzene carries out substitution reaction in preference to addition reactions. (2)

Answer 11

• if benzene were to undergo addition reaction, it would lose delocalisation and hence form a less stable product
• in substitution reactions, benzene retains its delocalisation

Question 12

explain why benzene is insoluble in water (2)

Answer 12

• the benzene ring is non- polar, hence only forms van der waals forces with water molecules
• these interactions re not strong enough for benzene to dissolve