Mechanisms Chem Unit 2 Pt 7
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What is an addition reaction?
Joining two or more molecules together to form a larger molecule. (eg. adding species across double-bond)
What is a hydrolysis reaction?
Splitting a molecule into two new molecules by adding H+ and OH- derived from water. (Often requires a catalyst) (haloalkanes can be hydrolysed to form an alkali)
What is polymerisation reaction?
Reaction involving two or more monomer molecules with double bond, which breaks and the monomers join to form a polymer - a giant molecule.
What is substitution reaction?
When one species is replaced by another in a molecule.
What is a oxidation reaction?
Any reaction in which an atom loses electrons.
What is a reduction reaction?
Any reaction in which an atom gains electrons.
What is an elimination reaction?
- When a small group of atoms breaks away from a larger molecule without being replaced.
- Involves removing atoms from adjacent carbon atoms and the formation of a double bond between them.
- (eg. you can remove halogen and hydrogen from halogenoalkane to form alkene).
What is a redox reaction?
Any reaction where electrons are transferred between two species.
You need to be able to write down mechanisms for a few reactions including those from unit 1 such as
- Free radical substitution of chlorine in alkanes to make chloroalkanes
- elctrophilic addition of bromine and hydrogen bromide to alkenes, to make bromoalkanes.
- LOOK AT NOTES, REVISION BOOK, and HAND-MADE FLASHCARDS!
Give a definition of electrophile.
- A species that attacks an area of high electron density (eg. double bond). It will form a covalent bond with a pair of electrons from the electron-rich site. They are election pair acceptors.
- They can be positive ions (eg. H+) or slightly positive atoms (eg. H in HBr) or neutral molecule once polarised (eg. Br-Br)
Give a definition of nucleophile.
- A species with a lone pair of electrons that is used to form a covalent bond. They attack a slightly positive atom on a molecule. They are election pair donors.
- Can be negative ions (eg. OH- or Cl-) or neutral species containing O or N (eg. water)
Give a definition of free radical.
An atom or group of atoms with an unpaired electron that is used to form a covalent bond. Formed from homolytic bond fission, they are very reactive.
What is heterolytic fission?
- Covalent bond breaks "unevenly". Both electrons from the shared electron pair move to one atom.
- Forms 2 species
- An electrophile (positively charged cation)
- A nucleophile (negatively charged anion)
What is homolytic fission?
- Cavalent bond breaking evenly. One electron from shared pair move to each atom.
- Forms - 2 electrically uncharged free radicals (atom with unpaired electron)
- Because of unpaired electron, free radicals are very reactive.
How does O2 and O3 absorb UV radiation?
- O2 absorbs UV radiation to form 2 oxygen free radicals. Oxygen free radicals then combine with O2 molecules to form O3.
- UV can also reverse formation of ozone by; O3 + UV --- O2 + Ofree-radical.
- An equilibrium is set up between oxygen molecules and oxygen free radicals, with ozone. This continuous making and destroying of ozone absorbs UV which is dangerous high energy radiation.
Why did the CFC's destroy the ozone layer?
- Cl free radicals were formed when CFC's were broken down by UV light.
- This Cl free radical act as a catalyst and react with ozone to form a free radical ClO and O2. This then reacted with more ozone to form O2 and Cl free radicals.
- Overall reaction: 2O3 ---- 3O2 (where Cl free radicals are the catalyst - as it is regenerated, it can attack many ozone molecules)
What are the benefits of CFC's? But why was it banned?
- Unreactive, non-flammable, non-toxic. Used in fire extinguishers, propellants in aerosols, and coolant gas in fridges.
- But, in Montreal Protocol, international treaty was signed to phase out use of CFC's because they were destroying the ozone layer.
Why do nitrogen oxides produced by aircraft and car engines, break down ozone too?
- Nitric oxide free radicals (NO-free radical) break down ozone into O2. They react in the same way as chlorine radicals do. They act as catalysts.
- Come from nitrogen oxides, which are produced by aircraft engines etc.
- NOo + O3 --- O2 + NO2o
- NO2o + O3 --- 2O2 + NOo
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