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Chemistry 2.11

AS edexcel chemistry mechanisms

QuestionAnswer
What are the 2 types of bond fission? Homolytic - a covalent bond splits evenly so one electron from the shared pair of electrons goes to each atom, forming free radicals. Heterolytic - both electrons from shared pair go to 1 atom, forming a cation and an anion
What is water an example of? A nucleophile - an electron-rich species that will donate a pair of electrons to form a new covalent bond, so is attracted to groups that accept electron pairs e.g. anion
What is a cation an example of? An electrophile - an electron-deficient species that will accept a pair of electrons from an electron-rich group to form a new covalent bond
What are free radicals? Atoms or molecules with an unpaired electron that can be sued to form a new covalent bond, so are highly reactive
What is a substitution reaction? Where 1 substituent is replaced by another
What is an oxidation reaction? The gain of oxygen (combustion), the loss of hydrogen, or the loss of electrons (OIL)
What is an elimination reaction? Where 2 substituents are removed from adjacent carbon atoms in a molecule of an organic substance
What is a reduction reaction? The loss of oxygen, the gain of hydrogen, the gain of electrons (RIG)
What is polymerisation? Where thousands of monomer molecules are joined together to form a long, polymeric chain
What is addition? Where 2 reactants react to form a single product
What is nucleophilic substitution? Where a halogen atom in a halogenoalkane is replaced by a nucleophile (e.g. OH-, CN-, NH2-, NH3, H2O). Involves heterolytic fission only. There must be polar bonds present
How do you name a reaction? S/A (substitution or addition) N/E (nucleophilic or electrophilic) then the number of species involved in step 1
What is an SN1 reaction? Nucleophilic substitution for tertiary halogenoalkanes. Carbon-halogen bond breaks (rate-determining step) forming tertiary carbocation and anion, nucleophile forms a dative covalent bond with positively charged carbon atom in carbocation
What is an SN2 reaction? Nucleophilic substitution for primary halogenoalkanes. Nucleophile starts to form dative covalent bond with carbon atom, electron pair in carbon-halogen bond are pushed closer to halogen atom in transition stage, halide ion and new molecule
What can't tertiary halogenoalkanes undero SN2? Due to steric hindrance caused by bulky carbon chains - nucleophile can't attack carbon atom
Why is it important to classify reagents? To make predictions about the reaction, minimise risk, safety precautions, know what to expect
How is ozone formed? 1. O3 -> O2 + O* (photolysis, broken down by UV radiation 200-300nm). 2. O2 -> 2O* (180-240nm). 3. O2 + O* -> O3 (exothermic reaction)
What is the function of ozone? Absorbs UV rays of wavelength 200-300nm, protects earth from harmful UV rays, converts light into heat energy
What are CFCs + their effect? CFCs are released from leaking refrigerators, inert, diffuse up to stratosphere, UV causes heterolytic fission of C-Cl bond forming Cl* free radical, a catalyst, chain reaction until it reacts with another Cl free radical or H atom
What is the mechanism of Cl free radical ozone depletion? Cl* + O3 -> ClO* + O2. O3 -> O2 + O*. O* + ClO* -> Cl* + O2. Overall: 2O3 -> 3O2
What are nitrous oxides + mechanism ? NOx, catalysts, free radicals, formed from lightning and combustion of fossil fuels in cars and jets (fly close to stratosphere). NO* + O3 -> O2 + NO2. O3 -> O2 + O*. O* + NO2 -> NO* + O2. Overall: 2O3 -> 3O2
Created by: 11043
 

 



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