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Chemistry 2.11
AS edexcel chemistry mechanisms
| Question | Answer |
|---|---|
| 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 |
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