Chemistry 2

unit 2 AS aqa

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  • Created by: rebecca
  • Created on: 09-05-12 14:55

Collision Theory

  • Reactions only occur when collisions take place between particles with sufficient energy
  • Most reactions do not lead to a reaction

Activation Energy - The minimum amount of energy required to start a reaction

  • Exothermic - reactants have more energy than products
  • Endothermic - products have more energy than reactants

Maxwell-Boltzman distribution

  • Increasing temperature shifts the curve to the right, more particles have EA

Temperature and rate of reaction

  • Increasing temperature increases rate, energy of each particle increases- more collisions

Concentration

  • More particles in a given volume - more collisions
  • Rate of Rx slows as reactants are used up
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Catalysts

  • Provide a different Rx pathway with a lower EA
  • Not used up in a reaction

Heterogeneous

  • In a different state to the reactants

Homogeneous

  • In the same states as the reactants
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Equilibria

Dynamic equilibria - The concentrations of reactants and products remain constant. Both forward and reverse reactions proceed at equal rates

  • Many chemical reactions are reversible

Le Chatelier's principle

  • If a system at equilibrum is disturbed, the equilibrium moves in the direction that most reduces the disruption

For exothermic reactions:

  • Increase temp, eq <-, yield decreases, system maintains lower temp
  • Decrease temp, eq ->, yield increases, system increases to original temp
  • Increase pressure, eq ->, yield increases, system tries to decrease pressure
  • Decrease pressure, eq <-, yield decreases, system tries to increase pressure
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Equilibria in industry

Methanol

  • 2H2 + CO <--> CH3OH  ^H=-90KJmol-1

Hydration of ethene

  • C2H5 +H20 <--> C2H5OH   ^H=-46KJmol-1
  • Pressure = 60-70 atm
  • Temp = 300C
  • Catalyst = Phosphoric acid

Liquids as fuels

  • Carbon neutral, from renewable resources, fewer pollutants, greener than petrol.
  • Methanol & Ethanol can be used as fuels on their own or added to petrol

Compromise temperature - between maximum yield and a faster reaction

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Redox Reactions

Oxidation - Loss of electrons

Reduction - Gain of electrons

Oxidising agent - Electron acceptor

Reducing agent - Electron donor

Equations 

  • Charges on each side must balance
  • numbers of atoms of each element must balance on each side

Oxidation states - the most electronegative has the negative oxidation state

  • H= +1 (Except in metal hydrides, -1)
  • Group 1 = +1
  • Group 2 = +2
  • Aluminium = +3
  • O = -2 (Except in peroxides and with F, -1)
  • Fluorine = -1
  • Chlorine = -1 (except with O and F, positive)
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Group 7 - the halogens

trends in physical properties:

  • electronegativity - decreases down the group - increased nuclear shielding & increased atomic radius
  • boiling point - increases down group - more e- - vdw stronger

trends in oxidising abilities

  • decreases down group
  • halogens are reduced because they are oxidising agents

trends in reducing abilities

  • increases down group
  • larger ions = more shielding = less nuclear charge on outer electrons -- electrons lost more readily
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halogens cont

reactions with conc H2SO4

  • NaCl (s) + H2SO4 --> NaHSO4 (s) + HCl
  • NaF (s) + H2SO4 --> NaHSO4 (s) + HF (this will etch glass)
  • NaBr +H2SO4 --> NaHSO4 + HBr 
  • 2H+ + 2Br- + H2SO4 --> SO2 +H2O +Br2

Identification of halide ions by silver nitrate

  • Dilute HNO3 is added to the halide solution to get rid of any soluble carbonate/OH- impurities
  • add a few drops of silver nitrate & a precipitate forms

Halide     Silver Fluoride  Silver Chloride  Silver Bromide  Silver Iodide

Colour      no ppt             White ppt        Cream ppt          Pale Yellow ppt

Further test   /            dissolves dilute      dissolves conc       insoluble

with NH3

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Group 7 - uses of chlorine

Cl2 + H2O  <---> HClO + HCl         Disproportionation 

0                            +1      -1

With NaOH

  • Cl2 + 2NaOH --> NaCl + H2O
  • NaCl is added to swimming pools instead of direct chlorination

In Sunlight

  • 2Cl2 + 2H2O --> 4HCl + O2
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Group 2

trends in chemical properties 

  • reactions with water >   M + 2H2O --> M(OH)2 + H2   (M is any G2 metal)
  • Solubilities of the hydroxides:
    •  
      •  
        • Mg(OH)2 Almost insoluble
        • Ca(OH)2 Sparingly soluble
        • Sr(OH)2 More soluble
        • Ba(OH)2 soluble & gives a strongly alkaline solution
  • Solubilities of the sulfates (test for sulfate ions = BaCl2)
    •  
      •  
        • MgSO4 Soluble
        • CaSO4 Less soluble
        • SrSO4  Sparingly soluble
        • BaSO4  Almost insoluble
  • Uses of Ca(OH)2 in agriculture - to treat acidic soils
  • Mg(OH)2 in medicine - indigestion treatment
  • Mg react slow with cold water, more readily with steam -> MgO + H2
  • BaSO4 in medicine - barium meals - shows lining of gut in X-rays 
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Group 2

trends in physical properties

  • atomic radius - increases down group
  • ionisation energy - all G2 loose 2e-, 1st & 2nd ionisation's decrease down group, more nuclear shielding = less nuclear charge on outer e-
  • melting point - high mpts = giant metallic structure, down group delocalised e- further from nucleus - mpts decrease from Ca (Mg lowest mpt - different lattice arrangement
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Extraction of metals - Principles

metals found as ores (oxides/sulfides)

Involves reduction, common reducing agents: Carbon monoxide, Carbon

Iron

  • CO is reducing agent
  • Fe2O3 +3CO --> 2Fe + 3CO2
  • C (s) + O2 (g) --> CO2 (g) ..... CO2 (s) + C (s) --> 2CO

Manganese Oxide reduced by Carbon

Copper

  • CuCO3 --> CuO + CO2
  • Heated with coke .... 2CuO + C --> 2Cu + CO2
  • with scrap iron ... Cu2+ + Fe --> Cu + Fe2+
  • Scrap iron is readily available and no CO2 is produced at this stage
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Haloalkanes

Free Radical Substitution 

  • CFCs end up in the atmosphere, decompose to give Cl2 <- decomposes ozone. (CFCs are in solvents and are now banned
  • Initiation by UV light --- Cl2 --> Cl• + Cl•
  • Propagation - 1) Cl• + CH4 --> HCL + •CH3 … 2) •CH3 + Cl2 --> CH3Cl +Cl•
  • Termination - Cl• + Cl• --> Cl2 …•CH3 + •CH3--> C2H5…Cl•+•CH3 -->CH3Cl

Nucleophilic Substitution

  • Haloalkanes contain polar C-X bonds
  • Nucleophiles: OH- CN- NH3
  • Nucleophile attracted to C, electrons to X. products: Haloalkane and X:-

Nucleophilic Elimination

  • OH- attracted to H, double bond forms. products: H2O, X:- and alkene
  • Substitution - OH- at room temp, dissolved in water
  • Elimination OH- at high temp, dissolved in ethanol
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Alkenes

Structure, bonding, reactivity

  • Unsaturated hydrocarbons
  • Planar double bond (centre of high e- density)
  • Exhibit E-Z stereoisomerism (E= opposite, Z= same)

Addition reactions

  • water and a H3PO4 catalyst to make alcohols
  • Bromine water = test for unsaturation 
  • Electrophilic addition: HBr H2SO4 Br2
  • H2C==CH2 + X2 --> H2CX--CH2X
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Polymerisation of Alkenes

Addition polymerisation

  • adding alkenes together
  • unreactive

Polyethene

Low Density

  • High branching - flexible (used for plastic bags & insulation)
  • High Pressure & High Temperature

High Density

  • Less chain branching - more rigid (used for washing up bowls, buckets & bottles)
  • Temp & pressure little above room conditions
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Polymerisation of alkenes

Polypropene

  • Thermo-plastic polymer, softens when heated so can be recycled & remoulded
  • e.g rope

Recycling

  • problems with plastic - non-biodegradeable
  • mechanical recycling - separate different types of plastics, wash & ground into small pellets, melt & remould
  • Feedstock recycling - heated to produce monomers
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Alcohols

CnH2n+1OH  suffix = ol

Classification & reactions

  • 1º = 1R group, OH at end of chain
  • 2º = 2R groups
  • 3º = 3R groups
  • Higher mpt & bpt compared to alkanes with same Mr
  • H bonding between molecules (shorter chain alcohols soluble in water)

Oxidation

  • 1º --> aldehydes --> carboxylic acids
  • 2º --> ketones
  • 3º --> dont oxidise easily
  • Oxidising agent = acidified potassium dichromate, orange --> green
  • Tollens reagent - aldehyde gives silver mirror 
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Alcohols - reactions

Eliminaton

  • alkenes formed from alcohols by acid catalysed elimination reaction
  • route to polymers without using monomers derived from oil

Ethanol production

From crude oil

  • cracking fractions produces ethene
  • ethene hydrated to form ethanol
  • CH2=CH2 + H2O --(phosphoric acid catalyst)--> C2H5OH

Fermentation

  • C6H12O6 --> 2C2H5OH + 2CO2
  • biofuel
  • at about 15% fermentation stops
  • Rate affected by temp 
  • air kept out, to prevent oxidation to ethanoic acid
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Carbon Neutral

  • No net annual carbon emissions
  • CO2 released when ethanol is burnt is balanced by the CO2 absorbed by the plant
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Analytical techniques

Mass spectrometry

  • high resolution mass spec used to determine the molecular mass of a compound 
  • uses an accurate mass of the compound

Infrared spectroscopy 

  • identify particular functional groups & identify impurities (use data sheet)
  • 'Fingerprinting' allows identification of a molecule (compare spectra)
  • certain groups absorb IR at characteristic frequencies 
  • CO2, water vapour & methane contribute to global warming - all good at absorbing energy - can emit IR in all directions - including back to Earth - too much heat energy is trapped
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