Chemistry 2

• 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

• 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

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

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

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)

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

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

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

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 

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

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

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

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

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

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

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 

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

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

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