CHEMISTRY UNIT 2

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  • Created by: charlie
  • Created on: 21-02-14 11:25

hydrocarbons

TYPE OF COMPOUND              FORMULA                  PREFIX                       SUFFIX 

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hydrocarbons (2)

NO. OF CARBONS       STEM 

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structural isomerism

CHAIN ISOMERS 

  • carbon skeleton arranged differently (chain/branch)
  • similar chemical properties + diff. physical properties 

POSITIONAL ISOMERS 

  • skeleton + functional group the same 
  • group attached to diff. carbon atoms 
  • diff. physical + possibly chemical properties 

FUNCTIONAL GROUP ISOMERS

  • same atoms arranged in different functional groups
  • very diff. chemical + physical properties 
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stereoisomerism

  • same structural formula but diff. arrangment in space 
  • alkenes because lack of rotaion around C=C 
  • E-isomer (trans-isomer) = lies across the double bond 
  • Z- isomer (cis-isomer) = lies both same side of double bond 
  • CANT USE IF >2 GROUPS (othe than hydrogen) AROUND DOUBLE BOND 
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empirical formula

  • 1) work out mass in 100g 
  • 2) divide by R.A.M = MOLES 
  • 3) gives ratios of atoms 

ALKENES = CnH2n

ALKANES = CnH(2n+2)

ALCOHOLS = CnH(2n+1)OH 

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structural + skeletal formula

e.g.  pent-2-ene 

DISPLAYED            

STRUCTURAL 

SKELETAL 

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hydrocarbons for crude oil

'compound containing elements of H + C only' 

CRUDE OIL - 'fossil fuel (natural decay of P + A) made of mixture of HC's'

FRACTIONAL DISTILLATION - bitumen / fuel oil / diesel oil / kerosene / gasoline / petroleum gas

inc. length = inc. BP as more VDW forces due to more points of contact 

branched = dec. BP as less VDW forces due to less contact (cannot get as close)

SHORT CHAIN HC'S UNDERGO COMPLETE COMBUSTION (CO2 + H2O)

LONGER HC'S UNDERGO INCOMPLETE COMBUSTION (CO + H2O)

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cracking

surplus long chain HC's (short chain in high demand)

catalytic cracking uses ZEOLITE CATALYST at 450d.c 

ISOMERISATION 

unbranched straight chain ---> branched 

REFORMING 

unbranched straight chains ---> cyclic HC's 

IMPROVING FUELS 

  • research octane number - how well fuel burns (100 is best)
  • branched + cyclic alkanes - more efficient combustion
  • hydrogen produced in REFORMING - ammonia (haber) + margarine production 
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fuels for the future

BIOFUEL - recently living material 

AGRICULTURAL crops grown for energy - sugar cane/****

ETHANOL - fermenting sugar burns efficiently with O2 --> CO2 + H2O 

ETHANOL BLENDS + petroleum - burns efficiently 

BIOETHANOL - plants 

BIODIESEL - rapseed blended with normal diesel 

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substitution reactions of alkanes

MECHANISM REACTION : 

  • C6H12 + Br2 --uv light--> C6H11Br + HBr 

HETEROLYTIC FISSION (different)

  • Br2 --uv light--> Br+ + Br - 

HOMOLYTIC FISSION (same)

  • Br2 --uv light--> Br . + Br .
  • unpaired e- wants to pair = VERY REACTIVE
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substitution reactions of alkanes (2)

1) INITIATION 

Br - Br ---> Br . + Br .

2) PROPAGATION 

Br . + C6H12 ---> C6H11 . + HBr 

C6H11 . + Br2 ---> C6H11Br + Br . 

3) TERMINATION 

C6H11 . + Br . ---> C6H11Br

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electophilic addition of alkenes

DIAGRAM 

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measuring enthalpy changes

energy (joules) = m c /\ T 

  • m = mass in grams of stuff heated (water)
  • c = S.H.C of susbtance (water 
  • /\ T = temp. change 

energy (joules) then (divide) by 1000 = KJ

then (divide) by moles = KJ/mol

if exothermic insert (-ve) sign  

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standard enthalpy change

DIAGRAM 

PRESSURE = 1 atm 

TEMP = 25 d.c or (298K) 

solutions CONCEN. = 1mol/dm3 

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Hess' law

USED IF 

  • reaction takes place by >1 route 
  • initial + final conditions are the same 
  • gives total enthalpy change for each route 

DIAGRAM

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Hess' law - enthalpy changes of formation

DIAGRAM 

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Hess' law -enthalpy changes of combustion

DIAGRAM 

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enthalpy

  • measure of heat content of a substance at constant pressure
  • cannot measure exact enthalpy of substance 

enthalpy change ( /_\ H) = enthalpy of products - enthalpy of  reactants 

EXOTHERMIC DIAGRAM 

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enthalpy (2)

ENDOTHERMIC DIAGRAM 

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bond energies

/_\ H = (sum of) bond enthalpies of bonds broken - (sum of) bond enthalpies of bonds formed 

USE HESS' LAW 

DIAGRAM 

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