Chemistry unit 5

chem 5 aqa

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  • Created by: rebecca
  • Created on: 31-05-12 17:37

Thermodynamics - definitions

∆Hf = 1 mole of compound formed from its elements under standard condtions, all reactants and products are in their standard states

∆Hat = 1 mole of gaseous atoms formed from the element in standard states

∆Hie = 1 mole of gaseous atoms converted to 1 mole gaseous ions, each with a single positive charge

∆Hea = 1mole of gaseous atoms converted to 1 mole of gaseous ions, each with a single negative charge

∆Hle = 1 mole of solid ionic compound is formed from its gaseous ions

∆Hhyd = water molecules surround 1 mole of gaseous ions

∆Hsol = 1 mole of solution dissolves in sufficient solvent to form a solution

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Born-Haber cycles

  • larger ions have smaller LE
  • Mg--> Mg2+ ... ionisation = 1st+2nd  ... 2nd is larger, more difficult to loose the 2nd e- from a positive ion
  • Cl2 <-- all quantities regaurding Cl are doubled

Hess' Law

  • enthalpy change of a reaction is the same whatever route is taken from reactants to products
  • if more energy given out --∆H is NEGATIVE
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Free-energy change and entropy change

Entropy

  • ∆H not sufficient enough to explain spontaneous change
  • entropy = randomness of a system, where product is more disordered ∆S is positive
  • JK-1mol-1
  • Increases with temperature
  • ∆S=products-reactants

∆G

  • two factors govern the feasibility of a rx - entropy change and enthalpy change
  • If ∆G is negative - rx IS feasible
  • if ∆G is positive - rx NOT feasible
  • ∆G= ∆H-T∆S
  • if ∆G=0 - the point at which a reaction is just feasible & how temp can be calculated (rx not feasible below this temp)
  • T = ∆H/∆S = kJK-1mol-1 (x1000)
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Periodicity

trends with water:

  • Na - 2Na + 2H2O --> 2NaOH + H2   -vigorous in cold water, molten ball forms, pH approx 12-14
  • Mg - slow with cold water ->>  Mg + 2H2O --> Mg(OH)2 + H2
  • with steam ->> Mg + H2O(g) --> MgO + H2

trends with oxygen

  • Na2O - vigorous rx - yellow flame - 2Na + 1/2O2 --> Na2O
  • MgO - vigorous rx - brilliant white flame - Mg + 1/2O2 --> MgO
  • Al2O3 - slow - 2Al + 1 1/2O2 --> Al2O3
  • SiO2 - slow - Si + O2 --> SiO2
  • P4O10 - spontaneously combusts - brilliant white flame - P4+5O2 -->P4O10
  • SO2 - burns steadily - blue flame - S + O2 --> SO2
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Periodicity - bonding & structure

Na2O, MgO, Al2O3 

  • metal oxides
  • MgO high mpt - forms 2+ ion
  • Giant ionic lattices - strong forces of attraction
  • Al2O3 - 3+ ion distort O's e- cloud - bonds partially covalent

SiO2

  • higher than other non-metal oxides
  • Giant macromolecular structure

P4O10, SO2

  • simple molecular structures
  • weak IMF's - VDW/ dipole-dipole
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periodicity - rx of oxides with water

Oxide            Bonding            ions present        approx pH

Na2O           Ionic                  Na+ OH-               13-14

MgO            Ionic                   Mg2+ OH-               10

Al2O3          covalent/ionic    INSOLUBLE              7

SiO2           covalent             INSOLUBLE              7

P4O10       covalent             H+  H2PO4-             0-1

SO2           covalent             H+  HSO3-               2-3

SO3           covalent            H+  HSO4-                0-1

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Periodicity - rx of oxides with acid/base

acid

  • Na2O + HCl --> 2NaCl + H2O
  • MgO + H2SO4 --> MgSO4 + H2O

Base

  • SiO2 + 2NaOH --> Na2SiO3 + 2H2O
  • P4O10 + 12NaOH --> 4Na3PO4 + 6H2O
  • SO2 + 2NaOH --> Na2SO3 + H2O
  • SO3 + 2NaOH --> Na2SO4 + H2O

Amphoteric oxides

  • Al2O3 + 3H2SO4 --> Al2(SO4)3 + 3H2O
  • Al2O3 + 2NaOH + 3H2O --> 2NaAl(OH)4
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redox equilibria

redox equations:

  • oxygen: -2 (-1 in peroxides, +2 in compounds with F)
  • hydrogen: +1 (-1 in metal hydrides)
  • fluorine: -1
  • chlorine: -1 (in compounds with F&O has positive values)
  • G1: +1
  • G2: +2
  • Al: +3

Oxidation number = e- lost or gained

Electrode potentials

  • standard hydrogen electrode, emf= 0 (any other cell connected has whatever emf is calculated
  • conditions: 298K, 100kPa, 1.00 dm-3
  • change in conc of ions or temperature will change electrical potential 
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redox: electrochemical series/cells

electrochemical series

  • cell representation: | = phase boundary, || = salt bridge
  • emf = E(right) - E(left)  more positive value is on the right
  • standard electrical potentials can be listed as an electrochemical series - most negative at top

Electrochemical cells: commercial source of electrical energy

  • non rechargeable: irreversible. EMF = E(most positive) - E(most negative)
  • rechargeable: reversible. EMF = E(most positive) - E(most negative)

Fuel cells : commercial

  • hydrogen is highly flammable & crude oil is non renewable
  • 2H+ + 2e- <---> H2      E=0V
  • 4H+ + O2 + 4e- <---> 2H2O       E=1.2V
  • overall: H2 --> 2H+ + 2e-       4H+ + O2 + 4e- --> 2H2O
  • 2H2 + O2 --> 2H2O
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Thermodynamics

properties

  • incomplete D-sublevel
  • form: complexes, coloured ions
  • have: variable oxidation states, catalytic activity

complex formation

  • ligand - an atom/ion/molecule that forms a coordinate bond with a transition metal using a lone pair of electrons. can be unidentate, bidentate or multi dentate 
  • a complex is a metal ion surrounded by ligands
  • coordination number - the number of coordinate bonds to a metal ion
  • Haem - iron(II) complex with multi dentate ligand. O2 forms weak bonds with it & is easily released. CO forms stronger bonds & isn't released
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transition metals

shapes of complex ions

  • octahedral with small ligands: coordination number = 6. e.g. H2O & NH3
  • tetrahedral with larger ligands: coordination number = 4 e.g. Cl-
  • square planar: coordination number = 4 e.g. cisplatin (anti cancer drug, forms coordinate bond with now of the DNA strands to stop it unwinding)
  • Ag+ commonly forms a linear complex - tollens reagent [Ag(NH3)2]+ 

formation of coloured ions

  • colour changes arise from changes in oxidation state & coordination number
  • absorption of visible light is used in spectroscopy to determine the concentration of coloured ions
  • electronic transitions from ground to excited ∆E=hv
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transition metals : catalysis

Heterogeneous

  • different phase
  • use a support medium to max SA & min costs
  • catalysts can be poisoned by impurities

V2O5 (contact process)

  • 2SO2 + O2 <---> 2SO3
  • SO2 + V2O5 --> So3 + V2O4
  • 2V2O4 + O2 --> 2V2O5

Cr2O3 (methanol production)

  • CH4 + H2O --> CO + 3H2
  • CO + 2H2 --> CH3OH

Fe (Haber process)

  • N2 + 3H2 <---> 2NH3
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transition metals - catalysis

Homogeneous

  • same phase
  • intermediate species

(S2O8)2- catalysed by Fe2+

  • (S2O8)2- + 2I- --> 2(SO4)2- + I2
  • S2O8 2- + 2Fe2+ --> 2SO4 2- + 2Fe3+
  • 2Fe3+ + 2I- --> 2Fe2+ + I2
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transition metals - variable oxidation states

2Co2+ + H2O2 --> 2Co3+ + 2OH-

MnO4- + 5Fe2+ + 8H+ --> 5Fe3+ + Mn2+ + 4H2O

Cr2O7 2- + 14H+ + 6Fe2+ --> 2Cr3+ + 7H2O + 6Fe3+

2Cr3+ + 10OH- + 3H2O2 --> 2CrO42- + 8H2O

[Co(H2O)6]2+ + NH3 --> [Co(H2O)4(OH)2] + exs NH3 --> [Co(NH3)6]2+ --stand in air--> [Co[NH3)6]3+

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