Chemistry unit 5

∆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

• 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

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)

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

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

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

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

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 

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

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

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

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

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

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+