REDOX

REDOX

1. oxidation

   • gain of oxygen atoms
     • 2 Mg (s) + O2 (g) -> 2 MgO (s)
       • Mg (reactant/starting material) is oxidised bc there is a gain of oxygen atom from Mg to form MgO
   • loss of hydrogen atoms
     • H2S (g) + Cl2 (g) -> S (s) + 2HCl (g)
       • H2S is oxidised bc loss of hydrogen from H2S to S
   • loss of electrons
     • 2Mg (s) + O2 (g) -> 2MgO (s)
       • Mg loss of 2 electrons from Mg to Mg ^2+
   • increase oxidation state/number
     • Cu + 4HNO3 -> Cu(No3)2 + 2H2O + 2NO2
       • Cu increase from O in Cu to +2 in Cu^2+

2. reduction

   • loss of oxygen atoms
     • 2Mg (s) + O2 (g) -> 2MgO (s)
       • O2 is reduced bc there is a loss of oxygen atom from O2 to MgO
   • gain of hydrogen atoms
     • H2S (g) + Cl2 (g) -> S (s) + HCl (g)
       • Cl2 is reduced bc of addition of hydrogen from Cl2 to HCL
   • gain in electrons
     • 2Mg (s) + O2 (g) -> 2MgO (s)
       • O2 gain of 2 electrons from O2 to O^2-
   • decrease in oxidation state
     • Cu + 4HNO3 -> Cu(NO3)2 + 2H2O + 2NO2
       • N decrease from + in HNO3 to +4 in NO2

3. oxidation number/state

   • charge an atom of an element would have if it existed as an ion in compound (even if it is covalently bonded)
   • oxidation number in an element is zero
     • Cu, Cl2
   • simple ions is same as charge on ion
     • K^+ = +1, S^2- = -2
   • hydrogen
     • with non metals is +1
       • Nh3
     • with metals is -1
       • NaH
   • oxygen
     • as an oxide is -2
       • Na2O
     • an a peroxide is -1
       • H2O2
   • atoms present in a formula of a compound add up to zero
     • CaCo3
       • Ca = +2
       • C = x
       • O3 = 3(-2) = -6
       • Ca + C + O3 = 0
         • +2 + x + (-6) = 0
           • x = -2 + 6 = +4
   • fluorine is -1 in ALL compounds
   • sum in polyatomic ion is equal to charge of ion
     • nitrate ion, NO3^-
       • (+5) + 3(-2) = -1

4. non redox reactions

   • precipitation
     • Ag^+ (aq) + Cl^- (aq) -> AgCl (s)
       • oxidation number of each element is unchanged
   • neutralisation
     • NaOH (aq) + HCl (aq) -> NaCl (aq) + H2O (l)
       • oxidation number of each element is unchanged

5. oxidising agent

   • causes oxidation in other substance, substance itself is reduced
   • potassium manganate(VII) , KMnO4
     • acidified aqueous ^^ turned from purple to colourless
       • brown ppt formed is most likely due to insufficient amt of acid used to acidify KMnO4 !!! repeat test with more acid added
       • Mn(VII) [+7] -> purple, Mn(II) -> colourless
   • potassium dichromate(VI) , K2Cr2O7
     • acidified aqueous ^^ turned from orange to green
       • Cr(VI) [+6] -> orange, Cr(III) [+3] -> green
     • prepare by adding equal vol of H2SO4 to aqueous potassium dichromate(VI)
     • add to portion of unknown substance
   • iron chlroride , FeCl3
     • aqueous ^^ turned from yellow/yellowish brown to pale green

6. reducing agent

   • causes reduction in other substance, substance itself is oxidised
   • used to test for oxidising agents
     • potassium iodide (KI)
       • acidified aqueous ^^ turns from colourless to reddish brown solution
         • may observe black solid at bottom of test tube if amt of iodine present is larger
         • prepared by adding equal vol of dilute sulfuric acid to aqueous KI
         • add to a portion of unknown substance
     • concentrated HCl
       • pale yellow and pungent gas turned moist blue litmus paper red then white (bleaching)
         • gas turned moist starch iodide paper blue-black and gas in Cl2
     • iron sulfate (FeSO4)
       • acidified aqueous ^^ changes from pale green to yellow/ yellowish-brown
         • Fe^2+ -> pale green, Fe^3+ -> yellow/yellowish brown