Redox Reactions

Question 1

Redox

Answer 1

Oxidation reactions always occur simultaneously with reduction reaction and as such, these reaction are termed redox reaction

Question 2

Transfer of electrons in redox

Answer 2

Oxidation involves a loss of electrons

Reduction involves a gain of electrons

Question 3

Change in oxidation number

Answer 3

Oxidation involves an increase in oxidation state/number

Reduction involves a decrease in oxidation state/number

Question 4

Oxidation number

Answer 4

• An atom in the elemental state is always zero
• Monoatomic ion is the charge on the ion
• Very electropositive/negative elements have fixed oxidation
  numbers

Question 5

Assigning oxidation number using structural formulae

Answer 5

• Pick a bond between atom of interest and another atom it is
  bonded to
• Imagine if it broke, the more electronegative atom will gain both
  electrons, thus its oxidation state -1 and vice versa
• Repeat for all bonds around atom of interest

Question 6

Reducing agents

Answer 6

Causes other reactant to reduce, itself oxidised

Question 7

Oxidising agent

Answer 7

Causes other to oxidise, itself reduced

Question 8

Disproportionation reaction agents

Answer 8

• Hydrogen peroxide (H₂O₂) and the nitrite ion (NO₂^-) can function as
  oxidising or reducing agents depending on if they react with a strong
  oxidising or reducing agent

Question 9

Balancing redox equations

Answer 9

1. Balance no. of atoms
2. Balance no. of Oxygen by adding H₂O
3. Balance the no. of Hydrogen by adding H⁺
4. Balance the charges by adding electrons
5. Sum up the two half equations
6. If in alkaline medium, add OH- to neutralise H⁺
7. Simplify

Question 10

Potassium Manganate(VII) vs Potassium Dichromate(VI)

Answer 10

• KMnO₄ is a stronger OA than
  K₂Cr₂O_y
• KMnO₄ has a more distinct colour change (pale pink to colourless) at the titration endpoint than
  KKMnO₄ Cr₂O₇
• However KMnO₄ cannot bes used titrate against reducing agents with chloride ions

Question 11

Iodine-Thiosulfate (Iodometric) Titration

Answer 11

• Brown Iodine is titrated against Sodium Thiosulfate
  Na₂S₂O₃ until a pale yellow
  colour (low [I₂])
• Starch indicator then added to get sharper change of colour from
  dark blue to colourless to be observed at the endpoint
• I₂ + 2S₂O₃^2- → 2I^- +2S₄O₆^2-
• Redox reaction:
  I₂ to 2I^-
  Sulfur oxidised to tetrathionate