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Chemistry IB HL (2016+) > Topic 9: Redox Processes > Flashcards

Topic 9: Redox Processes Flashcards

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1
Q

oxidation

A

loss of electrons

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2
Q

reduction

A

gain of electrons

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3
Q

rules for oxidation states

A
  • the sum of all oxidation states in a compound always corresponds to overall charge
  • in a compound, oxidation state usually corresponds to group no (except for transition metals)
  • H is +1 (except when bonded to a metal, in which case it’s -1)
  • O is -2 (except in H2O2, in which case it’s -1)
  • F is always -1
  • pure elements are always 0
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4
Q

oxidizing agent

A

AKA oxidant

  • readily accept electrons
  • thus gets reduced in a rxn
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5
Q

reducing agent

A

AKA reductant

  • readily donates electrons
  • thus gets oxidized in a rxn
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6
Q

oxidation and reduction in terms of oxidation states

A
  • oxidation state increases when oxidized
  • vice versa for reduction
  • if there’s no change in oxidation state in a rxn, the rxn is not a redox rxn
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7
Q

rules for balancing redox equations

A
  • when oxygen is deficient, add H2O (l)

- when hydrogen is deficient, add H+ (aq)

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8
Q

redox tendency trends in activity series

A

down the series:

  • ↓ reactivity
  • ↑ reducing ability
  • ↓ oxidizing ability
  • thus for metals, ↑ reactivity = ↑ reducing ability
  • as they are reductants, they undergo oxidation
  • more reactive metals can reduce less reactive metals
  • in redox rxns, this explains why H in acids can only be displaced by metals above it in the activity series
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9
Q

redox reactions: highly reactive metals + cold water

A

2M (s) + 2H2O (l) → 2M+ (aq) + 2OH- (aq) + H2 (g)

M can only be alkali metals

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10
Q

redox reactions: less reactive metals + steam

A

M (s) + 2H2O (l) → MOH* (aq) + H2 (g)

*dependent on M’s charge

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11
Q

redox tendency trends in Group VII

A

down the series:

  • ↓ reactivity
  • ↓ reducing ability
  • ↑ oxidizing ability
  • thus for non-metals, ↑ reactivity = ↑ oxidizing ability
  • as they are oxidants, they undergo reduction
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12
Q

winkler method

A
  • application of redox process

- used to measure BOD (biological oxygen demand)

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13
Q

BOD

A

Biological Oxygen Demand
- measure of the dissolved oxygen (in ppm) required to decompose organic matter in water over a set time period (usually 5 days)

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14
Q

implications behind BOD values

A

↑ pollution and ↑ BOD = cannot sustain aquatic life

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15
Q

winkler method - process

A
    • a measured vol of the sample is incubated for 5 days
      - microorganisms in the water oxidize the organic material
      - after 5 days, excess Mn (II) salt is added
  2. In alkaline conditions, Mn (II) is oxidized to Mn (IV) oxide by remaining oxygen:
    2Mn2+ (aq) + 4OH- (aq) + O2 (aq) → 2MnO2 (s) + 2H2O (l)
  3. KI is added, and in acidic conditions is oxidized by Mn (IV) oxide to form iodine:
    MnO2 (s) + 2I- (aq) + 4H+ (aq) → Mn2+ (aq) + I2 (aq) + 2H2O (l)
  4. The released iodine is titrated with sodium thiosulfate:
    I2 (aq) + 2S2O3 2- (aq) → S4O6 2- + 2I- (Aq)
  5. The oxygen concentration in the sample can be deduced by determining the number of moles of iodine released.
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16
Q

ppm

A

mass (in mg) dissolved in 1 dm^3 of water

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17
Q

half-cell

A

metal in contact with an aq soln of its own ions

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18
Q

voltaic cell

A
  • 2 diff half cells
  • connected to enable e- transfer during redox rxns
  • in order to produce energy (in the form of electricity)
  • cells are connected by a salt bridge and external wire to enable free movement of ions
  • ions flow through the salt bridge to complete the circuit and keep half-cells electrically neutral
  • in voltaic cells, anode is negative (where oxidation occurs)
19
Q

rules for writing cells

A
  • half-cell undergoing oxidation is always placed on the left
  • 2 vertical lines ( || ) are used to separate them
  • presented as electrode | before , after || before , after | electrode
20
Q

electrolyte

A
  • substance that doesn’t conduct electricity when solid

- but can conduct when liquid

21
Q

electrolytic cell

A
  • used to make non-spontaneous redox rxns occur
  • by supplying energy from an external power source
  • the electricity is passed through the electrolyte to allow electrical energy to convert into chemical energy
  • it’s industrially used to obtain reactive metals from their ores
  • in electrolytic cells, anode is positive
    e. g. electrolysis of molten NaCl
22
Q

redox titrations

A
  • used to determine the unknown concentration of a known analyte
  • finds equivalence point where the redox reaction has been stoichiometrically completed via e- transfer
  • doesn’t necessarily require an indicator (as some redox rxns change color at equivalence point)
23
Q

when redox titrations are used

A
  • food industry
  • pharmaceutical industry
  • water and environment analysis
24
Q

examples of common redox titrations

A
  • analysis of iron content using KMnO4 (potassium permanganate)
  • as part of the winkler method: iodine-thiosulfate redox titration with starch as an indicator
25
Q

redox half-equations of the iodine-thiosulfate reaction

A

oxidation: 2S2O3 2- –> S4O6 2- + 2e-
reduction: I2 + 2e- –> 2I-

!! remember: halogens are very strong oxidizing agents

26
Q

standard conditions to measure standard electrode potential

A
  • conc: 1 mol dm^-3
  • pressure: 100kPa
  • all pure substances
  • temp: 298K
  • if no metals are present or can conduct, electrodes must be Pt
27
Q

electrolysis: physical observations of bromine

A
  • brown liquid

- strong smell

28
Q

electrolysis: physical observations of copper

A

pinkish metallic solid

29
Q

electrolysis: physical observations of chlorine

A
  • greenish-yellow gas

- pungent smell

30
Q

electrolysis: physical observations of lead

A

grey metallic solid

31
Q

calculating electrolysis products

A

Q (charge) = lt (current x time)

no of moles of e-s = Q / F (F = faraday’s constant)

32
Q

electrolysis of aqueous solutions

A
  • ## H2O may be oxidized/reduced in place of one or both reactants
33
Q

factors affecting the outcome of electrolysis of aqueous solutions

A
  • relative cell potential values of the ions
  • relative concentrations of the ions
  • the nature of the electrode
34
Q

special consideration for the electrolysis of NaCl (aq)

A
  • at the cathode either OH- or Cl- may be produced

- Cl- is preferentially discharged when the concentration by mass of NaCl (aq) is > 25% of the overall solution

35
Q

electrolysis of aqueous solutions

A
  • ## H2O may be oxidized/reduced in place of one or both reactants
36
Q

factors affecting the outcome of electrolysis of aqueous solutions

A
  • relative cell potential values of the ions
  • relative concentrations of the ions
  • the nature of the electrode
37
Q

special consideration for the electrolysis of NaCl (aq)

A
  • at the cathode either OH- or Cl- may be produced
  • Cl- is preferentially discharged when the concentration by mass of NaCl (aq) is > 25% of the overall solution
  • otherwise OH- is discharged (as its E cell value is smaller)
38
Q

observations when Cl- is discharged in the electrolysis of NaCl (aq)

A
  • strong smell
  • bleaching effect on damp litmus paper
  • gas evolved at both anode (Cl2) and cathode (H2)
39
Q

observations when copper electrodes are used in the electrolysis of CuSO4 (aq)

A
  • pinky-brown metallic solid deposited on cathode
  • no change in intensity of blue color
  • anode disintegrates
  • no change in pH
40
Q

applications of the electrolysis of CuSO4 (aq) using copper electrodes

A

purification of copper:

  • uses electrolysis of a solution containing Cu2+ ions, an impure copper anode, and a small pure copper cathode
  • the anode disintegrates as Cu transfers to the cathode, which increases in mass
  • impurities collect as sludge at the bottom of the cell
41
Q

observations when inert electrodes are used in the electrolysis of CuSO4 (aq)

A
  • gas evolved at anode (O2)
  • decreased pH (due to release of H+ at anode)
  • loss of intensity of blue color (due to reduction of Cu2+)
  • pinky-brown metallic solid deposited on cathode
42
Q

observations when copper electrodes are used in the electrolysis of CuSO4 (aq)

A
  • pinky-brown metallic solid deposited on cathode
  • no change in intensity of blue color
  • anode disintegrates
  • no change in pH
43
Q

applications of the electrolysis of CuSO4 (aq) using copper electrodes

A

purification of copper:

  • uses electrolysis of a solution containing Cu2+ ions, an impure copper anode, and a small pure copper cathode
  • the anode disintegrates as Cu transfers to the cathode, which increases in mass
  • impurities collect as sludge at the bottom of the cell
44
Q

differences between voltaic and electrolytic cells

A
  • voltaic: spontaneous rxns with chemical –> electrical energy conversion
    electrolytic: non-spontaneous rxns with electrical –> chemical energy conversion
  • voltaic: 2 separate solns joined by salt bridge
    electrolytic: 1 soln, no salt bridge required

Chemistry IB HL (2016+) (13 decks)

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